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Los Alamos, NM, United States

Los Alamos National Laboratory is one of two laboratories in the United States where classified work towards the design of nuclear weapons is undertaken. The other, since 1952, is Lawrence Livermore National Laboratory. LANL is a United States Department of Energy national laboratory, managed and operated by Los Alamos National Security , located in Los Alamos, New Mexico. The laboratory is one of the largest science and technology institutions in the world. It conducts multidisciplinary research in fields such as national security, space exploration, renewable energy, medicine, nanotechnology, and supercomputing.LANL is the largest institution and the largest employer in northern New Mexico, with approximately 9,000 direct employees and around 650 contractor personnel. Additionally, there are roughly 120 DOE employees stationed at the laboratory to provide federal oversight of LANL's work and operations. Approximately one-third of the laboratory's technical staff members are physicists, one quarter are engineers, one-sixth are chemists and materials scientists, and the remainder work in mathematics and computational science, biology, geoscience, and other disciplines. Professional scientists and students also come to Los Alamos as visitors to participate in scientific projects. The staff collaborates with universities and industry in both basic and applied research to develop resources for the future. The annual budget is approximately US$2.2 billion. Wikipedia.

Zhang Y.,Rice University | Grady N.K.,Rice University | Grady N.K.,Los Alamos National Laboratory | Ayala-Orozco C.,Rice University | Halas N.J.,Rice University
Nano Letters | Year: 2011

Plasmonic nanostructures enable the generation of large electromagnetic fields confined to small volumes, potentially providing a route for the development of nanoengineered nonlinear optical media. A metal-capped hemispherical nanoparticle, also known as a nanocup, generates second harmonic light with increasing intensity as the angle between the incident fundamental beam and the nanocup symmetry axis is increased. Nanoparticle orientation also modifies the emission direction of the second harmonic light. With conversion efficiencies similar to those of inorganic SHG crystals, these structures provide a promising approach for the design and fabrication of stable, synthetic second-order nonlinear optical materials tailored for specific wavelengths. © 2011 American Chemical Society.

Chartrand R.,Los Alamos National Laboratory
IEEE Transactions on Signal Processing | Year: 2012

We develop new nonconvex approaches for matrix optimization problems involving sparsity. The heart of the methods is a new, nonconvex penalty function that is designed for efficient minimization by means of a generalized shrinkage operation. We apply this approach to the decomposition of video into low rank and sparse components, which is able to separate moving objects from the stationary background better than in the convex case. In the case of noisy data, we add a nonconvex regularization, and apply a splitting approach to decompose the optimization problem into simple, parallelizable components. The nonconvex regularization ameliorates contrast loss, thereby allowing stronger denoising without losing more signal to the residual. © 2012 IEEE.

Kapustinsky J.S.,Los Alamos National Laboratory
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2010

The Forward Silicon Vertex Tracker (FVTX) upgrade for the PHENIX detector at RHIC will extend the vertex capability of the central PHENIX Silicon Vertex Tracker (VTX). The FVTX is designed with adequate spatial resolution to separate decay muons coming from the relatively long-lived heavy quark mesons (Charm and Beauty), from prompt particles and the longer-lived pion and kaon decays that originate at the primary collision vertex. These heavy quarks can be used to probe the high-density medium that is formed in Au+Au collisions at RHIC. The FVTX is designed as two endcaps. Each endcap comprises four silicon disks covering opening angles from 10° to 35° to match the existing muon arm acceptance. Each disk consists of p-on-n, silicon wedges, with ac-coupled mini-strips on 75 μm radial pitch and projective length in the phi direction that increases with radius. A custom front-end chip, the FPHX, has been designed for the FVTX. The chip combines fast trigger capability with data push architecture in a low-power design. © 2009 Elsevier B.V. All rights reserved.

Guo H.,James Franck Institute | Chien C.-C.,Los Alamos National Laboratory | Levin K.,James Franck Institute
Physical Review Letters | Year: 2010

We show how in ultracold Fermi gases the difference between the finite temperature T structure factors, called S-(ω,q), associated with spin and density, reflects coherent order at all ω, q, kFa, and T. This observation can be exploited in two photon Bragg scattering experiments on gases which are subject to variable attractive interactions. Our calculations incorporate spin and particle number conservation laws which lead to compatibility at general T with two f-sum rules. Because of its generality a measurement of S-(ω,q) can be a qualitative, direct, in situ approach for establishing superfluid order.© 2010 The American Physical Society.

Silver G.L.,Los Alamos National Laboratory
Journal of Radioanalytical and Nuclear Chemistry | Year: 2011

Alternative methods for estimating the numerical value of the equilibrium-constant of the first hydrolysis reaction of tetravalent plutonium are illustrated. They are applied to recent data on Pu oxidation-state distributions in HCl solutions. The new estimates of the hydrolysis constant typically agree with the traditional values. © 2011 Akadémiai Kiadó, Budapest, Hungary.

Lin S.-Z.,Japan International Center for Materials Nanoarchitectonics | Hu X.,Los Alamos National Laboratory
New Journal of Physics | Year: 2012

The Josephson-like interband couplings in multi-band superconductivity exhibit degenerate energy minima, which support states with kinks in phase of superconductivity. When the interband couplings in systems of three or more components are frustrated, the time-reversal symmetry (TRS) can be broken, which generates another type of phase kink between the two time-reversal symmetry breaking (TRSB) pair states. In this work, we focus on these novel states of phase kinks, and investigate their stability, similarity, differences and physical consequences. The main results can be summarized as follows: (i) we find a new type of phase slip when the kink becomes unstable. (ii) In the kink region, TRS is broken and spontaneous magnetic fields are induced. (iii) In superconductors with TRSB, composite topological excitations associated with variations of both the superconductivity phase and amplitude can be created by local perturbations or due to proximity effect between normal metals. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Wohlberg B.,Los Alamos National Laboratory
IEEE Signal Processing Letters | Year: 2011

Exemplar-based methods, in which actual image blocks are used to fill in missing content, have achieved state of the art performance in image inpainting. The majority of these adopt a progressive approach, filling in the missing region inwards from the boundary. The final result is highly dependent on fill order, and while significant progress has been made on the choice of this order, the greedy nature of such a process leads to artifacts in some cases. The alternative exemplar-based approach proposed here is defined via joint optimization of a single functional, simultaneously assigning an estimated value to the entire inpainting region. The results are found to be highly competitive with other recent inpainting methods. © 2006 IEEE.

Seletskiy D.V.,University of New Mexico | Seletskiy D.V.,Air Force Research Lab | Hehlen M.P.,Los Alamos National Laboratory | Epstein R.I.,University of New Mexico | Sheik-Bahae M.,University of New Mexico
Advances in Optics and Photonics | Year: 2012

We review the field of laser cooling of solids, focusing our attention on the recent advances in cryogenic cooling of an ytterbium-doped fluoride crystal (Yb3+:YLiF4). Recently, bulk cooling in this material to 155 K has been observed upon excitation near the lowest-energy (E4-E5) crystal-field resonance of Yb3+. Furthermore, local cooling in the same material to a minimum achievable temperature of 110 K has been measured, in agreement with the predictions of the laser cooling model. This value is limited only by the current material purity. Advanced material synthesis approaches reviewed here would allow reaching temperatures approaching 80 K. Current results and projected improvements position optical refrigeration as the only viable all-solid-state cooling approach for cryogenic temperatures. © 2010 Optical Society of America.

Lindquist N.C.,University of Minnesota | Lindquist N.C.,Bethel University | Nagpal P.,Los Alamos National Laboratory | McPeak K.M.,ETH Zurich | And 2 more authors.
Reports on Progress in Physics | Year: 2012

Metallic nanostructures now play an important role in many applications. In particular, for the emerging fields of plasmonics and nanophotonics, the ability to engineer metals on nanometric scales allows the development of new devices and the study of exciting physics. This review focuses on top-down nanofabrication techniques for engineering metallic nanostructures, along with computational and experimental characterization techniques. A variety of current and emerging applications are also covered. © 2012 IOP Publishing Ltd.

Elliott S.R.,Los Alamos National Laboratory | Franz M.,University of British Columbia
Reviews of Modern Physics | Year: 2015

Ettore Majorana (1906-1938) disappeared while traveling by ship from Palermo to Naples in 1938. His fate has never been fully resolved and several articles have been written that explore the mystery itself. His demise intrigues us still today because of his seminal work, published the previous year, that established symmetric solutions to the Dirac equation that describe a fermionic particle that is its own antiparticle. This work has long had a significant impact in neutrino physics, where this fundamental question regarding the particle remains unanswered. But the formalism he developed has found many uses as there are now a number of candidate spin-1/2 neutral particles that may be truly neutral with no quantum number to distinguish them from their antiparticles. If such particles exist, they will influence many areas of nuclear and particle physics. Most notably the process of neutrinoless double beta decay can exist only if neutrinos are massive Majorana particles. Hence, many efforts to search for this process are underway. Majorana's influence does not stop with particle physics, however, even though that was his original consideration. The equations he derived also arise in solid-state physics where they describe electronic states in materials with superconducting order. Of special interest here is the class of solutions of the Majorana equation in one and two spatial dimensions at exactly zero energy. These Majorana zero modes are endowed with some remarkable physical properties that may lead to advances in quantum computing and, in fact, there is evidence that they have been experimentally observed. This Colloquium first summarizes the basics of Majorana's theory and its implications. It then provides an overview of the rich experimental programs trying to find a fermion that is its own antiparticle in nuclear, particle, and solid-state physics. © 2015 American Physical Society.

Booth T.E.,Los Alamos National Laboratory
Nuclear Science and Engineering | Year: 2010

This technical note shows that it is possible and effective to use Monte Carlo variancereduction methods for the probability of initiation problem. The benefits are threefold. First, the proper use of variance reduction obviates using an arbitrary definition of a "divergent chain. " Second, because chains of all lengths are allowed, there is no bias introduced by ignoring some long chains because they meet the divergent chain definition. Third, variance-reduction methods might drastically increase the efficiency of some of these calculations.

Mitri F.G.,Los Alamos National Laboratory | Fellah Z.E.A.,CNRS Laboratory of Mechanics and Acoustics
Ultrasonics | Year: 2014

The present analysis investigates the (axial) acoustic radiation force induced by a quasi-Gaussian beam centered on an elastic and a viscoelastic (polymer-type) sphere in a nonviscous fluid. The quasi-Gaussian beam is an exact solution of the source free Helmholtz wave equation and is characterized by an arbitrary waist w0 and a diffraction convergence length known as the Rayleigh range zR. Examples are found where the radiation force unexpectedly approaches closely to zero at some of the elastic sphere's resonance frequencies for kw0 ≤ 1 (where this range is of particular interest in describing strongly focused or divergent beams), which may produce particle immobilization along the axial direction. Moreover, the (quasi)vanishing behavior of the radiation force is found to be correlated with conditions giving extinction of the backscattering by the quasi-Gaussian beam. Furthermore, the mechanism for the quasi-zero force is studied theoretically by analyzing the contributions of the kinetic, potential and momentum flux energy densities and their density functions. It is found that all the components vanish simultaneously at the selected ka values for the nulls. However, for a viscoelastic sphere, acoustic absorption degrades the quasi-zero radiation force. © 2013 Elsevier B.V. All rights reserved.

Lee S.-B.,Carnegie Mellon University | Lebensohn R.A.,Los Alamos National Laboratory | Rollett A.D.,Carnegie Mellon University
International Journal of Plasticity | Year: 2011

A viscoplastic approach using the Fast Fourier Transform (FFT) method for obtaining local mechanical response is utilized to study microstructure-property relationships in composite materials. Specifically, three-dimensional, two-phase digital materials containing isotropically coarsened particles surrounded by a matrix phase, generated through a Kinetic Monte Carlo Potts model for Ostwald ripening, are used as instantiations in order to calculate the stress and strain-rate fields under uniaxial tension. The effects of the morphology of the matrix phase, the volume fraction and the contiguity of particles, and the polycrystallinity of matrix phase, on the stress and strain-rate fields under uniaxial tension are examined. It is found that the first moments of the stress and strain-rate fields have a different dependence on the particle volume fraction and the particle contiguity from their second moments. The average stresses and average strain-rates of both phases and of the overall composite have rather simple relationships with the particle volume fraction whereas their standard deviations vary strongly, especially when the particle volume fraction is high, and the contiguity of particles has a noticeable effect on the mechanical response. It is also found that the shape of stress distribution in the BCC hard particle phase evolves as the volume fraction of particles in the composite varies, such that it agrees with the stress field in the BCC polycrystal as the volume of particles approaches unity. Finally, it is observed that the stress and strain-rate fields in the microstructures with a polycrystalline matrix are less sensitive to changes in volume fraction and contiguity of particles. © 2010 Elsevier Ltd. All rights reserved.

Silva G.T.,Federal University of Alagoas | Mitri F.G.,Los Alamos National Laboratory
Physics in Medicine and Biology | Year: 2011

Vibro-acoustography (VA) is a medical imaging method based on the nonlinear interaction of two or more distinct ultrasound beams whose frequencies differ by several kHz. In turn, the interacting waves produce a difference-frequency signal which carries the information of the imaged tissue region. Two mechanisms are responsible for the difference-frequency generation (DFG) in VA, namely the dynamic (oscillatory) radiation force and the scattering of sound-by-sound. The role and importance of each phenomenon in VA is assessed here. A theoretical model based on Westervelt's equation for the DFG in the nonlinear scattering of two incident ultrasound waves by a small rigid sphere (compared to the incident wavelengths) is presented. Furthermore, a scattering experiment using VA is devised and the data show very good agreement with the proposed theory. The results reveal that the effect of scattering of sound-by-sound is the dominant component in the DFG in VA rather than the dynamic radiation force. © 2011 Institute of Physics and Engineering in Medicine.

Black-Schaffer A.M.,Uppsala University | Balatsky A.V.,Los Alamos National Laboratory | Balatsky A.V.,NORDITA
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We discuss the appearance of odd-frequency spin-triplet s-wave superconductivity, first proposed by Berezinskii, on the surface of a topological insulator proximity coupled to a conventional spin-singlet s-wave superconductor. Using both analytical and numerical methods, we show that this disorder robust odd-frequency state is present whenever there is an in-surface gradient in the proximity induced gap, including superconductor-normal state junctions. The time-independent order parameter for the odd-frequency superconductor is proportional to the in-surface gap gradient. The induced odd-frequency component does not produce any low-energy states. © 2012 American Physical Society.

Black-Schaffer A.M.,Uppsala University | Balatsky A.V.,Los Alamos National Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Using a three-dimensional microscopic lattice model of a strong topological insulator (TI) we study potential impurities and vacancies in surface, subsurface, and bulk positions. For all impurity locations we find impurity-induced resonance states with energy proportional to the inverse of the impurity strength, although the impurity strength needed for a low-energy resonance state increases with the depth of the impurity. For strong impurities and vacancies as deep as 15 layers into the material, resonance peaks will appear at and around the Dirac point in the surface energy spectrum, splitting the original Dirac point into two nodes located off-center. Furthermore, we study vacancy clusters buried deep inside the bulk and find zero-energy resonance states for both single and multiple-site vacancies. Only fully symmetric multiple-site vacancy clusters show resonance states expelled from the bulk gap. © 2012 American Physical Society.

Das T.,Los Alamos National Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Motivated by the recent discovery of two Q∼0 collective modes in single-layer HgBa 2CuO 4+δ, which are often taken as evidence of the orbital-current origin of a pseudogap, I examine an alternative and assumption-free scenario constrained by first-principles calculations. I find that in addition to the common CuO 2 band, a hybridized Hg-O state is present in the vicinity of the Fermi level and that it contributes to the low-energy ground state of this system. I calculate the spin-excitation spectrum based on the random-phase approximation in the superconducting state using a two-band model and show that a collective mode in the multiorbital channel arises at Q=0. This mode splits in energy yet remains at Q∼0 as a pseudogap develops breaking both translational and time-reversal symmetries. The observations of the dynamical mode and static moment in the pseudogap state are in good accord with experimental observations. Detection of Hg-O band via optical study, or magnetic moment in the Hg-O layer will be tests of this calculation. © 2012 American Physical Society.

Das T.,Los Alamos National Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We present a random-phase-approximation-based multilayer spin-susceptibility calculation for the trilayer YBa 2Cu 3O 6+y (YBCO) system (including bilayer CuO 2 planes and uniaxial CuO chain layer) in the superconducting state. We show that the observed in-plane anisotropy in the spin-excitation spectrum of YBCO-which is often interpreted as evidence for the electron nematic phase-can alternatively be explained via incorporating the uniaxial CuO chain's contribution. We demonstrate that the neutron spectra in YBCO is dominated by the contribution from the fourfold symmetric CuO 2 plane state as in other cuprates; however, it acquires an in-plane anisotropy via finite interlayer coupling with the chain state. The result rules out the claim that an electronic nematic phase is responsible for the pseudogap state in YBCO. © 2012 American Physical Society.

Wang Z.Q.,University of North Texas | Beyerlein I.J.,Los Alamos National Laboratory
International Journal of Plasticity | Year: 2011

Atomistic simulations have shown that a screw dislocation in body-centered cubic (BCC) metals has a complex non-planar atomic core structure. The configuration of this core controls their motion and is affected not only by the usual resolved shear stress on the dislocation, but also by non-driving stress components. Consequences of the latter are referred to as non-Schmid effects. These atomic and micro-scale effects are the reason slip characteristics in deforming single and polycrystalline BCC metals are extremely sensitive to the direction and sense of the applied load. In this paper, we develop a three-dimensional discrete dislocation dynamics (DD) simulation model to understand the relationship between individual dislocation glide behavior and macro-scale plastic slip behavior in single crystal BCC Ta. For the first time, it is shown that non-Schmid effects on screw dislocations of both {110} and {112} slip systems must be implemented into the DD models in order to predict the strong plastic anisotropy and tension-compression asymmetry experimentally observed in the stress-strain curves of single crystal Ta. Incorporation of fundamental atomistic information is critical for developing a physics-based, predictive meso-scale DD simulation tool that can connect length/time scales and investigate the underlying mechanisms governing the deformation of BCC metals. © 2011 Elsevier Ltd. All rights reserved.

Das T.,Los Alamos National Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We present magnetotransport calculations for YBa 2Cu 3O 7-δ (YBCO) materials to show that the electron-like metallic chain state gives both the negative Hall effect and in-plane anisotropic large Nernst signal. We show that the inevitable presence of the metallic 1D CuO chain layer lying between the CuO 2 bilayers in YBCO renders an electron-like Fermi surface in the doping range as wide as p=0.05 to overdoping. With underdoping, a pseudogap opening in the CuO 2 state reduces its hole-carrier contribution, and, therefore, the net electron-like quasiparticles dominate the transport properties, and a negative Hall resistance commences. We also show that the observation of in-plane anisotropy in the Nernst signal-which was taken as a definite evidence of the electronic "nematic" pseudogap phase-is naturally explained by including the "quasiuniaxial" metallic chain state. Finally, we comment on how the chain state can also lead to electron-like quantum oscillations. © 2012 American Physical Society.

Beaird R.,Louisiana State University | Vekhter I.,Louisiana State University | Zhu J.-X.,Los Alamos National Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We examine the effect of a single, nonmagnetic impurity in a multiband, extended s-wave superconductor allowing for anisotropy of the gaps on the Fermi surfaces. We derive analytic expressions for the Green's functions in the continuum and analyze the conditions for the existence of sharp impurity-induced resonant states. Underlying band structure is more relevant for the multiband than for the single-band case, and mismatch between the bands generically makes the formation of the impurity states less likely in the physical regime of parameters. We confirm these conclusions by numerically solving the impurity problem in a tight-binding parametrization of the bands relevant to pnictide superconductors. © 2012 American Physical Society.

Kolluri K.,Massachusetts Institute of Technology | Kolluri K.,Los Alamos National Laboratory | Demkowicz M.J.,Massachusetts Institute of Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Atomistic simulations are used to study the formation, migration, and clustering of delocalized vacancies and interstitials at a model fcc-bcc semicoherent interface formed by adjacent layers of Cu and Nb. These defects migrate between interfacial trapping sites through a multistep mechanism that may be described using dislocation mechanics. Similar mechanisms operate in the formation, migration, and dissociation of interfacial point defect clusters. Effective migration rates may be computed using the harmonic approximation of transition state theory with a temperature-dependent prefactor. Our results demonstrate that delocalized vacancies and interstitials at some interfaces may be viewed as genuine defects, albeit governed by mechanisms of higher complexity than conventional point defects in crystalline solids. © 2012 American Physical Society.

Black-Schaffer A.M.,NORDITA | Black-Schaffer A.M.,Uppsala University | Balatsky A.V.,Los Alamos National Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Topological insulators (TIs) are said to be stable against nonmagnetic impurity scattering due to suppressed backscattering in the Dirac surface states. We solve a lattice model of a three-dimensional TI in the presence of strong potential impurities and find that both the Dirac point and low-energy states are significantly modified: Low-energy impurity resonances are formed that produce a peak in the density of states near the Dirac point, which is destroyed and split into two nodes that move off center. The impurity-induced states penetrate up to ten layers into the bulk of the TI. These findings demonstrate the importance of bulk states for the stability of TIs and how they can destroy the topological protection of the surface. © 2012 American Physical Society.

Brandao F.G.S.L.,Federal University of Minas Gerais | Christandl M.,ETH Zurich | Yard J.,Los Alamos National Laboratory
Proceedings of the Annual ACM Symposium on Theory of Computing | Year: 2011

We present a quasipolynomial-time algorithm for solving the weak membership problem for the convex set of separable, i.e. non-entangled, bipartite density matrices. The algorithm decides whether a density matrix is separable or whether it is ∈-away from the set of the separable states in time exp(O(∈-2 log|A| log|B|)), where |A| and |B| are the local dimensions, and the distance is measured with either the Euclidean norm, or with the so-called LOCC norm. The latter is an operationally motivated norm giving the optimal probability of distinguishing two bipartite quantum states, each shared by two parties, using any protocol formed by quantum local operations and classical communication (LOCC) between the parties. We also obtain improved algorithms for optimizing over the set of separable states and for computing the ground-state energy of mean-field Hamiltonians. The techniques we develop are also applied to quantum Merlin-Arthur games, where we show that multiple provers are not more powerful than a single prover when the verifier is restricted to LOCC protocols, or when the verification procedure is formed by a measurement of small Euclidean norm. This answers a question posed by Aaronson et al. (Theory of Computing 5, 1, 2009) and provides two new characterizations of the complexity class QMA, a quantum analog of NP. Our algorithm uses semidefinite programming to search for a symmetric extension, as first proposed by Doherty, Parrilo and Spedialieri (Phys. Rev. A, 69, 022308, 2004). The bound on the runtime follows from an improved de Finetti-type bound quantifying the monogamy of quantum entanglement. This result, in turn, follows from a new lower bound on the quantum conditional mutual information and the entanglement measure squashed entanglement. © 2011 ACM.

Luo S.,University of California at Berkeley | Perelson A.S.,Los Alamos National Laboratory
Philosophical Transactions of the Royal Society B: Biological Sciences | Year: 2015

Antibody affinity maturation by somatic hypermutation of B-cell immunoglobulin variable region genes has been studied for decades in various model systems using well-defined antigens. While much is known about the molecular details of the process, our understanding of the selective forces that generate affinity maturation are less well developed, particularly in the case of a co-evolving pathogen such as HIV. Despite this gap in understanding, high-throughput antibody sequence data are increasingly being collected to investigate the evolutionary trajectories of antibody lineages in HIV-infected individuals. Here, we review what is known in controlled experimental systems about the mechanisms underlying antibody selection and compare this to the observed temporal patterns of antibody evolution in HIV infection. We describe howour current understanding of antibody selection mechanisms leaves questions about antibody dynamics in HIV infection unanswered. Without a mechanistic understanding of antibody selection in the context of a co-evolving viral population, modelling and analysis of antibody sequences in HIV-infected individuals will be limited in their interpretation and predictive ability. © 2015 The Author(s) Published by the Royal Society. All rights reserved.

Lin S.-Z.,Los Alamos National Laboratory
Journal of Physics Condensed Matter | Year: 2014

This article reviews theoretical and experimental work on the novel physics in multiband superconductors. Multiband superconductors are characterized by multiple superconducting energy gaps in different bands with interaction between Cooper pairs in these bands. The discovery of prominent multiband superconductors MgB2 and later iron-based superconductors, has triggered enormous interest in multiband superconductors. The most recently discovered superconductors exhibit multiband features. The multiband superconductors possess novel properties that are not shared with their single-band counterpart. Examples include: the time-reversal symmetry broken state in multiband superconductors with frustrated interband couplings; the collective oscillation of number of Cooper pairs between different bands, known as the Leggett mode; and the phase soliton and fractional vortex, which are the main focus of this review. This review presents a survey of a wide range of theoretical exploratory and experimental investigations of novel physics in multiband superconductors. A vast amount of information derived from these studies is shown to highlight unusual and unique properties of multiband superconductors and to reveal the challenges and opportunities in the research on the multiband superconductivity. © 2014 IOP Publishing Ltd.

Efimov A.,Los Alamos National Laboratory
Optics Express | Year: 2014

The modulus of the complex degree of coherence is directly measured at the output of a step-index multimode optical fiber using lateral-sheering, delay-dithering Mach-Zehnder interferometer. Pumping the multimode fiber with monochromatic light always results in spatiallycoherent output, whereas for the broadband pumping the modal dispersion of the fiber leads to a partially coherent output. While the coherence radius is a function of the numerical aperture only, the residual coherence outside the main peak is an interesting function of two dimensionless parameters: the number of non-degenerate modes and the ratio of the modal dispersion to the coherence time of the source. We develop a simple model describing this residual coherence and verify its predictions experimentally. © 2014 Optical Society of America.

Livescu D.,Los Alamos National Laboratory
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2013

A tentative review is presented of various approaches for numerical simulations of two-fluid gaseous mixtures at high density ratios, as they have been applied to the Rayleigh-Taylor instability (RTI). Systems exhibiting such RTI behaviour extend from atomistic sizes to scales where the continuum approximation becomes valid. Each level of description can fit into a hierarchy of theoretical models and the governing equations appropriate for each model, with their assumptions, are presented. In particular, because the compressible to incompressible limit of the Navier-Stokes equations is not unique and understanding compressibility effects in the RTI critically depends on having the appropriate basis for comparison, two relevant incompressible limits are presented. One of these limits has not been considered before. Recent results from RTI simulations, spanning the levels of description presented, are reviewed in connection to the material mixing problem. Owing to the computational limitations, most in-depth RTI results have been obtained for the incompressible case. Two such results, concerning the asymmetry of the mixing and small-scale anisotropy anomaly, as well as the possibility of a mixing transition in the RTI, are surveyed. New lines for further investigation are suggested and it is hoped that bringing together such diverse levels of description may provide new ideas and increased motivation for studying such flows. © 2013 The Author(s) Published by the Royal Society. All rights reserved.

Boffetta G.,University of Turin | Ecke R.E.,Los Alamos National Laboratory
Annual Review of Fluid Mechanics | Year: 2011

In physical systems, a reduction in dimensionality often leads to exciting new phenomena. Here we discuss the novel effects arising from the consideration of fluid turbulence confined to two spatial dimensions. The additional conservation constraint on squared vorticity relative to three-dimensional (3D) turbulence leads to the dual-cascade scenario of Kraichnan and Batchelor with an inverse energy cascade to larger scales and a direct enstrophy cascade to smaller scales. Specific theoretical predictions of spectra, structure functions, probability distributions, and mechanisms are presented, and major experimental and numerical comparisons are reviewed. The introduction of 3D perturbations does not destroy the main features of the cascade picture, implying that 2D turbulence phenomenology establishes the general picture of turbulent fluid flows when one spatial direction is heavily constrained by geometry or by applied body forces. Such flows are common in geophysical and planetary contexts, are beautiful to observe, and reflect the impact of dimensionality on fluid turbulence.

Silver G.L.,Los Alamos National Laboratory
Journal of Radioanalytical and Nuclear Chemistry | Year: 2011

The one-oxidation-state method is used to estimate the equilibrium constant of the first hydrolysis reaction of tetravalent plutonium. The analysis of the properties of plutonium near a triple point is an alternative approach to the estimation of the hydrolysis constant. © 2011 Akadémiai Kiadó, Budapest, Hungary.

Silver G.L.,Los Alamos National Laboratory
Journal of Radioanalytical and Nuclear Chemistry | Year: 2011

A simplified method for representing the disproportionation reactions of plutonium is illustrated. It applies to any N within the range (3 < N < 6) and at any pH that does not introduce precipitation or polymer-forming reactions. Recalculation of recent estimates of the first hydrolysis constant of the tetravalent plutonium ion improves their precision. © 2010 Akadémiai Kiadó, Budapest, Hungary.

Mitri F.G.,Los Alamos National Laboratory
Applied Physics Letters | Year: 2013

The possibility to trap a sphere in the near-field of a single-beam piston transducer is theoretically demonstrated. Conditions are found where a rigid, fluid, elastic, and viscoelastic sphere with arbitrary radius placed in the near-field and centered on the axis of a circular piezoelectric transducer vibrating uniformly, experiences a pulling force, so the acoustical waves act as a "tractor" beam. Numerical predictions illustrate the theory with particular emphasis on the distance from the source, the size of the transducer, and the elastic properties of the sphere. Those results can potentially suggest a simple and reliable method in designing acoustical tweezers. © 2013 AIP Publishing LLC.

Chatterjee A.,Indian Institute of Technology Kanpur | Voter A.F.,Los Alamos National Laboratory
Journal of Chemical Physics | Year: 2010

We present a novel computational algorithm called the accelerated superbasin kinetic Monte Carlo (AS-KMC) method that enables a more efficient study of rare-event dynamics than the standard KMC method while maintaining control over the error. In AS-KMC, the rate constants for processes that are observed many times are lowered during the course of a simulation. As a result, rare processes are observed more frequently than in KMC and the time progresses faster. We first derive error estimates for AS-KMC when the rate constants are modified. These error estimates are next employed to develop a procedure for lowering process rates with control over the maximum error. Finally, numerical calculations are performed to demonstrate that the AS-KMC method captures the correct dynamics, while providing significant CPU savings over KMC in most cases. We show that the AS-KMC method can be employed with any KMC model, even when no time scale separation is present (although in such cases no computational speed-up is observed), without requiring the knowledge of various time scales present in the system. © 2010 American Institute of Physics.

Shafran E.,University of Utah | Mangum B.D.,University of Utah | Mangum B.D.,Los Alamos National Laboratory | Gerton J.M.,University of Utah
Nano Letters | Year: 2010

Precision measurements of resonant energy transfer from isolated quantum dots (QDs) to individual carbon nanotubes (CNTs) exhibit unique features due to the one-dimensional nature of CNTs. In particular, excitons can be created at varying distances from the QD at different locations along the CNT length. This leads to large variations in energy transfer length scales for different QDs and a novel saturation of the energy transfer efficiency at ∼96%, seemingly independent of CNT chirality. © 2010 American Chemical Society.

Libisch F.,Vienna University of Technology | Huang C.,Los Alamos National Laboratory | Carter E.A.,Andlinger Center for Energy and the Environment
Accounts of Chemical Research | Year: 2014

ConspectusAb initio modeling of matter has become a pillar of chemical research: with ever-increasing computational power, simulations can be used to accurately predict, for example, chemical reaction rates, electronic and mechanical properties of materials, and dynamical properties of liquids. Many competing quantum mechanical methods have been developed over the years that vary in computational cost, accuracy, and scalability: density functional theory (DFT), the workhorse of solid-state electronic structure calculations, features a good compromise between accuracy and speed. However, approximate exchange-correlation functionals limit DFT's ability to treat certain phenomena or states of matter, such as charge-transfer processes or strongly correlated materials. Furthermore, conventional DFT is purely a ground-state theory: electronic excitations are beyond its scope. Excitations in molecules are routinely calculated using time-dependent DFT linear response; however applications to condensed matter are still limited.By contrast, many-electron wavefunction methods aim for a very accurate treatment of electronic exchange and correlation. Unfortunately, the associated computational cost renders treatment of more than a handful of heavy atoms challenging. On the other side of the accuracy spectrum, parametrized approaches like tight-binding can treat millions of atoms. In view of the different (dis-)advantages of each method, the simulation of complex systems seems to force a compromise: one is limited to the most accurate method that can still handle the problem size. For many interesting problems, however, compromise proves insufficient. A possible solution is to break up the system into manageable subsystems that may be treated by different computational methods. The interaction between subsystems may be handled by an embedding formalism.In this Account, we review embedded correlated wavefunction (CW) approaches and some applications. We first discuss our density functional embedding theory, which is formally exact. We show how to determine the embedding potential, which replaces the interaction between subsystems, at the DFT level. CW calculations are performed using a fixed embedding potential, that is, a non-self-consistent embedding scheme. We demonstrate this embedding theory for two challenging electron transfer phenomena: (1) initial oxidation of an aluminum surface and (2) hot-electron-mediated dissociation of hydrogen molecules on a gold surface. In both cases, the interaction between gas molecules and metal surfaces were treated by sophisticated CW techniques, with the remainder of the extended metal surface being treated by DFT. Our embedding approach overcomes the limitations of conventional Kohn-Sham DFT in describing charge transfer, multiconfigurational character, and excited states. From these embedding simulations, we gained important insights into fundamental processes that are crucial aspects of fuel cell catalysis (i.e., O2 reduction at metal surfaces) and plasmon-mediated photocatalysis by metal nanoparticles. Moreover, our findings agree very well with experimental observations, while offering new views into the chemistry. We finally discuss our recently formulated potential-functional embedding theory that provides a seamless, first-principles way to include back-action onto the environment from the embedded region. © 2014 American Chemical Society.

Lin S.-Z.,Los Alamos National Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

Motivated by the recent experimental observations on the skyrmion creation by cutting chiral stripe domains under a current drive [Jiang, Science 349, 283 (2015)SCIEAS0036-807510.1126/science.aaa1442], we study the mechanism of skyrmion generation by simulating the dynamics of stripe domains. Our theory for skyrmion generation is based on the fact that there are two half skyrmions attached to the ends of a stripe domain. These half skyrmions move due to the coupling between the skyrmion topological charge and current. As a consequence, the stripe domain is bent or stretched depending on the direction of motion of the half skyrmions. For a large current, skyrmions are created by chopping the stripe domains via strong bending or stretching. Our theory provides an explanation to the experiments and is supported by the new experiments. Furthermore, we predict that skyrmions can also be generated using a Bloch stripe domain under a spin transfer torque which can be realized in B20 compounds. © 2016 American Physical Society.

Delzanno G.L.,Los Alamos National Laboratory
Journal of Computational Physics | Year: 2015

A spectral method for the numerical solution of the multi-dimensional Vlasov-Maxwell equations is presented. The plasma distribution function is expanded in Fourier (for the spatial part) and Hermite (for the velocity part) basis functions, leading to a truncated system of ordinary differential equations for the expansion coefficients (moments) that is discretized with an implicit, second order accurate Crank-Nicolson time discretization. The discrete non-linear system is solved with a preconditioned Jacobian-Free Newton-Krylov method. It is shown analytically that the Fourier-Hermite method features exact conservation laws for total mass, momentum and energy in discrete form. Standard tests involving plasma waves and the whistler instability confirm the validity of the conservation laws numerically. The whistler instability test also shows that we can step over the fastest time scale in the system without incurring in numerical instabilities. Some preconditioning strategies are presented, showing that the number of linear iterations of the Krylov solver can be drastically reduced and a significant gain in performance can be obtained. © 2015 Elsevier Inc.

David Arnett W.,University of Arizona | Meakin C.,University of Arizona | Meakin C.,Los Alamos National Laboratory
Astrophysical Journal | Year: 2011

Three-dimensional (3D) hydrodynamic simulations of shell oxygen burning exhibit bursty, recurrent fluctuations in turbulent kinetic energy. These are shown to be due to a general instability of the convective cell, requiring only a localized source of heating or cooling. Such fluctuations are shown to be suppressed in simulations of stellar evolution which use the mixing-length theory. Quantitatively similar behavior occurs in the model of a convective roll (cell) of Lorenz, which is known to have a strange attractor that gives rise to chaotic fluctuations in time of velocity and, as we show, luminosity. Study of simulations suggests that the behavior of a Lorenz convective roll may resemble that of a cell in convective flow. We examine some implications of this simplest approximation and suggest paths for improvement. Using the Lorenz model as representative of a convective cell, a multiple-cell model of a convective layer gives total luminosity fluctuations which are suggestive of irregular variables (red giants and supergiants), and of the long secondary period feature in semiregular asymptotic giant branch variables. This "τ-mechanism" is a new source for stellar variability, which is inherently nonlinear (unseen in linear stability analysis), and one closely related to intermittency in turbulence. It was already implicit in the 3D global simulations of Woodward et al. This fluctuating behavior is seen in extended two-dimensional simulations of CNeOSi burning shells, and may cause instability which leads to eruptions in progenitors of core-collapse supernovae prior to collapse. © 2011. The American Astronomical Society. All rights reserved.

Bradley P.A.,Los Alamos National Laboratory
Physics of Plasmas | Year: 2014

An investigation of direct drive capsules with different shell thicknesses and gas fills was conducted to examine the amount of shock induced (Richtmyer-Meshkov) mix versus Rayleigh-Taylor mix from deceleration of the implosion. The RAGE (Eulerian) code with a turbulent mix model was used to model these capsules for neutron yields along with time-dependent mix amounts. The amount of Richtmyer-Meshkov induced mix from the shock breaking out of the shell is about 0.1 μg (0.15 μm of shell material), while the Rayleigh-Taylor mix is of order 1 μg and determines the mixed simulation yield. The simulations were able to calculate a yield over mix (YOM) ratio (experiment/mix simulation) between 0.5 and 1.0 for capsules with shell thicknesses ranging from 7.5 to 20 μm and with gas fills between 3.8 and 20 atm of D2 or DT. The simulated burn averaged Tion values typically lie with 0.5 keV of the data, which is within the measurement error. For capsules with shell thicknesses >25 μm, the YOM values drop to 0.10 ± 0.05, suggesting that some unmodeled effect needs to be accounted for in the thickest capsules. © 2014 AIP Publishing LLC.

Olatunji-Ojo O.,University of California at Berkeley | Taylor C.D.,Los Alamos National Laboratory
Philosophical Magazine | Year: 2013

To promote a greater understanding of the process and nature of metal passivation, we have performed a first-principles analysis of partially oxidized Ni(111) and Ni(311) surface and ultra-thin film NiO layers on Ni(111). We have adopted a bimodal theoretical strategy that considers the oxidation process using either a fixed generalized gradient approximation (GGA) functional for the description of all atoms in the system, or a perturbation approach, that perturbs the electronic structure of various Ni atoms in contact with oxygen by application of the GGA+U technique. This strategy allows us to assess the relative merits of the two approaches, and whether or not the two approaches are at variance with one another as concerns the process of metal passivation. We consider oxygen binding in the cases of isolated atomic adsorption, the development of an oxygen monolayer, and epitaxial NiO(111) monolayers and bilayers with various terminations. Selective application of GGA+U drives structural and charge-transfer processes at the interface, in particular, the octopolar reconstruction of high oxygen coverage pre-passive systems, which, in fact, template an epitaxial NiO(111)-oriented film. These outputs are observable through the development of cationic states in the nickel atoms at the interface, the emergence of a band gap in the projected density of states and in oxygen binding energies that approach the energy of oxide formation. © 2013 © 2013 Taylor & Francis.

Freund H.P.,Los Alamos National Laboratory
Physical Review Special Topics - Accelerators and Beams | Year: 2013

Free-electron laser amplifiers have been operated at high efficiency at wavelengths from the microwave through the visible. Typically, these amplifiers require long tapered sections and produce spent beams with large energy spreads that are 4-5 times the electronic efficiency. In addition, while optical guiding during exponential growth in the uniform wiggler section confines the optical mode, the guiding disappears in the tapered wiggler section resulting in a relatively large optical mode at the wiggler exit. Optical klystrons consist of a Modulator wiggler that induces a velocity modulation on the electron beam followed by a magnetic dispersive section that enhances the velocity modulation prior to injection into a second, radiator wiggler. Optical klystrons have been operated over a broad spectral range; however, no optical klystron has been built with a tapered radiator wiggler. A comparison between a optical klystron with a step-tapered Radiator wiggler and a conventional tapered wiggler amplifier is analyzed in this paper. The purpose of the step taper is to both enhance the efficiency and to extend the range of the exponential gain and so preserve the optical guiding over a longer interaction length. The step-tapered optical klystron and a tapered wiggler amplifier are compared for a nominal set of parameters to determine the differences in the efficiency, interaction length, spent beam energy spread, and the size of the optical mode at the wiggler exit.

Yang X.,Los Alamos National Laboratory
Bulletin of the Seismological Society of America | Year: 2011

By modeling synthetic Pn amplitudes, I and my colleagues in 2007 proposed a Pn geometric-spreading model (Y2007) that takes into account the spherical shape of the Earth. In this study, I used a set of observed Pn amplitudes from the tectonically active regions of Asia to evaluate the performance of Y2007 and to develop new, observation-based Pn spreading models. Even though Y2007 provides improved geometric-spreading correction of Pn amplitudes over the traditional power-law model, the corrected amplitudes exhibit undesirable decay rate variations. To address this issue, I used a procedure to develop Pn spreading models based on observed data. I first correct the Pn amplitudes for attenuation using an average quality factor Q estimated from Y2007-corrected Pn amplitudes. I then develop a spreading model, which is a simplified version of Y2007, by fitting the corrected amplitudes. Compared with Y2007, the new spreading model significantly reduces amplitude variations, particularly at short distances. To more accurately model the complex data behavior, I also developed a segmented spreading model in which separate sets of model parameters are derived for amplitudes in different distance ranges. The spreading models developed in this study account for radially symmetric elastic and other effects, such as velocity gradient, forwarding scattering, and potential depth-dependent attenuation variation, as well as wavefront expansion and the spherical shape of the Earth. Using the new model for spreading correction results in better attenuation isolation and allows amplitudes in a broader distance range to be used in the accurate mapping of lateral attenuation variations. The method I employed in this study could be used as a general procedure to develop observation-based Pn spreading models for other regions.

Murphy J.W.,University of Washington | Meakin C.,Los Alamos National Laboratory | Meakin C.,University of Arizona
Astrophysical Journal | Year: 2011

Simulations of core-collapse supernovae (CCSNe) result in successful explosions once the neutrino luminosity exceeds a critical curve, and recent simulations indicate that turbulence further enables explosion by reducing this critical neutrino luminosity. We propose a theoretical framework to derive this result and take the first steps by deriving the governing mean-field equations. Using Reynolds decomposition, we decompose flow variables into background and turbulent flows and derive self-consistent averaged equations for their evolution. As basic requirements for the CCSN problem, these equations naturally incorporate steady-state accretion, neutrino heating and cooling, non-zero entropy gradients, and turbulence terms associated with buoyant driving, redistribution, and dissipation. Furthermore, analysis of two-dimensional (2D) CCSN simulations validate these Reynolds-averaged equations, and we show that the physics of turbulence entirely accounts for the differences between 1D and 2D CCSN simulations. As a prelude to deriving the reduction in the critical luminosity, we identify the turbulent terms that most influence the conditions for explosion. Generically, turbulence equations require closure models, but these closure models depend upon the macroscopic properties of the flow. To derive a closure model that is appropriate for CCSNe, we cull the literature for relevant closure models and compare each with 2D simulations. These models employ local closure approximations and fail to reproduce the global properties of neutrino-driven turbulence. Motivated by the generic failure of these local models, we propose an original model for turbulence which incorporates global properties of the flow. This global model accurately reproduces the turbulence profiles and evolution of 2D CCSN simulations. © 2011. The American Astronomical Society. All rights reserved.

Baalrud S.D.,University of Iowa | Daligault J.,Los Alamos National Laboratory
Physics of Plasmas | Year: 2014

A method for extending traditional plasma transport theories into the strong coupling regime is presented. Like traditional theories, this is based on a binary scattering approximation, but where physics associated with many body correlations is included through the use of an effective interaction potential. The latter is simply related to the pair-distribution function. Modeling many body effects in this manner can extend traditional plasma theory to orders of magnitude stronger coupling. Theoretical predictions are tested against molecular dynamics simulations for electron-ion temperature relaxation as well as diffusion in one component systems. Emphasis is placed on the connection with traditional plasma theory, where it is stressed that the effective potential concept has precedence through the manner in which screening is imposed. The extension to strong coupling requires accounting for correlations in addition to screening. Limitations of this approach in the presence of strong caging are also discussed. © 2014 AIP Publishing LLC.

Ringler T.,Los Alamos National Laboratory | Gent P.,U.S. National Center for Atmospheric Research
Ocean Modelling | Year: 2011

It is now over 40. years since a closure for the effects of mesoscale eddies in terms of Ertel potential vorticity was first proposed. The consequences of the closure that treats potential vorticity exactly the same as a passive tracer in isopycnal coordinates are explored in this paper. This leads to a momentum equation to predict the mean velocity. While the momentum equation is not unique due to the presence of an undefined potential function, the total energy equation is used to constrain its functional form. The inviscid form of the proposed eddy closure nearly conserves total energy; the error in conservation of total energy is proportional to the time derivative of the bolus velocity. The proposed eddy closure retains Kelvin's circulation theorem with mean potential vorticity conserved along particle trajectories following the transport (mean. +. bolus) velocity field. The relative vorticity component of the potential vorticity being diffused along isopycnals leads to terms that look like viscous stress, but these terms do not satisfy two important conditions of standard viscous closures. A numerical model based on this closure is developed, and idealized simulations in a re-entrant zonal channel are conducted to evaluate the merit of the proposed closure. When comparing various eddy closures to an eddy-resolving reference solution, the closure that both transports and diffuses potential vorticity performs marginally better than its peers, particularly with respect to the core zonal jet structure and position. However, these favorable results are obtained only if a potential vorticity diffusion coefficient is used that is smaller than the coefficient used to compute the bolus velocity. Based on these results, we conjecture that extending eddy-closures to include potential vorticity dynamics is possible, but will require the use of a closure parameter that varies temporally and spatially. © 2011 Elsevier Ltd.

Medin Z.,Los Alamos National Laboratory | Cumming A.,McGill University
Astrophysical Journal Letters | Year: 2014

We show that convection driven by chemical separation can significantly affect the cooling light curves of accreting neutron stars after they go into quiescence. We calculate the thermal relaxation of the neutron star ocean and crust including the thermal and compositional fluxes due to convection. After the inward propagating cooling wave reaches the base of the neutron star ocean, the ocean begins to freeze, driving chemical separation. The resulting convection transports heat inward, giving much faster cooling of the surface layers than found assuming the ocean cools passively. The light curves including convection show a rapid drop in temperature weeks after outburst. Identifying this signature in observed cooling curves would constrain the temperature and composition of the ocean as well as offer a real time probe of the freezing of a classical multicomponent plasma. © 2014. The American Astronomical Society. All rights reserved.

Neher R.A.,University of California at Santa Barbara | Leitner T.,Los Alamos National Laboratory
PLoS Computational Biology | Year: 2010

The evolutionary dynamics of HIV during the chronic phase of infection is driven by the host immune response and by selective pressures exerted through drug treatment. To understand and model the evolution of HIV quantitatively, the parameters governing genetic diversification and the strength of selection need to be known. While mutation rates can be measured in single replication cycles, the relevant effective recombination rate depends on the probability of coinfection of a cell with more than one virus and can only be inferred from population data. However, most population genetic estimators for recombination rates assume absence of selection and are hence of limited applicability to HIV, since positive and purifying selection are important in HIV evolution. Yet, little is known about the distribution of selection differentials between individual viruses and the impact of single polymorphisms on viral fitness. Here, we estimate the rate of recombination and the distribution of selection coefficients from time series sequence data tracking the evolution of HIV within single patients. By examining temporal changes in the genetic composition of the population, we estimate the effective recombination to be ρ = 1.4±0.6×10-5 recombinations per site and generation. Furthermore, we provide evidence that the selection coefficients of at least 15% of the observed non-synonymous polymorphisms exceed 0.8% per generation. These results provide a basis for a more detailed understanding of the evolution of HIV. A particularly interesting case is evolution in response to drug treatment, where recombination can facilitate the rapid acquisition of multiple resistance mutations. With the methods developed here, more precise and more detailed studies will be possible as soon as data with higher time resolution and greater sample sizes are available.

Saito S.,Japan National Institute of Information and Communications Technology | Peter Gary S.,Los Alamos National Laboratory
Physics of Plasmas | Year: 2012

Two-dimensional particle-in-cell simulations have been carried out to study electron beta dependence of decaying whistler turbulence and electron heating in a homogeneous, collisionless magnetized plasma. Initially, applied whistler fluctuations at relatively long wavelengths cascade their energy into shorter wavelengths. This cascade leads to whistler turbulence with anisotropic wavenumber spectra which are broader in directions perpendicular to the background magnetic field than in the parallel direction. Comparing the development of whistler turbulence at different electron beta values, it is found that both the wavenumber spectrum anisotropy and electron heating anisotropy decrease with increasing electron beta. This indicates that higher electron beta reduces the perpendicular energy cascade of whistler turbulence. Fluctuation energy dissipation by electron Landau damping responsible for the electron parallel heating becomes weaker at higher electron beta, which leads to more isotropic heating. It suggests that electron kinetic processes are important in determining the properties of whistler turbulence. This kinetic property is applied to discuss the generation of suprathermal strahl electron distributions in the solar wind. © 2012 American Institute of Physics.

Nishida Y.,Los Alamos National Laboratory
Physical Review Letters | Year: 2012

A three-component Fermi gas near a broad Feshbach resonance does not have a universal ground state due to the Thomas collapse, while it does near a narrow Feshbach resonance. We explore its universal phase diagram in the plane of the inverse scattering length 1/akF and the resonance range R *kF. For a large R*kF, there exists a Lifshitz transition between superfluids with and without an unpaired Fermi surface as a function of 1/akF. With decreasing R *kF, the Fermi surface coexisting with the superfluid can change smoothly from that of atoms to trimers ("atom-trimer continuity"), corresponding to the quark-hadron continuity in a dense nuclear matter. Eventually, there appears a finite window in 1/akF where the superfluid is completely depleted by the trimer Fermi gas, which gives rise to a pair of quantum critical points. The boundaries of these three quantum phases are determined in regions where controlled analyses are possible and are also evaluated based on a mean-field plus trimer model. © 2012 American Physical Society.

Das T.,Los Alamos National Laboratory
Physical Review Letters | Year: 2012

We propose and formulate an interaction induced staggered spin-orbit order as a new emergent phase of two-dimensional Fermi gases. We show that when some form of inherent spin splitting via Rashba-type spin-orbit coupling renders two helical Fermi surfaces to become significantly 'nested' a Fermi surface instability arises. To lift this degeneracy, a spontaneous symmetry-breaking spin-orbit density wave develops, causing a surprisingly large quasiparticle gapping with chiral electronic states. Since the staggered spin-orbit order is associated with a condensation energy, quantified by the gap value, destroying such spin-orbit interaction costs sufficiently large perturbation field or temperature or dephasing time. The BiAg2 surface state is shown to be a representative system for realizing such novel spin-orbit interaction with tunable and large strength, and the spin splitting is decoupled from charge excitations. These functional properties are relevant for spin electronics, spin caloritronics, and spin-Hall effect applications. © 2012 American Physical Society.

Vitev I.,Los Alamos National Laboratory | Zhang B.-W.,Hua Zhong Normal University
Physical Review Letters | Year: 2010

We demonstrate that jet observables are highly sensitive to the characteristics of the vacuum and the in-medium QCD parton showers and propose techniques that exploit this sensitivity to constrain the mechanism of quark and gluon energy loss in strongly interacting plasmas. As a first example, we calculate the inclusive jet cross section in high-energy nucleus-nucleus collisions to O(αs3). Theoretical predictions for the medium-induced jet broadening and the suppression of the jet production rate due to cold and hot nuclear matter effects in Au+Au and Cu+Cu reactions at RHIC are presented. © 2010 The American Physical Society.

Mayeur J.R.,Los Alamos National Laboratory | McDowell D.L.,Georgia Institute of Technology
International Journal of Plasticity | Year: 2014

We compare and contrast the governing equations and numerical predictions of two higher-order theories of extended single crystal plasticity, specifically, Gurtin type and micropolar models. The models are presented within a continuum thermodynamic setting, which facilitates identification of equivalent terms and the roles they play in the respective models. Finite element simulations of constrained thin films are used to elucidate the various scale-dependent strengthening mechanisms and their effect of material response. Our analysis shows that the two theories contain many analogous features and qualitatively predict the same trends in mechanical behavior, although they have substantially different points of departure. This is significant since the micropolar theory affords a simpler numerical implementation that is less computationally expensive and potentially more stable. © 2014 Elsevier Ltd. All rights reserved.

Salehinia I.,Washington State University | Wang J.,Los Alamos National Laboratory | Bahr D.F.,Purdue University | Zbib H.M.,Washington State University
International Journal of Plasticity | Year: 2014

Experimental studies show that metal-ceramic multilayers can have high strength, high strain hardening and measurable plasticity when the ceramic layer is a few nanometers thick. Using molecular dynamics simulations we studied deformation mechanisms in metal-ceramic multilayers and the role of interface structure and layer thickness on mechanical behavior. NbC/Nb multilayers were investigated numerically using the molecular dynamics (MD) method with empirical interatomic potentials. The interface dislocation structure was characterized by combining MD simulations and atomically informed Frank-Bilby theory. Two sets of pure edge misfit dislocations have been identified. Plastic deformation in NbC/Nb multilayers commences first in the metal layers by nucleation and glide of lattice dislocations initiating from interface misfit dislocations. These dislocations glide in the Nb layer and are deposited at the interface. The deposited dislocations facilitate slip transmission from the Nb layer to the NbC layer. The critical strain corresponding to dislocation nucleation is insensitive to layer thickness but depends on interface dislocation structure. The strain hardening and the peak flow strength of NbC/Nb multilayers are associated with the slip transmission from Nb to NbC, and are correlated to the interfacial dislocations, Nb layer thickness, and NbC layer thickness. The flow strength decreases with increasing Nb layer thickness and decreasing the NbC layer thickness. © 2014 Elsevier Ltd. All rights reserved.

Mau Y.,Ben - Gurion University of the Negev | Hagberg A.,Los Alamos National Laboratory | Meron E.,Ben - Gurion University of the Negev
Physical Review Letters | Year: 2012

Spatial periodic forcing of pattern-forming systems is an important, but lightly studied, method of controlling patterns. It can be used to control the amplitude and wave number of one-dimensional periodic patterns, to stabilize unstable patterns, and to induce them below instability onset. We show that, although in one spatial dimension the forcing acts to reinforce the patterns, in two dimensions it acts to destabilize or displace them by inducing two-dimensional rectangular and oblique patterns. © 2012 American Physical Society.

Colgan J.,Los Alamos National Laboratory | Pindzola M.S.,Auburn University
Physical Review Letters | Year: 2012

We explore the complete breakup of the Li atom after absorption of a single photon, the purest example of the so-called four-body Coulomb problem. The resulting strongly correlated three-electron continuum is investigated by calculating the angular distributions of the ionized electrons using advanced close-coupling techniques. We find that the distributions are dominated by the Coulomb interactions between the electrons, that multiple break-up processes can be identified, and that the complex dynamics of the fragmentation process are evident for most scattering geometries. © 2012 American Physical Society.

Watson M.C.,University of California at Santa Barbara | Brandt E.G.,University of California at Santa Barbara | Welch P.M.,Los Alamos National Laboratory | Brown F.L.H.,University of California at Santa Barbara
Physical Review Letters | Year: 2012

Thermal fluctuations of lipid orientation are analyzed to infer the bending rigidity of lipid bilayers directly from molecular simulations. Compared to the traditional analysis of thermal membrane undulations, the proposed method is reliable down to shorter wavelengths and allows for determination of the bending rigidity using smaller simulation boxes. The requisite theoretical arguments behind this analysis are presented and verified by simulations spanning a diverse range of lipid models from the literature. © 2012 American Physical Society.

Solenov D.,U.S. Navy | Velizhanin K.A.,Los Alamos National Laboratory
Physical Review Letters | Year: 2012

Chemical functionalization of graphene holds promise for various applications ranging from nanoelectronics to catalysis, drug delivery, and nanoassembly. In many applications it is important to be able to transport adsorbates on graphene in real time. We propose to use electromigration to drive the adsorbate transport across the graphene sheet. To assess the efficiency of electromigration, we develop a tight-binding model of electromigration of an adsorbate on graphene and obtain simple analytical expressions for different contributions to the electromigration force. Using experimentally accessible parameters of realistic graphene-based devices as well as electronic structure theory calculations to parametrize the developed model, we argue that electromigration on graphene can be efficient. As an example, we show that the drift velocity of atomic oxygen covalently bound to graphene can reach ∼1cm/s. © 2012 American Physical Society.

Jiang L.,Los Alamos National Laboratory | Presho M.,Texas A&M University
Multiscale Modeling and Simulation | Year: 2012

In this paper we use a splitting technique to develop new multiscale basis functions for the multiscale finite element method (MsFEM). The multiscale basis functions are iteratively generated using a Green's kernel. The Green's kernel is based on the first differential operator of the splitting. The proposed MsFEM is applied to deterministic elliptic equations and stochastic elliptic equations, and we show that the proposed MsFEM can considerably reduce the dimension of the random parameter space for stochastic problems. By combining the method with sparse grid collocation methods, the need for a prohibitive number of deterministic solves is alleviated. We rigorously analyze the convergence of the proposed method for both the deterministic and stochastic elliptic equations. Computational complexity discussions are also offered to supplement the convergence analysis. A number of numerical results are presented to confirm the theoretical findings. © 2012 Society for Industrial and Applied Mathematics.

Patton H.J.,Los Alamos National Laboratory
Geophysical Journal International | Year: 2016

Surface wave magnitude Ms for a compilation of 72 nuclear tests detonated in hard rock media for which yields and burial depths have been reported in the literature is shown to scale with yieldWas a+b×log[W],where a=2.50±0.08 and b=0.80±0.05. While the exponent b is consistent with an Ms scaling model for fully coupled, normal containment-depth explosions, the intercept a is offset 0.45 magnitude units lower than the model. The cause of offset is important to understand in terms of the explosion source. Hard rock explosions conducted in extensional and compressional stress regimes show similar offsets, an indication that the tectonic setting inwhich an explosion occurs plays no role causing the offset. The scaling model accounts for the effects of source medium material properties on the generation of 20-s period Rayleigh wave amplitudes. Aided by thorough characterizations of the explosion and tectonic release sources, an extensive analysis of the 1963 October 26 Shoal nuclear test detonated in granite 27 miles southeast of Fallon NV shows that the offset is consistent with the predictions of a material damage source model related to non-linear stress wave interactions with the free surface. This source emits Rayleigh waves with polarity opposite to waves emitted by the explosion. The Shoal results were extended to analyse surface waves from the 1962 February 15 Hardhat nuclear test, the 1988 September 14 Soviet Joint Verification Experiment, and the anomalous 1979 August 18 northeast Balapan explosion which exhibits opposite polarity, azimuth-independent source component U1 compared to an explosion. Modelling these tests shows that Rayleigh wave amplitudes generated by the damage source are nearly as large as or larger than amplitudes from the explosion. As such, destructive interference can be drastic, introducing metastable conditions due to the sensitivity of reduced amplitudes to Rayleigh wave initial phase angles of the explosion and damage sources. This meta-stability is a likely source of scatter inMs-yield scaling observations. The agreement of observed scaling exponent b with the model suggests that the damage source strength does not vary much with yield, in contrast to explosions conducted in weak media where Ms scaling rates are greater than the model predicts, and the yield dependence of the damage source strength is significant. This difference in scaling behaviour is a consequence of source medium material properties. © The Author 2016.

Scheinker A.,Los Alamos National Laboratory | Scheinker A.,University of California at San Diego | Krstic M.,University of California at San Diego
IEEE Transactions on Automatic Control | Year: 2013

Employing extremum seeking (ES) for seeking minima of control Lyapunov function (CLF) candidates, we develop 1) the first systematic design of ES controllers for unstable plants, 2) a simple non-model based universal feedback law that emulates, in an average sense, the LgV " controllers" for stabilization with inverse optimality, and 3) a new strategy for stabilization of systems with unknown control directions, as an alternative to Nussbaum gain controllers that lack exponential stability, lack transient performance guarantees, and lack robustness to changes in the control direction. The stability analysis that underlies our designs is inspired by an analysis approach synthesized in a recent work by Dürr, Stankovic, and Johansson, which combines a Lie bracket averaging result of Gurvits and Li with a semiglobal practical stability result under small parametric perturbations by Moreau and Aeyels. © 2012 IEEE.

Whalen D.J.,Carnegie Mellon University | Fryer C.L.,Los Alamos National Laboratory
Astrophysical Journal Letters | Year: 2012

The existence of 109 M ⊙ black holes (BHs) in massive galaxies by z ∼ 7 is one of the great unsolved mysteries in cosmological structure formation. One theory argues that they originate from the BHs of Pop III stars at z ∼ 20 and then accrete at the Eddington limit down to the epoch of reionization, which requires that they have constant access to rich supplies of fuel. Because early numerical simulations suggested that Pop III stars were ≳100 M ⊙, the supermassive black hole (SMBH) seeds considered up to now were 100-300 M ⊙. However, there is a growing numerical and observational consensus that some Pop III stars were tens of solar masses, not hundreds, and that 20-40 M ⊙ BHs may have been much more plentiful at high redshift. However, we find that natal kicks imparted to 20-40 M ⊙ Pop III BHs during formation eject them from their halos and hence their fuel supply, precluding them from Eddington-limit growth. Consequently, SMBHs are far less likely to form from low-mass Pop III stars than from very massive ones. © © 2012. The American Astronomical Society. All rights reserved.

Ticknor C.,Los Alamos National Laboratory
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We study two-body dipolar scattering in two dimensions with a tilted polarization axis. This tilt reintroduces the anisotropic interaction in a controllable manner. As a function of this polarization angle, we present the scattering rates in both the threshold and semiclassical regimes. Additionally, we study the properties of the molecular bound states as a function of the polarization angle. © 2011 American Physical Society.

Schoenborn B.P.,Los Alamos National Laboratory
Acta Crystallographica Section D: Biological Crystallography | Year: 2010

The first neutron diffraction data were collected from crystals of myoglobin almost 42 years ago using a step-scan diffractometer with a single detector. Since then, major advances have been made in neutron sources, instrumentation and data collection and analysis, and in biochemistry. Fundamental discoveries about enzyme mechanisms, bio-logical complex structures, protein hydration and H-atom positions have been and continue to be made using neutron diffraction. The promise of neutrons has not changed since the first crystal diffraction data were collected. Today, with the developments of beamlines at spallation neutron sources and the use of the Laue method for data collection, the field of neutrons in structural biology has renewed vitality. © 2010 International Union of Crystallography Printed in Singapore - all rights reserved.

Hollingsworth J.A.,Los Alamos National Laboratory
Coordination Chemistry Reviews | Year: 2014

It is now well known that the optical properties of semiconductor nanocrystal quantum dots (NQDs) - absorption onset and position of the photoluminescence maximum - can be precisely controlled by simple tuning of particle size within the quantum-confinement regime. More recently, however, the field has evolved beyond straightforward particle-size control to embrace more complex NQD heterostructures. As a result of the inclusion of internal, nanoscale compositional interfaces, heterostructured NQDs afford opportunities for enhanced, emergent and even multi-functional behavior and properties. A common structural motif for achieving such 'engineered' NQDs is to envelop the NQD core within a shell of a different composition. Herein, a summary of our recent research in the development, synthesis, characterization and application of 'core/shell' NQDs is provided. In the first part, enhancement of properties is demonstrated through our work in lead chalcogenide core/shell NQDs. In a subsequent section, emergence of novel properties resulting from specific combinations of core and shell physical and electronic structures is described in the context of non-blinking behavior and suppressed Auger recombination realized for our "giant" NQDs. Examples in this case entail both CdSe and InP cores. Application of these ultra-stable NQDs in the area of light-emission technologies is also demonstrated and discussed. Finally, multi-functionality is shown for the case of a coupled magnetic-semiconductor Co/CdSe core/shell nanocrystal system. © 2013 Elsevier B.V.

Rosten E.,University of Cambridge | Porter R.,Los Alamos National Laboratory | Drummond T.,University of Cambridge
IEEE Transactions on Pattern Analysis and Machine Intelligence | Year: 2010

The repeatability and efficiency of a corner detector determines how likely it is to be useful in a real-world application. The repeatability is important because the same scene viewed from different positions should yield features which correspond to the same real-world 3D locations [1]. The efficiency is important because this determines whether the detector combined with further processing can operate at frame rate. Three advances are described in this paper. First, we present a new heuristic for feature detection and, using machine learning, we derive a feature detector from this which can fully process live PAL video using less than 5 percent of the available processing time. By comparison, most other detectors cannot even operate at frame rate (Harris detector 115 percent, SIFT 195 percent). Second, we generalize the detector, allowing it to be optimized for repeatability, with little loss of efficiency. Third, we carry out a rigorous comparison of corner detectors based on the above repeatability criterion applied to 3D scenes. We show that, despite being principally constructed for speed, on these stringent tests, our heuristic detector significantly outperforms existing feature detectors. Finally, the comparison demonstrates that using machine learning produces significant improvements in repeatability, yielding a detector that is both very fast and of very high quality. © 2010 IEEE.

Quinn H.,Los Alamos National Laboratory
IEEE Transactions on Nuclear Science | Year: 2014

Many space programs depend on cutting-edge technology to increase computational power without increasing the power or weight of the payload. For these types of programs, component testing is necessary to ensure that the components do not fail while deployed. Radiation testing for advanced technology components can be challenging since the underlying architecture and organization of the component might be complex. This paper will cover a review of component testing best practices for total ionizing dose and single-event effect testing from recent years. © 2014 IEEE.

Kang Z.-B.,Los Alamos National Laboratory | Xiao B.-W.,Central China Normal University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We study the Sivers single spin asymmetry of the Drell-Yan lepton pair production in the small-x regime. We find that in the corresponding kinematic region the spin asymmetry calculated in the small-x approach is consistent with either the usual transverse-momentum-dependent factorization formalism or the collinear factorization formalism, respectively. We estimate the Sivers asymmetry for both polarized p+p and p+A collisions and argue that the Drell-Yan production is an interesting and unique probe for both the transverse spin physics and the small-x saturation effect. © 2013 American Physical Society.

Atanasov V.,Sofia University | Saxena A.,Los Alamos National Laboratory
Journal of Physics Condensed Matter | Year: 2011

Adopting a purely two-dimensional relativistic equation for graphene's carriers contradicts the Heisenberg uncertainty principle since it requires setting the off-the-surface coordinate of a three-dimensional wavefunction to zero. Here we present a theoretical framework for describing graphene's massless relativistic carriers in accordance with this most fundamental of all quantum principles. A gradual confining procedure is used to restrict the dynamics onto a surface and normal to the surface parts, and in the process the embedding of this surface into the three-dimensional world is accounted for. As a result an invariant geometric potential arises in the surface part which scales linearly with the mean curvature and shifts the Fermi energy of the material proportional to bending. Strain induced modification of the electronic properties or 'straintronics' is clearly an important field of study in graphene. This opens an avenue to producing electronic devices: micro-and nano-electromechanical systems (MEMS and NEMS), where the electronic properties are controlled by geometric means and no additional alteration of graphene is necessary. The appearance of this geometric potential also provides us with clues as to how quantum dynamics looks in the curved space-time of general relativity. In this context we explore a two-dimensional cross-section of the wormhole geometry, realized with graphene as a solid state thought experiment. © 2011 IOP Publishing Ltd.

Olson G.L.,Los Alamos National Laboratory
Journal of Computational Physics | Year: 2011

An existing solution method for solving the multigroup radiation equations, linear multifrequency-grey acceleration, is here extended to be second order in time. This method works for simple diffusion and for flux-limited diffusion, with or without material conduction. A new method is developed that does not require the solution of an averaged grey transport equation. It is effective solving both the diffusion and P1 forms of the transport equation. Two dimensional, multi-material test problems are used to compare the solution methods. © 2011 Elsevier Inc.

Ekdahl C.,Los Alamos National Laboratory
Journal of the Optical Society of America A: Optics and Image Science, and Vision | Year: 2011

Flash radiography of large hydrodynamic experiments driven by high explosives is a venerable diagnostic technique in use at many laboratories. The size of the radiographic source spot is often quoted as an indication of the resolving power of a particular flash-radiography machine. A variety of techniques for measuring spot size have evolved at the different laboratories, as well as different definitions of spot size. Some definitions are highly dependent on the source spot intensity distributions, and not necessarily well correlated with resolution. The concept of limiting resolution based on bar target measurements is introduced, and shown to be equivalent to the spatial wavenumber at a modulation transfer function value of 5%. This resolution is shown to be better correlated with the full width at half-maximum of the spot intensity distribution than it is with other definitions of spot size. © 2011 Optical Society of America.

Nisoli C.,Los Alamos National Laboratory
New Journal of Physics | Year: 2016

The impact of the Ising model on the development of so many diverse branches of theoretical physics can hardly be overstated. A group in Uppsala (Arnalds et al 2016 New J. Phys. 18 023008) has shown how to realize it at the nanoscale. Their work could open new paths to the study of critical phenomena, out of equilibrium kinetics, disorder and glassy behavior in a real yet controllable system. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Chlistunoff J.,Los Alamos National Laboratory
Journal of Physical Chemistry C | Year: 2011

Catalytic activity of heat-treated iron and polyaniline-based oxygen reduction catalysts was studied in aqueous acidic media using the rotating ring disk (RRDE) technique and linear potential scan voltammetry employing stationary electrodes. The stationary voltammograms of the catalyst exhibit the presence of a reversible surface red-ox reaction at 0.647 V vs RHE. It is shown that molecular oxygen reversibly adsorbs on the catalyst surface at potentials more positive than the formal potential of the surface red-ox couple and that the adsorption occurs through either the oxidized form of this couple or an atom in its close proximity. The Tafel plots for oxygen reduction reaction (ORR) exhibit variable slopes ranging from 60 mV dec-1 at the lowest overpotentials to more than 240 mV dec-1 at high overpotentials. The kinetic data obtained from the RRDE experiments for various catalyst loadings and from the linear potential scan voltammetry of adsorbed oxygen demonstrate that the high Tafel slopes originate from intrinsic features of the reduction mechanism rather than incomplete catalyst utilization. It is postulated that the surface red-ox couple is FeIII/FeII and that it takes an active part in ORR in the whole range of overpotentials. The proposed ORR mechanism involves a simple mediation by the FeIII/FeII couple at low overpotentials and a concerted process of charge transfer and oxygen-oxygen bond splitting at high overpotentials. © 2011 American Chemical Society.

The long-standing problem related to prompt analyses in continuous air sampling or monitoring has been the wellknown interference of the radon- and thoron-progeny codeposited on the filtration media with any potential suspect radionuclides. The solutions to this problem have been quite diverse, and have included, for example, simple gross-alpha screening, the use of beta-to-alpha ratios, and/or the use of alpha spectral analyses. In the context of week-long retrospective continuous air sampling, this paper will explain, in detail, the technical basis for the use of the simple gross-alpha screening, beta-to-alpha ratio, and alpha spectrometry techniques and demonstrate the efficacy (or lack thereof) of these methods with simple examples. Although the most sensitive analysis technique for week-long retrospective continuous air samples is no doubt a long-lived count performed typically after at least a four-day decay period, when necessary, certain prompt and semi-prompt techniques discussed here can approach a sensitivity that is within about an order of magnitude of the long-lived count. Copyright © 2011 Health Physics Society.

Fair J.M.,Los Alamos National Laboratory | Rivas A.L.,University of New Mexico
Transboundary and Emerging Diseases | Year: 2015

Most infectious disease surveillance methods are not well fit for early detection. To address such limitation, here we evaluated a ratio- and Systems Biology-based method that does not require prior knowledge on the identity of an infective agent. Using a reference group of birds experimentally infected with West Nile virus (WNV) and a problem group of unknown health status (except that they were WNV-negative and displayed inflammation), both groups were followed over 22 days and tested with a system that analyses blood leucocyte ratios. To test the ability of the method to discriminate small data sets, both the reference group (n = 5) and the problem group (n = 4) were small. The questions of interest were as follows: (i) whether individuals presenting inflammation (disease-positive or D+) can be distinguished from non-inflamed (disease-negative or D-) birds, (ii) whether two or more D+ stages can be detected and (iii) whether sample size influences detection. Within the problem group, the ratio-based method distinguished the following: (i) three (one D- and two D+) data classes; (ii) two (early and late) inflammatory stages; (iii) fast versus regular or slow responders; and (iv) individuals that recovered from those that remained inflamed. Because ratios differed in larger magnitudes (up to 48 times larger) than percentages, it is suggested that data patterns are likely to be recognized when disease surveillance methods are designed to measure inflammation and utilize ratios. © 2013 Blackwell Verlag GmbH.

Mussack K.,Los Alamos National Laboratory | Dappen W.,University of Southern California
Astrophysical Journal | Year: 2011

The solar abundance controversy inspires renewed investigations of the basic physics used to develop solar models. Here we examine the correction to the proton-proton reaction rate due to dynamic screening effects. Starting with the dynamic screening energy from the molecular-dynamic simulations of Mao et al., we compute a reaction-rate correction for dynamic screening. We find that, contrary to static screening theory, this dynamic screening does not significantly change the reaction rate from that of the bare Coulomb potential. © 2011. The American Astronomical Society. All rights reserved.

Sambasivan S.,Los Alamos National Laboratory | Kapahi A.,University of Iowa | Udaykumar H.S.,University of Iowa
Journal of Computational Physics | Year: 2013

Techniques are presented to solve problems involving high speed material interactions that can lead to large deformations followed by fragmentation. To simulate such problems in an Eulerian framework on a fixed Cartesian mesh, interfaces (free surfaces as well as interacting material interfaces) are tracked as levelsets; to resolve shocks and interfaces, a quadtree adaptive mesh is employed. This paper addresses issues associated with the treatment of all interfaces as sharp entities by defining ghost fields on each side of the interface. Collisions between embedded objects are resolved using an efficient collision detection algorithm and appropriate interfacial conditions are supplied. Key issues of supplying interfacial conditions at the precise location of the sharp interface and populating the ghost cells with physically consistent values during and beyond fragmentation events are addressed. Numerous examples pertaining to impact, penetration, void collapse and fragmentation phenomena are presented along with careful benchmarking to establish the validity, accuracy and versatility of the approach. © 2012 Elsevier Inc.

Perelson A.S.,Los Alamos National Laboratory | Guedj J.,French Institute of Health and Medical Research
Nature Reviews Gastroenterology and Hepatology | Year: 2015

Mathematically modelling changes in HCV RNA levels measured in patients who receive antiviral therapy has yielded many insights into the pathogenesis and effects of treatment on the virus. By determining how rapidly HCV is cleared when viral replication is interrupted by a therapy, one can deduce how rapidly the virus is produced in patients before treatment. This knowledge, coupled with estimates of the HCV mutation rate, enables one to estimate the frequency with which drug resistant variants arise. Modelling HCV also permits the deduction of the effectiveness of an antiviral agent at blocking HCV replication from the magnitude of the initial viral decline. One can also estimate the lifespan of an HCV-infected cell from the slope of the subsequent viral decline and determine the duration of therapy needed to cure infection. The original understanding of HCV RNA decline under interferon-based therapies obtained by modelling needed to be revised in order to interpret the HCV RNA decline kinetics seen when using direct-acting antiviral agents (DAAs). There also exist unresolved issues involving understanding therapies with combinations of DAAs, such as the presence of detectable HCV RNA at the end of therapy in patients who nonetheless have a sustained virologic response. © 2015 Macmillan Publishers Limited.

Bochev P.,Sandia National Laboratories | Ridzal D.,Sandia National Laboratories | Shashkov M.,Los Alamos National Laboratory
Journal of Computational Physics | Year: 2013

We develop a fast, efficient and accurate optimization-based algorithm for the high-order conservative and local-bound preserving remap (constrained interpolation) of a scalar conserved quantity between two close meshes with the same connectivity. The new formulation is as robust and accurate as the flux-variable flux-target optimization-based remap (FVFT-OBR) [1,2] yet has the computational efficiency of an explicit remapper. The coupled system of linear inequality constraints, resulting from the flux form of remap, is the main efficiency bottleneck in FVFT-OBR. While advection-based remappers use the flux form to directly enforce mass conservation, the optimization setting allows us to treat mass conservation as one of the constraints. To take advantage of this fact, we consider an alternative mass-variable mass-target (MVMT-OBR) formulation in which the optimization variables are the net mass updates per cell and a single linear constraint enforces the conservation of mass. In so doing we change the structure of the OBR problem from a global linear-inequality constrained QP to a singly linearly constrained QP with simple bounds. Using the structure of the MVMT-OBR problem, and the fact that in remap the old and new grids are close, we are able to develop a simple, efficient and easily parallelizable optimization algorithm for the primal MVMT-OBR QP. Numerical studies on a variety of affine and non-affine grids confirm that MVMT-OBR is as accurate and robust as FVFT-OBR, but has the same computational cost as the explicit, state-of-the-art FCR. © 2013 Elsevier Inc.

Waltz J.,Los Alamos National Laboratory
Journal of Computational Physics | Year: 2013

Operator splitting and time accuracy in Lagrange plus remap solution methods for the hydrodynamics equations are investigated. The time accuracy of the common solution approach is shown, both analytically and numerically, to be limited to first order due to operator splitting errors, low-order time integration of the remap terms, and other postulated first-order errors, even if the Lagrange step is second-order accurate in time. Additional numerical studies are used to demonstrate how these errors can be eliminated with an unsplit treatment that solves the remap terms directly. The Discontinuous Remap Method, in which a new mesh is generated during the remap step, also is shown to be first-order accurate in time. © 2013.

Joggerst C.,University of California at Santa Cruz | Joggerst C.,Los Alamos National Laboratory | Whalen D.J.,Carnegie Mellon University
Astrophysical Journal | Year: 2011

The observational signatures of the first cosmic explosions and their chemical imprint on second-generation stars both crucially depend on how heavy elements mix within the star at the earliest stages of the blast. We present numerical simulations of the early evolution of Population III (Pop III) pair-instability supernovae (PISNe) with the new adaptive mesh refinement code CASTRO. In stark contrast to 15-40M⊙ core-collapse primordial supernovae, we find no mixing in most 150-250 M⊙ PISNe out to times well after breakout from the surface of the star. This may be the key to determining the mass of the progenitor of a primeval supernova, because vigorous mixing will cause emission lines from heavy metals such as Fe and Ni to appear much sooner in the light curves of core-collapse supernovae than in those of pair-instability explosions. Our models are consistent with observations of SN2007bi, the most likely PISN candidate found to date, which show that heavy elements in the interior of the SN are not mixed at all with helium in the outer envelope. Our results also imply that unlike low-mass Pop III supernovae, whose collective metal yields can be directly compared to the chemical abundances of extremely metal-poor stars, further detailed numerical simulations will be required to determine the nucleosynthetic imprint of very massive Pop III stars on their direct descendants. © 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

Kapahi A.,University of Iowa | Sambasivan S.,Los Alamos National Laboratory | Udaykumar H.S.,University of Iowa
Journal of Computational Physics | Year: 2013

This work presents a three-dimensional, Eulerian, sharp interface, Cartesian grid technique for simulating the response of elasto-plastic solid materials to hypervelocity impact, shocks and detonations. The mass, momentum and energy equations are solved along with evolution equations for deviatoric stress and plastic strain using a third-order finite difference scheme. Material deformation occurs with accompanying nonlinear stress wave propagation; in the Eulerian framework the boundaries of the deforming material are tracked in a sharp fashion using level-sets and the conditions on the immersed boundaries are applied by suitable modifications of a ghost fluid approach. The dilatational response of the material is modeled using the Mie-Gruneisen equation of state and the Johnson-Cook model is employed to characterize the material response due to rate-dependent plastic deformation. Details are provided on the treatment of the deviatoric stress ghost state so that physically correct boundary conditions can be applied at the material interfaces. An efficient parallel algorithm is used to handle computationally intensive three-dimensional problems. The results demonstrate the ability of the method to simulate high-speed impact, penetration and fragmentation phenomena in three dimensions. © 2013 Elsevier Inc..

Rauscher S.A.,Los Alamos National Laboratory | Kucharski F.,Abdus Salam International Center For Theoretical Physics | Enfield D.B.,University of Miami
Journal of Climate | Year: 2011

This paper addresses several hypotheses designed to explain why AOGCM simulations of future climate in the third phase of the Coupled Model Intercomparison Project (CMIP3) feature an intensified reduction of precipitation over the Meso-America (MA) region. While the drying is consistent with an amplification of the subtropical high pressure cells and an equatorward contraction of convective regions due to the "upped ante" for convection in awarmer atmosphere, the physicalmechanisms behind the intensity and robustness of theMAdrying signal have not been fully explored. Regional variations in sea surface temperature (SST) warmingmay play a role. First, SSTs over the tropical North Atlantic (TNA) do not warm as much as the surrounding ocean. The troposphere senses a TNAthat is cooler than the tropical Pacific, potentially exciting aGill-type response, increasing the strength of theNorth Atlantic subtropical high. Second, thewarmENSO-like state simulated in the eastern tropical Pacific could decrease precipitation over MA, as warm ENSO events are associated with drying over MA. The authors use the International Centre for Theoretical Physics (ICTP) AGCM to investigate the effects of these regional SST warming variations on the projected drying over MA. First, the change of SSTs [Special Report on Emissions Scenarios (SRES) A1B's Twentieth-Century Climate in Coupled Model (A1B-20C)] in the ensemble average of the CMIP3 models is applied to determine if the ICTP AGCM can replicate the future drying. Then the effects of 1) removing the reduced warming over the TNA, 2) removing the warm ENSO-event-like pattern in the eastern tropical Pacific, and 3) applying uniform SST warming throughout the tropics are tested. The ICTPAGCMcan reproduce the general pattern and amount of precipitation over MA. Simulations in which the CMIP3 A1B-20C ensemble-average SSTs are added to climatological SSTs show drying of more than 20% over the MA region, similar to the CMIP3 ensemble average. Replacing the relatively cooler SSTs over the TNA excites a Gill response consistent with an off-equatorial heating anomaly, showing that the TNA relative cooling is responsible for about 16% (31%) of the drying in late spring (early summer). The warm ENSO-like SST pattern over the eastern Pacific also affects precipitation over the MA region, with changes of 19% and 31% in March-June (MMJ) and June-August (JJA), respectively. This work highlights the importance of understanding even robust signals in the CMIP3 future scenario simulations, and should aid in the design and analysis of future climate change studies over the region. © 2011 American Meteorological Society.

David Arnett W.,University of Arizona | Meakin C.,University of Arizona | Meakin C.,Los Alamos National Laboratory
Astrophysical Journal | Year: 2011

Two-dimensional (2D) hydrodynamical simulations of progenitor evolution of a 23 M ⊙ star, close to core collapse (in ∼1 hr in one dimension (1D)), with simultaneously active C, Ne, O, and Si burning shells, are presented and contrasted to existing 1D models (which are forced to be quasi-static). Pronounced asymmetries and strong dynamical interactions between shells are seen in 2D. Although instigated by turbulence, the dynamic behavior proceeds to sufficiently large amplitudes that it couples to the nuclear burning. Dramatic growth of low-order modes is seen as well as large deviations from spherical symmetry in the burning shells. The vigorous dynamics is more violent than that seen in earlier burning stages in the three-dimensional (3D) simulations of a single cell in the oxygen burning shell, or in 2D simulations not including an active Si shell. Linear perturbative analysis does not capture the chaotic behavior of turbulence (e.g., strange attractors such as that discovered by Lorenz), and therefore badly underestimates the vigor of the instability. The limitations of 1D and 2D models are discussed in detail. The 2D models, although flawed geometrically, represent a more realistic treatment of the relevant dynamics than existing 1D models, and present a dramatically different view of the stages of evolution prior to collapse. Implications for interpretation of SN1987A, abundances in young supernova remnants, pre-collapse outbursts, progenitor structure, neutron star kicks, and fallback are outlined. While 2D simulations provide new qualitative insight, fully 3D simulations are needed for a quantitative understanding of this stage of stellar evolution. The necessary properties of such simulations are delineated. © 2011. The American Astronomical Society. All rights reserved.

Colgan J.,Los Alamos National Laboratory | Emmanouilidou A.,University College London | Pindzola M.S.,Auburn University
Physical Review Letters | Year: 2013

We examine the angular distributions of all three electrons ionized from Li by a single photon near the triple ionization threshold using a fully quantum-mechanical treatment. We find strong evidence for a T-shape break-up pattern at a 5 eV excess energy as previously predicted by quasiclassical simulations. This finding is in conflict with the expected Wannier break-up dynamics of three electrons moving at mutual angles of 120, which is expected to hold at energies a few eV above threshold. We use our quantum-mechanical approach to explore the physical mechanisms behind this unusual break-up configuration. © 2013 American Physical Society.

Joglekar Y.N.,Indiana University - Purdue University Indianapolis | Saxena A.,Los Alamos National Laboratory
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We study the properties of a parity- and time-reversal- (PT) symmetric tight-binding chain of size N with position-dependent hopping amplitude. In contrast to the fragile PT-symmetric phase of a chain with constant hopping and imaginary impurity potentials, we show that, under very general conditions, our model is always in the PT-symmetric phase. We numerically obtain the energy spectrum and the density of states of such a chain, and show that they are widely tunable. By studying the size dependence of inverse participation ratios, we show that although the chain is not translationally invariant, most of its eigenstates are extended. Our results indicate that tight-binding models with non-Hermitian, PT-symmetric hopping have a robust PT-symmetric phase and rich dynamics which may be explored in coupled waveguides. © 2011 American Physical Society.

Valone S.M.,Los Alamos National Laboratory
Journal of Chemical Theory and Computation | Year: 2011

Charge flow in materials at the atomistic level is controlled through chemical potential equalization among its constituents. Consequently employing this concept in a simulation requires some model of chemical potential. Current atomistic models of chemical potential, such as the Iczkowski-Margrave (IM) model, are built largely on heuristic arguments and depend linearly on the net charge of each constituent. To gain new insight into the IM model, a many-electron model Hamiltonian is constructed at the atomistic level that is commensurate with the IM model, as opposed to one designed at the one-electron level. For a three-state, two-fragment system, the essential electronegativity and the chemical hardness energies are recovered. However, the model Hamiltonian imparts new charge dependencies not found in the IM model. Decidedly nonlinear, transitional or hopping contributions in those new dependencies are shown to be critical to regulating charge flow. Other modifications to the IM model are illustrated with simple two- and three-fragment systems, involving as many as five states, that act as paradigms for general materials models. Including more than three states in the three-fragment example introduces local bonding refinements to the Mulliken electronegativity and chemical hardness. © 2011 American Chemical Society.

Booth T.E.,Los Alamos National Laboratory
Journal of Computational Physics | Year: 2011

The usual Monte Carlo approach to the thermal radiative transfer problem is to view Monte Carlo as a solution technique for the nonlinear thermal radiative transfer equations. The equations contain time derivatives which are approximated by introducing small time steps. An alternative approach avoids time steps by using Monte Carlo to directly sample the time at which the next event occurs. That is, the time is advanced on a natural event-by-event basis rather than by introducing an artificial time step. © 2010 Elsevier Inc.

Densmore J.D.,Los Alamos National Laboratory
Journal of Computational Physics | Year: 2011

We perform an asymptotic analysis of the spatial discretization of radiation absorption and re-emission in Implicit Monte Carlo (IMC), a Monte Carlo technique for simulating nonlinear radiative transfer. Specifically, we examine the approximation of absorption and re-emission by a spatially continuous artificial-scattering process and either a piecewise-constant or piecewise-linear emission source within each spatial cell. We consider three asymptotic scalings representing (i) a time step that resolves the mean-free time, (ii) a Courant limit on the time-step size, and (iii) a fixed time step that does not depend on any asymptotic scaling. For the piecewise-constant approximation, we show that only the third scaling results in a valid discretization of the proper diffusion equation, which implies that IMC may generate inaccurate solutions with optically large spatial cells if time steps are refined. However, we also demonstrate that, for a certain class of problems, the piecewise-linear approximation yields an appropriate discretized diffusion equation under all three scalings. We therefore expect IMC to produce accurate solutions for a wider range of time-step sizes when the piecewise-linear instead of piecewise-constant discretization is employed. We demonstrate the validity of our analysis with a set of numerical examples. © 2010 Elsevier Inc.

Tang X.Z.,Los Alamos National Laboratory
Journal of Computational Physics | Year: 2011

A new formulation is presented for numerically computing the helical Chandrasekhar-Kendall modes in an axisymmetric torus. It explicitly imposes ∇ · B= 0 and yields a standard matrix eigenvalue problem, which can then be solved by standard matrix eigenvalue techniques. Numerical implementation and computational results are shown for an axisymmetric torus typical of reversed field pinch and spherical tokamak. © 2010 Elsevier Inc.

Rodriguez M.A.,Los Alamos National Laboratory | Shinavier J.,Rensselaer Polytechnic Institute
Journal of Informetrics | Year: 2010

Many, if not most network analysis algorithms have been designed specifically for single-relational networks; that is, networks in which all edges are of the same type. For example, edges may either represent "friendship," "kinship," or "collaboration," but not all of them together. In contrast, a multi-relational network is a network with a heterogeneous set of edge labels which can represent relationships of various types in a single data structure. While multi-relational networks are more expressive in terms of the variety of relationships they can capture, there is a need for a general framework for transferring the many single-relational network analysis algorithms to the multi-relational domain. It is not sufficient to execute a single-relational network analysis algorithm on a multi-relational network by simply ignoring edge labels. This article presents an algebra for mapping multi-relational networks to single-relational networks, thereby exposing them to single-relational network analysis algorithms.

Li X.,University of Hawaii at Manoa | Li X.,Los Alamos National Laboratory | Jia Yi.,Tsinghua University | Cao A.,University of Hawaii at Manoa | Cao A.,Peking University
ACS Nano | Year: 2010

The integration of organic and inorganic building blocks into novel nanohybrids is an important tool to exploit innovative materials with desirable functionalities. For this purpose, carbon nanotube- nanoparticle nanoarchitectures are intensively studied. We report here an efficient noncovalent chemical route to density-controllably and uniformly assemble single-walled carbon nanotubes with CdS nanoparticles. The methodology not only promises the resulting hybrids will be solution-processable but also endows the hybrids with distinct optoelectronic properties including tunable photoresponse mediated by amine molecules. On the basis of these merits, reliable thin-film photoswitches and light-driven chemical sensors are demonstrated, which highlights the potential of tailored hybrids in the development of new tunable optoelectronic devices and sensors. © 2010 American Chemical Society.

Prior computations have predicted the time-averaged acoustic radiation force on fluid spheres in water when illuminated by an acoustic high-order Bessel beam (HOBB) of quasi-standing waves. These computations are extended to the case of a rigid sphere in water which perfectly mimics a fluid sphere in air. Numerical results for the radiation force function of a HOBB quasi-standing wave tweezers are obtained for beams of zero, first and second order, and discussed with particular emphasis on the amplitude ratio describing the transition from progressive waves to quasi-standing waves behavior. This investigation may be helpful in the development of acoustic tweezers and methods for manipulating objects in reduced gravity environments and space related applications. © 2010 Elsevier Ltd. All rights reserved.

Singh R.,Oklahoma State University | Singh R.,Los Alamos National Laboratory | Al-Naib I.A.I.,University of Marburg | Koch M.,University of Marburg | Zhang W.,Oklahoma State University
Optics Express | Year: 2011

We report on the occurrence of sharp Fano resonances in planar terahertz metamaterials by introducing a weak asymmetry in a two gap split ring resonator. As the structural symmetry of the metamaterial is broken a Fano resonance evolves in the low-frequency flank of the symmetric fundamental dipole mode resonance. This Fano resonance can have much higher Q factors than that known from single gap split ring resonators. Supporting simulations indicate a Q factor of 50 for lowest degree of asymmetry. The Q factor decreases exponentially with increasing asymmetry. Hence, minute structural variations allow for a tuning of the Fano resonance. Such sharp resonances could be exploited for biochemical sensing. Besides, the strong current oscillations excited at the Fano resonance frequency could lead to the design of novel terahertz narrow band emitters. © 2011 Optical Society of America.

Somma R.D.,Los Alamos National Laboratory | Nagaj D.,Slovak Academy of Sciences | Kieferova M.,Slovak Academy of Sciences
Physical Review Letters | Year: 2012

We study the glued-trees problem from A.M. Childs, R. Cleve, E. Deotto, E. Farhi, S. Gutmann, and D. Spielman, in Proceedings of the 35th Annual ACM Symposium on Theory of Computing (ACM, San Diego, CA, 2003), p.59. in the adiabatic model of quantum computing and provide an annealing schedule to solve an oracular problem exponentially faster than classically possible. The Hamiltonians involved in the quantum annealing do not suffer from the so-called sign problem. Unlike the typical scenario, our schedule is efficient even though the minimum energy gap of the Hamiltonians is exponentially small in the problem size. We discuss generalizations based on initial-state randomization to avoid some slowdowns in adiabatic quantum computing due to small gaps. © 2012 American Physical Society.

Fagan J.A.,U.S. National Institute of Standards and Technology | Khripin C.Y.,U.S. National Institute of Standards and Technology | Silvera Batista C.A.,U.S. National Institute of Standards and Technology | Simpson J.R.,Towson University | And 3 more authors.
Advanced Materials | Year: 2014

Aqueous two-phase extraction is demonstrated to enable isolation of single semiconducting and metallic single-wall carbon nanotube species from a synthetic mixture. The separation is rapid and robust, with remarkable tunability via modification of the surfactant environment set for the separation. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Wang H.,McMaster University | Wu P.D.,McMaster University | Tome C.N.,Los Alamos National Laboratory | Huang Y.,Northwestern University
Journal of the Mechanics and Physics of Solids | Year: 2010

A large strain elastic-viscoplastic self-consistent (EVPSC) model for polycrystalline materials is developed. At single crystal level, both the rate sensitive slip and twinning are included as the plastic deformation mechanisms, while elastic anisotropy is accounted for in the elastic moduli. The transition from single crystal plasticity to polycrystal plasticity is based on a completely self-consistent approach. It is shown that the differences in the predicted stress-strain curves and texture evolutions based on the EVPSC and the viscoplastic self-consistent (VPSC) model proposed by Lebensohn and Tomé (1993) are negligible at large strains for monotonic loadings. For the deformations involving unloading and strain path changes, the EVPSC predicts a smooth elasto-plastic transition, while the VPSC model gives a discontinuous response due to lack of elastic deformation. It is also demonstrated that the EVPSC model can capture some important experimental features which cannot be simulated by using the VPSC model. © 2010 Elsevier Ltd. All rights reserved.

Frauenfelder H.,Los Alamos National Laboratory
Chemical Physics | Year: 2010

Fifty years ago, some proteins such as myoglobin appeared to be simple. As they were studied more deeply with sophisticated tools and over extended ranges of time, temperature, pressure, and in different environments, the simplicity turned into complexity and even today no protein is fully understood. Protein assemblies, cells, and extended biological systems (the brain!) display even more complexity. Here I sketch some of the steps that led to the present, still incomplete, understanding of the physics of a "simple" protein, myoglobin. The number of papers concerning myoglobin is vast. The choice made here is therefore exceedingly biased but I hope that it gives some insight into the present picture. © 2010 Elsevier B.V. All rights reserved.

Frauenfelder H.,Los Alamos National Laboratory
Physical Biology | Year: 2014

Stan Ulam, the famous mathematician, said once to Hans Frauenfelder: 'Ask not what Physics can do for biology, ask what biology can do for physics'. The interaction between biologists and physicists is a two-way street. Biology reveals the secrets of complex systems, physics provides the physical tools and the theoretical concepts to understand the complexity. The perspective gives a personal view of the path to some of the physical concepts that are relevant for biology and physics (Frauenfelder et al 1999 Rev. Mod. Phys. 71 S419-S442). Schrödinger's book (Schrödinger 1944 What is Life? (Cambridge: Cambridge University Press)), loved by physicists and hated by eminent biologists (Dronamraju 1999 Genetics 153 1071-6), still shows how a great physicist looked at biology well before the first protein structure was known. © 2014 IOP Publishing Ltd.

Wen X.-D.,Cornell University | Wen X.-D.,Los Alamos National Laboratory | Hoffmann R.,Cornell University | Ashcroft N.W.,Cornell University
Journal of the American Chemical Society | Year: 2011

In a theoretical study, benzene is compressed up to 300 GPa. The transformations found between molecular phases generally match the experimental findings in the moderate pressure regime (<20 GPa): phase I (Pbca) is found to be stable up to 4 GPa, while phase II (P43212) is preferred in a narrow pressure range of 4-7 GPa. Phase III (P21/c) is at lowest enthalpy at higher pressures. Above 50 GPa, phase V (P21 at 0 GPa; P21/c at high pressure) comes into play, slightly more stable than phase III in the range of 50-80 GP, but unstable to rearrangement to a saturated, four-coordinate (at C), one-dimensional polymer. Actually, throughout the entire pressure range, crystals of graphane possess lower enthalpy than molecular benzene structures; a simple thermochemical argument is given for why this is so. In several of the benzene phases there nevertheless are substantial barriers to rearranging the molecules to a saturated polymer, especially at low temperatures. Even at room temperature these barriers should allow one to study the effect of pressure on the metastable molecular phases. Molecular phase III (P21/c) is one such; it remains metastable to higher pressures up to ∼200 GPa, at which point it too rearranges spontaneously to a saturated, tetracoordinate CH polymer. At 300 K the isomerization transition occurs at a lower pressure. Nevertheless, there may be a narrow region of pressure, between P = 180 and 200 GPa, where one could find a metallic, molecular benzene state. We explore several lower dimensional models for such a metallic benzene. We also probe the possible first steps in a localized, nucleated benzene polymerization by studying the dimerization of benzene molecules. Several new (C6H6)2 dimers are predicted. © 2011 American Chemical Society.

Li J.,University of Geneva | Martin I.,Los Alamos National Laboratory | Buttiker M.,University of Geneva | Morpurgo A.F.,University of Geneva
Nature Physics | Year: 2011

The edges of graphene-based systems possess unusual electronic properties, originating from the non-trivial topological structure associated with the pseudospinorial character of the electron wavefunctions. These properties, which have no analogue for electrons described by the Schrödinger equation in conventional systems, have led to the prediction of many striking phenomena, such as gate-tunable ferromagnetism and valley-selective transport. In most cases, however, the predicted phenomena are not expected to survive the strong structural and chemical disorder that unavoidably affects the edges of real graphene devices. Here, we present a theoretical investigation of the intrinsic low-energy states at the edges of electrostatically gapped bilayer graphene, and find that the contribution of edge modes to the linear conductance of realistic devices remains sizable even for highly imperfect edges. This contribution may dominate over that of the bulk for sufficiently clean devices, such as those based on suspended bilayer graphene samples. Our results illustrate the robustness of those phenomena whose origin is rooted in the topology of the electronic band structure, even in the absence of specific protection mechanisms. © 2011 Macmillan Publishers Limited. All rights reserved.

Deffner S.,Los Alamos National Laboratory
New Journal of Physics | Year: 2016

Fluctuation theorems are symmetry relations for the probability to observe an amount of entropy production in a finite-time process. In a recent paper Pigeon et al (2016 New. J. Phys. 18 013009) derived fluctuation theorems for harmonic networks by means of the large deviation theory. Their novel approach is illustrated with various examples of experimentally relevant systems. As a main result, however, Pigeon et al provide new insight how to consistently formulate quantum stochastic thermodynamics, and provide new and robust tools for the study of the thermodynamics of quantum harmonic networks. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Gutfraind A.,Los Alamos National Laboratory
PLoS ONE | Year: 2010

Complex socioeconomic networks such as information, finance and even terrorist networks need resilience to cascades - to prevent the failure of a single node from causing a far-reaching domino effect. We show that terrorist and guerrilla networks are uniquely cascade-resilient while maintaining high efficiency, but they become more vulnerable beyond a certain threshold. We also introduce an optimization method for constructing networks with high passive cascade resilience. The optimal networks are found to be based on cells, where each cell has a star topology. Counterintuitively, we find that there are conditions where networks should not be modified to stop cascades because doing so would come at a disproportionate loss of efficiency. Implementation of these findings can lead to more cascade-resilient networks in many diverse areas.

Patton H.J.,Los Alamos National Laboratory | Taylor S.R.,Rocky Mountain Geophysics, Inc.
Journal of Geophysical Research: Solid Earth | Year: 2011

Classical explosion source theory relates isotropic seismic moment to the steady state level of the reduced displacement potential. The theoretical isotropic moment for an incompressible source region Mt is proportional to cavity volume Vc created by pressurization of materials around the point of energy release. Source medium damage due to nonlinear deformations caused by the explosion will also induce volume change Vd and radiate seismic waves as volumetric, double-couple, and compensated linear vector dipole (CLVD) body force systems. A new source model is presented where K is a relative measure of moment MCLVD with respect to the net moment from volumetric sources Vc and V d. K values from moment tensor inversions steadily decrease from ∼2.5 at lower yields to ∼1.0 for the highest-yield shots on Pahute Mesa. A value of 1.0 implies MCLVD = 0 and, by inference, small V d. We hypothesize that the extent to which damage adds (or subtracts) volumetric moment is controlled by material properties and dynamics of stress wave rebound, shock wave interactions with the free surface, gravitational unloading, and slapdown of spalled near-surface layers. This hypothesis is tested by comparing measurements of isotropic moment M̂I with estimates of Mt based on Vc scaling relationships and velocity-density models. The results support the hypothesis and the conclusion that M̂I represents the "apparent explosion moment" since it has contributions from direct effects due to cavity formation and indirect effects due to material damage. Implications for yield estimation using M̂I are discussed in general and for the North Korean tests. Copyright 2011 by the American Geophysical Union.

Rinehart J.D.,University of California at Berkeley | Kozimor S.A.,Los Alamos National Laboratory | Long J.R.,University of California at Berkeley
Angewandte Chemie - International Edition | Year: 2010

(Figure Presented) Hard-core uranium chemistry: The 3,5dimethylpyrazolate anion can be activated by uranium (III) to form 4-ketimidopent-2ene-2-imido (kipi3-) units, which are isoelectronic to acetylacetonate. Three related tetranuclear uranium cluster compounds were isolated (see picture), of which two are mixed valent. © 2010 Wiley-VCH Verlag GmbH S. Co. KGaA.

Birn J.,Los Alamos National Laboratory | Nakamura R.,Austrian Academy of Sciences | Panov E.V.,Austrian Academy of Sciences | Hesse M.,NASA
Journal of Geophysical Research: Space Physics | Year: 2011

Using three-dimensional MHD simulations of magnetic reconnection in the magnetotail, we investigate the fate of earthward bursty bulk flows (BBFs). The flow bursts are identified as entropy-depleted magnetic flux tubes ("bubbles") generated by the severance of a plasmoid via magnetic reconnection. The onset of fast reconnection coincides closely with a drastic entropy reduction at the onset of lobe reconnection. The fact that, in the simulation, the Alfvn speed does not change significantly at this time suggests that the destabilization of ballooning/interchange modes is important in driving faster reconnection as well as in providing cross-tail structure. In the initial phase, the BBFs are associated with earthward propagating dipolarization fronts. When the flow is stopped nearer to Earth, the region of dipolarization expands both azimuthally and tailward. Tailward flows are found to be associated with a rebound of the earthward flow and with reversed vortices on the outside of the flow. Earthward and tailward flows are also associated with expansion and contraction of the near plasma sheet. All of these features are consistent with recent satellite observations by Cluster and the Time History of Events and their Macroscopic Interactions during Substorms (THEMIS) mission. Copyright 2011 by the American Geophysical Union.

Tang X.Z.,Los Alamos National Laboratory
Plasma Physics and Controlled Fusion | Year: 2011

In a sheath-limited high-temperature and low-density plasma, energy and particle loss to an absorbing wall can set up a temperature anisotropy in which the normal-to-the-wall temperature is significantly lower than that of the parallel-to-the-wall directions, even for an upstream plasma source with isotropic temperature. This temperature anisotropy excites the Weibel instability and introduces a self-generated magnetic field which is parallel to the wall surface. The self-generated magnetic field modifies the sheath/presheath plasma in two ways upon saturation: (1) it suppresses the net energy loss rate to the absorbing wall, primarily through the ion channel and (2) it transfers energy between different degrees of freedom and reduces the plasma temperature anisotropy. © 2011 IOP Publishing Ltd.

ReVelle D.O.,Los Alamos National Laboratory
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2010

We have incorporated horizontal winds into ray-mode theory including the full spectrum of acoustic-gravity waves for a perfectly stratified, range-independent, steady-state model atmosphere for frequencies from 10-4 to ∼10 Hz. This approach has also been applied to a specific atmospheric propagation problem that has long defied a solution, namely counter-wind propagation arrivals at a location ∼300 km up-wind of the source. Our modified ray-mode theory predicts reliable up-wind solutions, but only if small-scale sound speed fluctuations were added to the mean seasonal sound speed profiles. Since full-waveguide theory and modified ray-mode mode theory incorporate diffraction and scattering propagation effects, we have performed additional analyses to determine the mechanism through which these fluctuations produce the up-wind signals. We have concluded that the dominant mechanism is through diffraction due to the presence of semi-permanent turbulence and internal gravity waves located near the stratopause.

Barouch D.H.,Beth Israel Deaconess Medical Center | Barouch D.H.,Massachusetts Institute of Technology | Korber B.,Los Alamos National Laboratory | Korber B.,Santa Fe Institute
Annual Review of Medicine | Year: 2010

Despite more than 25 years of concerted worldwide research, the development of a safe and effective HIV-1 vaccine remains elusive. Prototype antibody-based and T cell-based HIV-1 vaccines have failed to show efficacy in clinical trials to date. Next-generation HIV-1 vaccine candidates are in various stages of preclinical and clinical development, but key scientific obstacles pose major challenges for the field. Critical hurdles include the enormous global diversity of the virus and the challenges associated with generating broadly reactive neutralizing antibody and cellular immune responses. We review the current state of the HIV-1 vaccine field and outline strategies that are being explored to overcome these roadblocks. © 2010 by Annual Reviews All rights reserved.

Hecht M.W.,Los Alamos National Laboratory
Ocean Modelling | Year: 2010

We identify a potentially severe source of spurious cooling within and below the thermocline. The effect involves an interplay between tracer advection scheme and eddy parameterization: a dispersive advection scheme generates spurious warm and cold extrema, and then the tracer mixing scheme is relied upon to moderate those extrema. Noise suppression is less robust when the eddy parameterization consists of the more physically based use of eddy-induced transport and isopycnal tracer mixing. Convection occurs in response to the spurious warm and cold extrema generated by the dispersive advection scheme, driving a persistent cooling below the thermocline. When choosing an advection scheme for ocean climate modelling this effect should be considered as a significant concern associated with the use of dispersive centered advection. © 2010 Elsevier Ltd.

Taylor C.D.,Los Alamos National Laboratory
Journal of Nuclear Materials | Year: 2011

Surface properties of Tc-rich and Fe-rich portions of the Tc-Fe binary alloy phase diagram were computed in this work on the basis of density functional theory. Tc and Fe were found to have minimal degrees of mixing in the parent phases, consistent with the experimentally derived phase diagram. The influence of oxygen on surface phase stability was also studied, with no significant impact on surface segregation or degree of surface mixing. Oxygen adsorption was shown to change the ordering of surface facets in Tc, such that the pyramidal phase becomes lower in energy than the prismatic phase, even with low coverage of oxygen. No evidence for increased surface segregation upon oxidation was found for the solid-solution phases. A potential-pH surface Pourbaix diagram was derived for Tc and H, OH and O adsorbed sub-monolayers were shown to be precursors to oxide formation. While Tc and Fe have similar reactivities and properties in their parent phases, and hence, also in solid-solution, the properties of the intermetallic are expected to be significantly different due to the size-mismatch between the elements. © 2010 Elsevier B.V. All rights reserved.

Arrowsmith S.J.,Los Alamos National Laboratory | Johnson J.B.,New Mexico Institute of Mining and Technology | Drob D.P.,U.S. Navy | Hedlin M.A.H.,University of California at San Diego
Reviews of Geophysics | Year: 2010

The field of seismoacoustics is emerging as an important discipline in its own right, owing to the value of colocated seismic and infrasound arrays that sample elastic energy propagating in both the solid Earth and the atmosphere. The fusion of seismic and infrasonic data provides unique constraints for studying a broad range of topics including the source physics of natural and man-made events, interaction of mechanical waves in Earth's crust and atmosphere, source location and characterization, and inversion of atmospheric and shallow subsurface properties. This review article traces the seismoacoustic wavefield from source to receiver. Beginning at the source, we review the latest insights into the physics of natural and anthropogenic sources that have arisen from the analysis of seismoacoustic data. Next, a comparative review of 3-D models of the atmosphere and solid Earth and the latest algorithms for modeling the propagation of mechanical waves through these media provides the framework for a discussion of the seismoacoustic path. The optimal measurement of seismic and acoustic waves, including a discussion of instrumentation, as well as of array configurations and regional networks, is then outlined. Finally, we focus on broad research applications where the analysis of seismoacoustic data is starting to yield important new results, such as in the field of nuclear explosion monitoring. This review is intended to provide a primer on the field of seismoacoustics for seismologists or acousticians, while also providing a more general review of what constraints seismoacoustics can uniquely provide for understanding geophysical phenomena. © 2010 by the American Geophysical Union.

Borovsky J.E.,Los Alamos National Laboratory
Journal of Geophysical Research: Space Physics | Year: 2010

Using ACE, Helios, and OMNI2 measurements, the direction vectors of the solar wind magnetic field are statistically analyzed. Two populations of direction vectors are found: a Gaussian distribution about the Parker spiral direction and an isotropic population. Examination of the isotropic population finds ejecta, long-duration non-Parker spiral intervals, magnetic depressions, heliospheric-current-sheet crossings, and spillover from the Gaussian population. Via numerical experiments, spillover in spherical coordinates from the Gaussian population into the isotropic population is explored and quantified. ACE measurements find that the angular width of the Gaussian Parker spiral population increases with solar wind speed. Examining the properties of the two populations year by year, no clear solar-cycle trends are found. Inside the compression regions of corotating interaction regions, the longitudinal width of the Gaussian Parker spiral population decreases by about a factor of two, while the latitudinal width of that population is approximately unchanged. Helios measurements find that the angular width of the Gaussian Parker spiral population decreases closer to the Sun, and the isotropic fraction decreases. The flux tube model of the solar wind structure is compared with spacecraft measurements: the model approximately agrees with the Helios behavior versus the distance from the Sun, and the model approximately agrees with the ACE behavior in the corotating interaction region compressions. Copyright 2010 by the American Geophysical Union.

Welling D.T.,Los Alamos National Laboratory | Ridley A.J.,University of Michigan
Journal of Geophysical Research: Space Physics | Year: 2010

A persistent, unresolved problem in terrestrial magnetospheric physics is determining the dominant source and associated entry mechanism for plasma in the Earth's magnetosphere. This study uses the multispecies MHD code, Block Adaptive Tree Solar Wind Roe-Type Upwind Scheme (BATS-R-US), to investigate this issue. Two proton species, ionospheric origin and solar wind origin, are defined in the system and the evolution of each population is followed under different idealized solar wind conditions. It is found that during southward oriented interplanetary magnetic field (IMF), the dominant source is ionospheric plasma entering deep down tail through reconnecting field lines. During northward IMF, the dominant source is solar wind plasma entering through the flanks of the magnetosphere. This two-mode behavior is tested through data-model comparisons of real world simulations. Comparisons of model results against Los Alamos National Laboratory Magnetospheric Plasma Analyzer density, pressure, and inferred oxygen content support the conclusions of the idealized results. © 2010 by the American Geophysical Union.

Podesta J.J.,Los Alamos National Laboratory
Physics of Plasmas | Year: 2010

Large amplitude transient growth of kinetic scale perturbations in stable collisionless magnetized plasmas has recently been demonstrated using a linearized Landau fluid model. Initial perturbations with lengthscales of the order of the ion gyroradius were shown to have transient timescales that in some cases were long compared to the ion gyroperiod, Ωit≫1. Moreover, it was suggested that such perturbations are not rare but instead form a large class within the set of all possible initial conditions. For collisionless plasmas, the Vlasov-Maxwell equations provide a more complete description of kinetic physics and the existence of transient growth of solutions for the linearized Vlasov-Maxwell system is an interesting question. The existence of transient growth of solutions is demonstrated here for a special case of the Vlasov-Maxwell equations, namely, the one dimensional Vlasov-Poisson system. The analysis is different from the standard approach of nonmodal analysis since the initial value problem is described by a Volterra integral equation of the second kind, reflecting the fact that the time evolution of the system depends on the memory of the state from time zero through time t. For the case of a thermal equilibrium plasma, it is shown how initial conditions may be constructed to obtain solutions that grow linearly in time; the duration of this growth is the time required for a thermal electron to traverse the wavelength of the initial perturbation, a timescale that can last for many plasma periods 2π/ωpe, thus demonstrating the existence of transient growth of solutions for the linearized Vlasov-Poisson system. The results suggest that the phenomenon of transient growth may be a common feature of the linearized Vlasov-Maxwell system as well as for Landau fluid models. © 2010 American Institute of Physics.

Harrison N.,Los Alamos National Laboratory | Sebastian S.E.,University of Cambridge
New Journal of Physics | Year: 2014

We address the origin of the recently discovered close correspondence between the charge ordering wave vectors and the momentum-space separation between the tips of the Fermi arcs seen in angle-resolved photoemission measurements in underdoped high-temperature superconducting cuprates. This observation has been interpreted as a signature of charge order forming as an instability of pre-existing Fermi arcs of a different origin. We calculate the Fermi surface spectral weight for a charge density-wave model, considering a Fermi surface, charge ordering wave vectors and short correlation lengths similar to those found experimentally. We show that the observation of wave vectors spanning the tips of remnant Fermi surface sections is a natural consequence of a Fermi surface having been reconstructed by charge order. The presence of short-range charge order therefore cannot be ruled out as a potential origin of the observed Fermi arcs. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Rielage K.,Los Alamos National Laboratory
AIP Conference Proceedings | Year: 2012

The MiniCLEAN dark matter direct detection experiment is a single-phase liquid argon detector, instrumented with photomultiplier tubes to observe scintillation light from a 150 kg fiducial mass. This detector design strategy emphasizes scalability to target masses of order 10 tons or more. The projected light yield is >5 photo-electrons per keV, which allows pulse shape discrimination to separate the electron background from a WIMP-induced nuclear recoil signal. MiniCLEAN is also designed for a liquid neon target, which in the event of a positive signal will provide a unique test of the expected A 2 dependence of the WIMP interaction rate. This paper reviews the experimental technique and current status of MiniCLEAN. © 2012 American Institute of Physics.

Mitri F.G.,Los Alamos National Laboratory | Silva G.T.,Federal University of Alagoas
Wave Motion | Year: 2011

In this paper, the off-axial acoustic scattering of a high-order Bessel vortex beam by a rigid immovable (fixed) sphere is investigated. It is shown here that shifting the sphere off the axis of wave propagation induces a dependence of the scattering on the azimuthal angle. Theoretical expressions for the incident and scattered field from a rigid immovable sphere are derived. The near- and far-field acoustic scattering fields are expressed using partial wave series involving the spherical harmonics, the scattering coefficients of the sphere, the half-conical angle of the wave number components of the beam, its order and the beam-shape coefficients. The scattering coefficients of the sphere and the 3D scattering directivity plots in the near- and far-field regions are evaluated using a numerical integration procedure. The calculations indicate that the scattering directivity patterns near the sphere and in the far-field are strongly dependent upon the position of the sphere facing the incident high-order Bessel vortex beam. © 2011 Elsevier B.V.

Haynes C.T.,Queen Mary, University of London | Burgess D.,Queen Mary, University of London | Camporeale E.,Los Alamos National Laboratory
Astrophysical Journal | Year: 2014

Knowledge of turbulent behavior at sub-proton scales in magnetized plasmas is important for a full understanding of the energetics of astrophysical flows such as the solar wind. We study the formation of electron temperature anisotropy due to reconnection in the turbulent decay of sub-proton scale fluctuations using two-dimensional, particle-in-cell plasma simulations with a realistic electron-proton mass ratio and a guide field perpendicular to the simulation plane. A power spectrum fluctuation with approximately power-law form is created down to scales of the order of the electron gyroradius. We identify the signatures of collisionless reconnection at sites of X-point field geometry in the dynamic magnetic field topology, which gradually relaxes in complexity. The reconnection sites are generally associated with regions of strong parallel electron temperature anisotropy. The evolving topology of magnetic field lines connected to a reconnection site allows for the spatial mixing of electrons accelerated at multiple, spatially separated reconnection regions. This leads to the formation of multi-peaked velocity distribution functions with strong parallel temperature anisotropy. In a three-dimensional system that can support the appropriate wave vectors, the multi-peaked distribution functions would be expected to be unstable to kinetic instabilities, contributing to dissipation. The proposed mechanism of anisotropy formation is also relevant to space and astrophysical systems where the evolution of the plasma is constrained by linear temperature anisotropy instability thresholds. The presence of reconnection sites leads to electron energy gain, nonlocal velocity space mixing, and the formation of strong temperature anisotropy; this is evidence of an important role for reconnection in the dissipation of turbulent fluctuations. © 2014. The American Astronomical Society. All rights reserved..

Lebensohn R.A.,Los Alamos National Laboratory | Cazacu O.,The Reef
International Journal of Solids and Structures | Year: 2012

In this paper, the combined effects of texture and asymmetric single-crystal plastic deformation mechanisms on the dilatational response of voided polycrystals are assessed for the first time. To this end, a full-field dilatational viscoplastic Fast Fourier Transform (FFT)-based approach is used to generate gauge surfaces for porous polycrystals deforming by twinning at single crystal level, which are compared to yield surfaces obtained according to a recent analytical criterion for porous materials. Both approaches are cross-validated, revealing unusual features of the dilatational response, namely, a lack of symmetry of the surfaces with respect to both the hydrostatic and deviatoric axes. This strong sensitivity to the third invariant of the stress deviator is associated to the anisotropy and the tension-compression asymmetry of the plastic response of the matrix. © 2012 Elsevier Ltd. All rights reserved.

Terwilliger T.C.,Los Alamos National Laboratory
Acta Crystallographica Section D: Biological Crystallography | Year: 2010

A method for rapidly building Β-sheets into electron-density maps is presented. Β-Strands are identified as tubes of high density adjacent to and nearly parallel to other tubes of density. The alignment and direction of each strand are identified from the pattern of high density corresponding to carbonyl and CΒ atoms along the strand averaged over all repeats present in the strand. The Β-strands obtained are then assembled into a single atomic model of the Β-sheet regions. The method was tested on a set of 42 experimental electron-density maps at resolutions ranging from 1.5 to 3.8 Å. The Β-sheet regions were nearly completely built in all but two cases, the exceptions being one structure at 2.5 Å resolution in which a third of the residues in Β-sheets were built and a structure at 3.8 Å in which under 10% were built. The overall average r.m.s.d. of main-chain atoms in the residues built using this method compared with refined models of the structures was 1.5 Å. © 2010 International Union of Crystallography.

Weijer W.,Los Alamos National Laboratory | van Sebille E.,University of New South Wales
Journal of Climate | Year: 2014

The impact of Agulhas leakage variability on the strength of the Atlantic meridional overturning circulation (AMOC) in the Community Climate System Model, version 4 (CCSM4) is investigated. In this model an advective connection exists that transports salinity anomalies from the Agulhas region into the North Atlantic on decadal (30-40 yr) time scales. However, there is no identifiable impact of Agulhas leakage on the strength of theAMOC, suggesting that the salinity variations are too weak to significantly modify the stratification in the North Atlantic. It is argued that this study is inconclusive with respect to an impact of Agulhas leakage on theAMOC. Salinity biases leave the South Atlantic and IndianOceans too homogeneous, in particular erasing the observed salinity front in the Agulhas retroflection region. Consequently, salinity variability in the southeastern South Atlantic is found to be much weaker than observed. © 2014 American Meteorological Society.

Popa N.C.,National Institute of Materials Physics Bucharest | Balzar D.,University of Denver | Vogel S.C.,Los Alamos National Laboratory
Journal of Applied Crystallography | Year: 2014

A new approach for the determination of the elastic macro strain and stress in textured polycrystals by diffraction is presented. It consists of expanding the strain tensor weighted by texture in a series of generalized spherical harmonics where the ground state is defined by the strain/stress state in an isotropic sample in the Voigt model. In contrast to similar expansions already reported by other authors, this new approach provides expressions valid for any sample and crystal symmetries and can easily be implemented in whole powder pattern fitting, including Rietveld refinement. An earlier article [Popa & Balzar (2001). J. Appl. Cryst. 34, 187-195] reported a similar model, but with a spherical harmonics expansion around the hydrostatic strain/stress state of the isotropic polycrystal. The availability of several different models is beneficial in order to allow one to select the representation in which the ground state is the closest to the actual stress state in the sample. © 2014 International Union of Crystallography.

Fernandes R.M.,Columbia University | Fernandes R.M.,Los Alamos National Laboratory | Millis A.J.,Columbia University
Physical Review Letters | Year: 2013

Motivated by the recent experimental detection of Néel-type [(π, π)] magnetic fluctuations in some iron pnictides, we study the impact of competing (π, π) and (π, 0) spin fluctuations on the superconductivity of these materials. We show that, counterintuitively, even short-range, weak Néel fluctuations strongly suppress the s+- state, with the main effect arising from a repulsive contribution to the s +- pairing interaction, complemented by low-frequency inelastic scattering. Further increasing the strength of the Néel fluctuations leads to a low-Tc d-wave state, with a possible intermediate s+id phase. The results suggest that the absence of superconductivity in a series of hole-doped pnictides is due to the combination of short-range Néel fluctuations and pair-breaking impurity scattering and also that Tc of optimally doped pnictides could be further increased if residual (π, π) fluctuations were reduced. © 2013 American Physical Society.

Plank T.,Lamont Doherty Earth Observatory | Kelley K.A.,University of Rhode Island | Zimmer M.M.,Los Alamos National Laboratory | Hauri E.H.,Carnegie Institution of Washington | Wallace P.J.,University of Oregon
Earth and Planetary Science Letters | Year: 2013

The last 15yr have seen an explosion of data on the volatile contents of magmas parental to arc volcanoes. This has occurred due to the intense study of melt inclusions trapped in volcanic phenocrysts, aliquots of magma that have presumably escaped degassing during eruption. The surprising first-order result is the narrow range in H2O concentrations in the least degassed melt inclusions from each volcano. Nearly all arc volcanoes are sourced with mafic magmas that contain 2-6wt% H2O. The average for each arc varies even less, from 3.2 (for the Cascades) to 4.5 (for the Marianas), with a global average of 3.9±0.4wt% H2O. Significant variations occur from volcano to volcano within each arc, but the means are indistinguishable within one s.d. The narrow range and common average value for H2O are in stark contrast to the concentrations of most other subduction tracers, such as Nb or Ba, which vary by orders of magnitude. A modulating process, either in the crust or mantle, is likely responsible for the restricted range in the H2O contents of arc melt inclusions. One possibility is that melt inclusion H2O values reflect vapor saturation at the last storage depth in the crust prior to eruption. In this scenario, magmas rise from the mantle with variable H2O contents (>4wt%), become vapor-saturated and start degassing, and continue to degas up until the depth at which they stall. If the stalling depths are ~6km, which is common for storage depths beneath volcanoes, magmas would be saturated at ~4wt% H2O, and melt inclusions, most of which become closed during further ascent, would thus record ≤4wt% H2O. Another possibility is that the mantle melting process modulates water content in the melt such that magmas rise out of the mantle with ~4wt% H2O. A strong relationship between the water content of the source, H2O(o) and the degree of melting (F) maintains nearly constant water contents in the melt for a restricted range in mantle temperature. Magmas with 3-4wt% H2O can be generated at ~50° below the dry solidus for a wide range in F and H2O(o). The narrow range in wedge temperatures may be another manifestation of a planet with average upper mantle of 1400°C potential temperature. The characteristic mean and range of H2O contents of arc magmas has implications for both the volatile fuel for explosive eruptions and the mass balance of H2O recycled through subduction zones. © 2012 Elsevier B.V.

Gamberg L.,Pennsylvania State University | Kang Z.-B.,Los Alamos National Laboratory | Prokudin A.,Jefferson Lab
Physical Review Letters | Year: 2013

We analyze the spin asymmetry for single inclusive jet production in proton-proton collisions collected by the AnDY experiment at the Relativistic Heavy Ion Collider and the Sivers asymmetry data from semi-inclusive deep inelastic scattering experiments. In particular, we consider the role color gauge invariance plays in determining the process dependence of the Sivers effect. We find that after carefully taking into account the initial-state and final-state interactions between the active parton and the remnant of the polarized hadron, the calculated jet spin asymmetry based on the Sivers functions extracted from HERMES and COMPASS experiments is consistent with the AnDY experimental data. This provides a first indication for the process dependence of the Sivers effect in these processes. We also make predictions for both direct photon and Drell-Yan spin asymmetry, to further test the process dependence of the Sivers effect in future experiments. © 2013 American Physical Society.

Ardeljan M.,University of New Hampshire | Beyerlein I.J.,Los Alamos National Laboratory | Knezevic M.,University of New Hampshire
Journal of the Mechanics and Physics of Solids | Year: 2014

We present a multiscale model for anisotropic, elasto-plastic, rate- and temperature-sensitive deformation of polycrystalline aggregates to large plastic strains. The model accounts for a dislocation-based hardening law for multiple slip modes and links a single-crystal to a polycrystalline response using a crystal plasticity finite element based homogenization. It is capable of predicting local stress and strain fields based on evolving microstructure including the explicit evolution of dislocation density and crystallographic grain reorientation. We apply the model to simulate monotonic mechanical response of a hexagonal close-packed metal, zirconium (Zr), and a body-centered cubic metal, niobium (Nb), and study the texture evolution and deformation mechanisms in a two-phase Zr/Nb layered composite under severe plastic deformation. The model predicts well the texture in both co-deforming phases to very large plastic strains. In addition, it offers insights into the active slip systems underlying texture evolution, indicating that the observed textures develop by a combination of prismatic, pyramidal, and anomalous basal slip in Zr and primarily {110}〈111〉 slip and secondly {112}〈111〉 slip in Nb. © 2014 Elsevier Ltd. All rights reserved.

Daligault J.,Los Alamos National Laboratory
Journal of Statistical Physics | Year: 2011

We present a theory for the construction of renormalized kinetic equations to describe the dynamics of classical systems of particles in or out of equilibrium. A closed, self-consistent set of evolution equations is derived for the single-particle phase-space distribution function f, the correlation function C=〈δfδf〉, the retarded and advanced density response functions χR,A=δf/δφ to an external potential φ, and the associated memory functions ΣR,A,C. The basis of the theory is an effective action functional Ω of external potentials φ that contains all information about the dynamical properties of the system. In particular, its functional derivatives generate successively the single-particle phase-space density f and all the correlation and density response functions, which are coupled through an infinite hierarchy of evolution equations. Traditional renormalization techniques (involving Legendre transform and vertex functions) are then used to perform the closure of the hierarchy through memory functions. The latter satisfy functional equations that can be used to devise systematic approximations that automatically imply the conservation laws of mass, momentum and energy. The present formulation can be equally regarded as (i) a generalization to dynamical problems of the density functional theory of fluids in equilibrium and (ii) as the classical mechanical counterpart of the theory of non-equilibrium Green's functions in quantum field theory. It unifies and encompasses previous results for classical Hamiltonian systems with any initial conditions. For equilibrium states, the theory reduces to the equilibrium memory function approach used in the kinetic theory of fluids in thermal equilibrium. For non-equilibrium fluids, popular closures of the BBGKY hierarchy (e. g. Landau, Boltzmann, Lenard-Balescu-Guernsey) are simply recovered and we discuss the correspondence with the seminal approaches of Martin-Siggia-Rose and of Rose and we discuss the correspondence with the seminal approaches of Martin-Siggia-Rose and of Rose. © 2011 Springer Science+Business Media, LLC.

Chavez D.E.,Los Alamos National Laboratory | Bottaro J.C.,SRI International | Petrie M.,SRI International | Parrish D.A.,U.S. Navy
Angewandte Chemie - International Edition | Year: 2015

This study presents the synthesis and characterization of a fused, tricyclic 1,2,3,4-tetrazine ring system. The molecule is synthesized in a three-step process from 5,5′-dinitro-bis,1,2,4-triazole via a di-N-amino compound. Oxidation to form the azo-coupled fused tricyclic 1,2,3,4-tetrazine is achieved using tert-butyl hypochlorite as the oxidant. The di-N-amino compound and the desired fused tricyclic 1,2,3,4-triazine display interesting thermal behavior and are predicted to be high-performance energetic materials. Ring of fire: Double N-amination of the bis(tetraethylammonium) salt of 3,3′-dinitro-5,5′-bi-1,2,4-triazole results in a thermally stable di-N-aminated energetic material that is insensitive to impact, spark, and friction. Subsequent oxidation leads to a fused, tricyclic 1,2,3,4-tetrazine ring system (see figure) with excellent density and explosive performance properties. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ben-Naim E.,Los Alamos National Laboratory | Krapivsky P.L.,Boston University
Physical Review Letters | Year: 2014

We study extreme-value statistics of Brownian trajectories in one dimension. We define the maximum as the largest position to date and compare maxima of two particles undergoing independent Brownian motion. We focus on the probability P(t) that the two maxima remain ordered up to time t and find the algebraic decay P∼t-β with exponent β=1/4. When the two particles have diffusion constants D1 and D2, the exponent depends on the mobilities, β=(1/π)arctanD2/D1. We also use numerical simulations to investigate maxima of multiple particles in one dimension and the largest extension of particles in higher dimensions. © 2014 American Physical Society.

Huang J.,Los Alamos National Laboratory | Liu T.,Hong Kong University of Science and Technology | Wang L.-T.,University of Chicago | Yu F.,Fermi National Accelerator Laboratory
Physical Review Letters | Year: 2014

We reveal a set of novel decay topologies for the 125 GeV Higgs boson in supersymmetry which are initiated by its decay into a pair of neutralinos, and discuss their collider search strategies. This category of exotic Higgs decays is characterized by the collider signature: visible objects+ET, with ET dominantly arising from escaping dark matter particles. Their benchmark arises naturally in the Peccei-Quinn symmetry limit of the minimal supersymmetric standard model singlet extensions, which is typified by the coexistence of three light particles: singletlike scalar h1 and pseudoscalar a1, and singlinolike neutralino χ1, all with masses of 10GeV, and the generic suppression of the exotic decays of the 125 GeV Higgs boson h2h1h1, a1a1 and χ1χ1, however. As an illustration, we study the decay topology: h2χ1χ2, where the binolike χ2 decays to h1χ1 or a1χ1, and h1/a1ff̄, with ff̄=+-, bb̄. In the dimuon case (mh1/a11GeV), a statistical sensitivity of S/B>6 can be achieved easily at the 8 TeV LHC, assuming (ppWh2)/(ppWhSM)Br(h2+-χ1χ1)=0.1. In the bb̄ case (mh1/a145GeV), 600fb1 data at the 14 TeV LHC can lead to a statistical sensitivity of S/B>5, assuming (ppZh2)/(ppZhSM) Br(h2bb̄χ1χ1)=0.5. These exotic decays open a new avenue for exploring new physics couplings with the 125 GeV Higgs boson at colliders. © 2014 American Physical Society.

Horigane K.,University of Virginia | Llobet A.,Los Alamos National Laboratory | Louca D.,University of Virginia
Physical Review Letters | Year: 2014

SrFeO2 is an insulating antiferromagnet with a remarkably high transition temperature in spite of its quasi-two-dimensional crystal structure. The magnetic exchange coupling is, however, very sensitive to a local mode involving transverse displacements of O and Fe, resulting in zigzag patterns of distortion. The buckling driven by rising temperatures is enhanced just as the Fe magnetic moment is reduced, implying a strong spin-lattice coupling. It is suggested that the undulations lead to orbital disorder by distorting the three possible paths to exchange interactions. © 2014 American Physical Society.

Brandao F.G.S.L.,Federal University of Minas Gerais | Christandl M.,ETH Zurich | Yard J.,Los Alamos National Laboratory
Communications in Mathematical Physics | Year: 2011

Squashed entanglement is a measure for the entanglement of bipartite quantum states. In this paper we present a lower bound for squashed entanglement in terms of a distance to the set of separable states. This implies that squashed entanglement is faithful, that is, it is strictly positive if and only if the state is entangled. We derive the lower bound on squashed entanglement from a lower bound on the quantum conditional mutual information which is used to define squashed entanglement. The quantum conditional mutual information corresponds to the amount by which strong subadditivity of von Neumann entropy fails to be saturated. Our result therefore sheds light on the structure of states that almost satisfy strong subadditivity with equality. The proof is based on two recent results from quantum information theory: the operational interpretation of the quantum mutual information as the optimal rate for state redistribution and the interpretation of the regularised relative entropy of entanglement as an error exponent in hypothesis testing. The distance to the set of separable states is measured in terms of the LOCC norm, an operationally motivated norm giving the optimal probability of distinguishing two bipartite quantum states, each shared by two parties, using any protocol formed by local quantum operations and classical communication (LOCC) between the parties. A similar result for the Frobenius or Euclidean norm follows as an immediate consequence. The result has two applications in complexity theory. The first application is a quasipolynomial-time algorithm solving the weak membership problem for the set of separable states in LOCC or Euclidean norm. The second application concerns quantum Merlin-Arthur games. Here we show that multiple provers are not more powerful than a single prover when the verifier is restricted to LOCC operations thereby providing a new characterisation of the complexity class QMA. © 2011 Springer-Verlag.

Roytershteyn V.,University of California at San Diego | Daughton W.,Los Alamos National Laboratory | Karimabadi H.,University of California at San Diego | Mozer F.S.,University of California at Berkeley
Physical Review Letters | Year: 2012

Using fully kinetic 3D simulations of magnetic reconnection in asymmetric antiparallel configurations, we demonstrate that an electromagnetic lower-hybrid drift instability (LHDI) localized near the X line can substantially modify the reconnection mechanism in the regimes with large asymmetry, a moderate ratio of electron to ion temperature, and low plasma β. However, the mode saturates at a small amplitude in the regimes typical of Earth's magnetopause. In these cases, LHDI-driven turbulence is predominantly localized along the separatrices on the low-β side of the current sheet, in agreement with spacecraft observations. © 2012 American Physical Society.

Steiner A.W.,Michigan State University | Steiner A.W.,University of Washington | Gandolfi S.,Los Alamos National Laboratory
Physical Review Letters | Year: 2012

Using a phenomenological form of the equation of state of neutron matter near the saturation density which has been previously demonstrated to be a good characterization of quantum Monte Carlo simulations, we show that currently available neutron star mass and radius measurements provide a significant constraint on the equation of state of neutron matter. At higher densities we model the equation of state by using polytropes and a quark matter model. We show that observations offer an important constraint on the strength of the three-body force in neutron matter, and thus some theoretical models of the three-body force may be ruled out by currently available astrophysical data. In addition, we obtain an estimate of the symmetry energy of nuclear matter and its slope that can be directly compared to the experiment and other theoretical calculations. © 2012 American Physical Society.

Daligault J.,Los Alamos National Laboratory
Physical Review Letters | Year: 2012

Molecular dynamics simulations are used to investigate the diffusion properties of one-component plasmas and binary ionic mixtures from the weakly to the strongly coupled regimes. A physically motivated model for the diffusivities is proposed that reproduces the simulation data and gives insight into the nature of ionic motions and interactions in plasmas across the coupling regimes. The model extends the widely used Chapman-Spitzer theory from the weakly to the moderately coupled regime. In the strongly coupled regime, diffusion is modeled in terms of thermally activated jumps between equilibrium positions separated by an energy barrier. The basic ideas discussed are applicable to the study of other transport coefficients. © 2012 American Physical Society.

Weijer W.,Los Alamos National Laboratory
Geophysical Research Letters | Year: 2010

The Australian-Antarctic Basin (AAB) is known for its high levels of intraseasonal variability; sea-surface height variability exceeds background values by factors of 2 over thousands of kilometers. This paper addresses the hypothesis that this variability is caused by trapping of barotropic energy by the basin geometry. Analysis of a multi-year integration of a shallow-water model shows that the variability is dominated by a single, large-scale statistical mode that is highly coherent over the entire AAB. The flow associated with this mode is northwestward along the Southeast Indian Ridge, southward in the Kerguelen Abyssal Plain, and eastward in the southern AAB. The mode is interpreted as an almost-free topographically trapped mode, as it is confined by contours of potential vorticity that almost entirely enclose the AAB. The apex of the Wilkes Abyssal Plain represents the strongest barrier to the modal circulation: here velocities are strongest, making it a key area for dissipation of kinetic energy through bottom friction and eddy viscosity. Copyright © 2010 by the American Geophysical Union.

Dai W.,Central China Normal University | Vitev I.,Los Alamos National Laboratory | Zhang B.-W.,Central China Normal University
Physical Review Letters | Year: 2013

In collisions of ultrarelativistic nuclei, photon-tagged jets provide a unique opportunity to compare jet production and modification due to parton shower formation and propagation in strongly interacting matter at vastly different center-of-mass energies. We present first results for the cross sections of jets tagged by an isolated photon to O(αemα s2) in central Au+Au reactions with √sNN=200 GeV at RHIC and central Pb+Pb reactions with √sNN=2.76 TeV at LHC. We evaluate the increase in the transverse momentum imbalance of the observed γ+jet state, induced by the dissipation of the parton shower energy due to strong final-state interactions. Theoretical predictions to help interpret recent and upcoming experimental data are presented. © 2013 American Physical Society.

Sabbatini J.,University of Queensland | Zurek W.H.,Los Alamos National Laboratory | Davis M.J.,University of Queensland
Physical Review Letters | Year: 2011

The miscibility-immiscibility phase transition in binary Bose-Einstein condensates (BECs) can be controlled by a coupling between the two components. Here we propose a new scheme that uses coupling-induced pattern formation to test the Kibble-Zurek mechanism (KZM) of topological-defect formation in a quantum phase transition. For a binary BEC in a ring trap we find that the number of domains forming the pattern scales as a function of the coupling quench rate with an exponent as predicted by the KZM. For a binary BEC in an elongated harmonic trap we find a different scaling law due to the transition being spatially inhomogeneous. We perform a "quantum simulation" of the harmonically trapped system in a ring trap to verify the scaling exponent. © 2011 American Physical Society.

Afnan I.R.,Flinders University | Gibson B.F.,Los Alamos National Laboratory
Physical Review C - Nuclear Physics | Year: 2010

Background: Direct measurement of the electric dipole moment (EDM) of the neutron is in the future; measurement of a nuclear EDM may well come first. The deuteron is one nucleus for which exact model calculations are feasible. Purpose: We explore the model dependence of deuteron EDM calculations. Methods: Using a separable potential formulation of the Hamiltonian, we examine the sensitivity of the deuteron EDM to variation in the nucleon-nucleon interaction. We write the EDM as the sum of two terms, the first depending on the target wave function with plane-wave intermediate states, and the second depending on intermediate multiple scattering in the 3P1 channel, the latter being sensitive to the off-shell behavior of the 3P1 amplitude. Results: We compare the full calculation with the plane-wave approximation result, examine the tensor force contribution to the model results, and explore the effect of short-range repulsion found in realistic, contemporary potential models of the deuteron. Conclusions: Because one-pion exchange dominates the EDM calculation, separable potential model calculations will provide an adequate description of the H2 EDM until such time as a measurement better than 10% is obtained. © 2010 The American Physical Society.

Wehling T.O.,University of Bremen | Black-Schaffer A.M.,Uppsala University | Balatsky A.V.,NORDITA | Balatsky A.V.,Los Alamos National Laboratory
Advances in Physics | Year: 2014

A wide range of materials, like d-wave superconductors, graphene, and topological insulators, share a fundamental similarity: their low-energy fermionic excitations behave as massless Dirac particles rather than fermions obeying the usual Schrödinger Hamiltonian. This emergent behavior of Dirac fermions in condensed matter systems defines the unifying framework for a class of materials we call "Dirac materials." In order to establish this class of materials, we illustrate how Dirac fermions emerge in multiple entirely different condensed matter systems and we discuss how Dirac fermions have been identified experimentally using electron spectroscopy techniques (angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy). As a consequence of their common low-energy excitations, this diverse set of materials shares a significant number of universal properties in the low-energy (infrared) limit. We review these common properties including nodal points in the excitation spectrum, density of states, specific heat, transport, thermodynamic properties, impurity resonances, and magnetic field responses, as well as discuss many-body interaction effects. We further review how the emergence of Dirac excitations is controlled by specific symmetries of the material, such as time-reversal, gauge, and spin-orbit symmetries, and how by breaking these symmetries a finite Dirac mass is generated. We give examples of how the interaction of Dirac fermions with their distinct real material background leads to rich novel physics with common fingerprints such as the suppression of back scattering and impurity-induced resonant states. © 2014 Taylor & Francis.

Taylor C.D.,Los Alamos National Laboratory
Journal of Physical Chemistry C | Year: 2014

Advanced models for alloy corrosion should take into account the properties of the oxide film that provides the first line of corrosion resistance. Predictive models for alloy corrosion are particularly desirable for the evaluation of candidate alloy forms for the disposition of spent nuclear fuel. Since technetium is one of the long-lived fission products generated in a nuclear reactor, we consider herein the mechanism of technetium oxidation, beginning with computing the properties of the oxide using a number of first-principles approaches, and then moving to oxygen chemisorption and multilayer oxide growth. For TcO2, electronic structure elements were improved with the use of the Hubbard U method, but this choice led to poor structural predictions. Conventional GGA, on the other hand, gave good structural and mechanical agreement with the hybrid density functional calculations. We examined a number of surface terminations for this oxide phase and found that the "striped" nonpolar surface termination was preferred. The surface energy for this configuration was calculated to be 1.41 J/m2. Oxygen chemisorption to Tc(0001) is very strong, implying that formation of a full surface monolayer of chemisorbed oxygen is preferred to the nucleation of oxide islands. The formation of multilayers of TcO2 on Tc leads to surface configurations that resemble the charge state, bonding patterns, and configuration dependence of TcO2 at an early stage. The films are strongly adherent, with adhesion energies of TcO2(001) on Tc(0001) computed to be -2.50 J/m2 for 0.50 monolayer (ML), -3.86 J/m2 for 1.0 ML, and -2.51 J/m2 for 2.0 ML. © 2014 American Chemical Society.

Rodriguez M.A.,Los Alamos National Laboratory
Knowledge-Based Systems | Year: 2011

The Resource Description Framework (RDF) is a semantic network data model that is used to create machine-understandable descriptions of the world and is the basis of the Semantic Web. This article discusses the application of RDF to the representation of computer software and virtual computing machines. The Semantic Web is posited as not only a web of data, but also as a web of programs and processes. © 2010 Elsevier B.V. All rights reserved.

Panaitescu A.,Los Alamos National Laboratory
Astrophysical Journal | Year: 2015

We develop a numerical formalism for calculating the distribution with energy of the (internal) pairs formed in a relativistic source from unscattered MeV-TeV photons. For gamma-ray burst (GRB) afterglows, this formalism is more suitable if the relativistic reverse shock that energizes the ejecta is the source of the GeV photons. The number of pairs formed is set by the source GeV output (calculated from the Fermi-LAT fluence), the unknown source Lorentz factor, and the unmeasured peak energy of the LAT spectral component. We show synchrotron and inverse-Compton light curves expected from pairs formed in the shocked medium and identify some criteria for testing a pair origin of GRB optical counterparts. Pairs formed in bright LAT afterglows with a Lorentz factor in the few hundreds may produce bright optical counterparts () lasting for up to one hundred seconds. The number of internal pairs formed from unscattered seed photons decreases very strongly with the source Lorentz factor, thus bright GRB optical counterparts cannot arise from internal pairs if the afterglow Lorentz factor is above several hundreds. © 2015. The American Astronomical Society. All rights reserved.

Hitchhiking and severe bottleneck effects have impact on the dynamics of genetic diversity of a population by inducing homogenization at a single locus and at the genome-wide scale, respectively. As a result, identification and differentiation of the signatures of such events from DNA sequence data at a single locus is challenging. This paper develops an analytical framework for identifying and differentiating recent homogenization events at multiple neutral loci in low recombination regions. The dynamics of genetic diversity at a locus after a recent homogenization event is modeled according to the infinite-sites mutation model and the Wright-Fisher model of reproduction with constant population size. In this setting, I derive analytical expressions for the distribution, mean, and variance of the number of polymorphic sites in a random sample of DNA sequences from a locus affected by a recent homogenization event. Based on this framework, three likelihood-ratio based tests are presented for identifying and differentiating recent homogenization events at multiple loci. Lastly, I apply the framework to two data sets. First, I consider human DNA sequences from four non-coding loci on different chromosomes for inferring evolutionary history of modern human populations. The results suggest, in particular, that recent homogenization events at the loci are identifiable when the effective human population size is 50000 or greater in contrast to 10000, and the estimates of the recent homogenization events are agree with the "Out of Africa" hypothesis. Second, I use HIV DNA sequences from HIV-1-infected patients to infer the times of HIV seroconversions. The estimates are contrasted with other estimates derived as the mid-time point between the last HIV-negative and first HIV-positive screening tests. The results show that significant discrepancies can exist between the estimates.

Medin Z.,Los Alamos National Laboratory | Cumming A.,McGill University
Astrophysical Journal | Year: 2015

We discuss the effect of convection driven by chemical separation at the ocean-crust boundary of accreting neutron stars. We extend the steady-state results of Medin & Cumming to transient accretors, by considering the time-dependent cases of heating during accretion outbursts and cooling during quiescence. During accretion outbursts, inward heat transport has only a small effect on the temperature profile in the outer layers until the ocean is strongly enriched in light elements, a process that takes hundreds of years to complete. During quiescence, however, inward heat transport rapidly cools the outer layers of the ocean while keeping the inner layers hot. We find that this leads to a sharp drop in surface emission at around a week followed by a gradual recovery as cooling becomes dominated by the crust. Such a dip should be observable in the light curves of these neutron star transients, if enough data is taken at a few days to a month after the end of accretion. If such a dip is definitively observed, it will provide strong constraints on the chemical composition of the ocean and outer crust. © 2015. The American Astronomical Society. All rights reserved..

Gary S.P.,Los Alamos National Laboratory | Chang O.,University of Southern California | Wang J.,University of Southern California
Astrophysical Journal | Year: 2012

This manuscript describes the first ensemble of three-dimensional (3D) particle-in-cell (PIC) plasma simulations of whistler turbulence. The computational model represents a collisionless, homogeneous, magnetized plasma on which an initial spectrum of relatively long wavelength whistler fluctuations is imposed. The simulations represent a range of initial fluctuation amplitudes and follow the temporal evolution of the system as it decays into a broadband, anisotropic, turbulent spectrum at shorter wavelengths via a forward cascade. The resulting 3D turbulence is similar in many ways to whistler turbulence from previous two-dimensional (2D) PIC simulations, although the anisotropies in 3D are stronger than in comparable 2D runs. The most important difference is that reduced magnetic fluctuation spectra from the 3D simulations show a clear break in the perpendicular wavenumber (k ⊥) spectra. Spectra at small k ⊥ are relatively steep, but spectra at larger k ⊥ are even steeper, similar in character to magnetic spectra at electron scales recently measured in the solar wind. © 2012. The American Astronomical Society. All rights reserved.

Delgado F.,International Iberian Nanotechnology Laboratory | Batista C.D.,Los Alamos National Laboratory | Fernandez-Rossier J.,International Iberian Nanotechnology Laboratory
Physical Review Letters | Year: 2013

Spin chains are among the simplest physical systems in which electron-electron interactions induce novel states of matter. Here we propose to combine atomic scale engineering and spectroscopic capabilities of state of the art scanning tunnel microscopy to probe the fractionalized edge states of individual atomic scale S=1 spin chains. These edge states arise from the topological order of the ground state in the Haldane phase. We also show that the Haldane gap and the spin-spin correlation length can be measured with the same technique. © 2013 American Physical Society.

Harrison N.,Los Alamos National Laboratory | Sebastian S.E.,University of Cambridge
Physical Review Letters | Year: 2011

A multiple wave vector (Q) reconstruction of the Fermi surface is shown to yield a profoundly different electronic structure to that characteristic of single wave vector reconstruction, despite their proximity in energy. We consider the specific case in which ordering is generated by Q x=[2πa,0] and Qy=[0,2πb] (in which a=b=14)-similar to those identified in neutron diffraction and scanning tunneling microscopy experiments-and more generally show that an isolated pocket adjacent to the nodal point knodal=[±π2,±π2] is a protected feature of such a multiple-Q model, potentially corresponding to the nodal "Fermi arcs" observed in photoemission and the small size of the electronic heat capacity found in high magnetic fields-importantly, containing electron carriers which can yield negative Hall and Seebeck coefficients observed in high magnetic fields. © 2011 American Physical Society.

Fermi surface models applied to the underdoped cuprates predict the small pocket area to be strongly dependent on doping whereas quantum oscillations in YBa2Cu3O6+x find precisely the opposite to be true-seemingly at odds with the Luttinger volume. We show that such behavior can be explained by an incommensurate antinodal Fermi surface nesting-type instability-further explaining the doping-dependent superstructures seen in cuprates using scanning tunneling microscopy. We develop a Fermi surface reconstruction scheme involving orthogonal density waves in two dimensions and show that their incommensurate behavior requires momentum-dependent coupling. A cooperative modulation of the charge and bond strength is therefore suggested. © 2011 American Physical Society.

We derive an exact solution of an explicitly time-dependent multichannel model of quantum mechanical nonadiabatic transitions. In the limit Nâ‰1, where N is the number of states, we find that the survival probability of the initially populated state remains finite despite an almost arbitrary choice of a large number of parameters. This observation proves that quantum mechanical nonadiabatic transitions among a large number of states can effectively keep memory about the initial state of the system. This property can lead to a strongly nonergodic behavior even in the thermodynamic limit of some systems with a broad distribution of coupling constants and the lack of energy conservation. © 2013 American Physical Society.

Miller E.K.,Los Alamos National Laboratory
IEEE Antennas and Propagation Magazine | Year: 2011

A procedure called FARS (Far-field Analysis of Radiation Sources) was previously described by the author as a means of determining the quantitative contribution per unit length or per unit area to the power radiated from a perfect electric conductor in the frequency domain. It is based on a source-integral expression for the fields of an object of interest. Extension of frequency-domain FARS (FDFARS) to the time domain (TDFARS) is presented here, together with some representative results. Just as frequency-domain and time-domain solutions provide complementary perspectives of general electromagnetic phenomena, so do FDFARS and TDFARS for their particular applications. For example, while a frequency-domain result explicitly demonstrates the effects of standing waves, a time-domain result has the advantage of separating various contributions to the far field due to their different time delays. The motivation for both FDFARS and TDFARS is to provide information about radiation from a perfect electric conductor. A brief description of TDFARS is given here, and demonstrated for some simple wire geometries. © 2006 IEEE.

Hammond G.E.,Pacific Northwest National Laboratory | Lichtner P.C.,Los Alamos National Laboratory
Water Resources Research | Year: 2010

High-resolution, three-dimensional, reactive flow and transport simulations are carried out to describe the migration of hexavalent uranium [U(VI)] at the Hanford 300 Area bordering the Columbia River and to better understand the persistence of the uranium plume at the site. The computer code PFLOTRAN developed under a DOE SciDAC-2 project is employed in the simulations that are executed on ORNL's Cray XT4/XT5 supercomputer Jaguar. The conceptual model used in the simulations is based on the recognition of three distinct phases or time periods in the evolution of the U(VI) plume. These correspond to (1) initial waste emplacement; (2) initial presence of both labile and nonlabile U(VI) with an evolved U(VI) plume extending from the source region to the river boundary, representing present-day conditions; and (3) the complete removal of all nonlabile U(VI) and labile U(VI) in the vadose zone. This work focuses primarily on modeling Phase II using equilibrium and multirate sorption models for labile U(VI) and a continuous source release of nonlabile U(VI) in the South Process Pond through dissolution of metatorbernite as a surrogate mineral. For this case, rapid fluctuations in the Columbia River stage combined with the slow release of nonlabile U(VI) from contaminated sediment are found to play a predominant role in determining the migration behavior of U(VI) with sorption only a second-order effect. Nevertheless, a multirate model was essential in explaining breakthrough curves obtained from laboratory column experiments using the same sediment and is demonstrated to be important in Phase III. The calculations demonstrate that U(VI) is discharged to the river at a highly fluctuating rate in a ratchet-like behavior as the river stage rises and falls. The high-frequency fluctuations must be resolved in the model to calculate the flux of U(VI) at the river boundary. By time averaging the instantaneous flux to average out noise superimposed on the river stage fluctuations, the cumulative U(VI) flux to the river is found to increase approximately linearly with time. The flow rate and U(VI) flux are highly sensitive to the conductance boundary condition that describes the river-sediment interface. By adjusting the conductance coefficient to give a better match to the measured piezometric head, good agreement was obtained with field studies for both the mean flux of water of 109 kg/yr and U(VI) of 25 kg/yr at the river-aquifer boundary for a computational domain encompassing the South Process Pond. Finally, it is demonstrated that, through global mass conservation, the U(VI) leach rate from the source region is related to the U(VI) flux at the river boundary. Copyright 2010 by the American Geophysical Union.

Zurek W.H.,Los Alamos National Laboratory
Physical Review Letters | Year: 2011

Symmetry of entangled states under a swap of outcomes (" envariance") implies their equiprobability and leads to Born's rule p k=|ψk2. Here I show the converse: I demonstrate that the amplitude of a state given by a superposition of sequences of events that share the same total count (e.g., n detections of 0 and m of 1 in a spin-12 measurement) is proportional to the square root of the fraction-square root of the relative frequency-of all the equiprobable sequences of 0's and 1's with that n and m. © 2011 American Physical Society.

Yu R.,Rice University | Zhu J.-X.,Los Alamos National Laboratory | Si Q.,Rice University
Physical Review Letters | Year: 2011

The degree of electron correlations remains a central issue in the iron-based superconductors. The parent iron pnictides are antiferromagnetic, and their bad-metal behavior has been interpreted in terms of proximity to a Mott transition. We study such a transition in multiorbital models on modulated lattices containing an ordered pattern of iron vacancies, using a slave-rotor method. We show that the ordered vacancies lead to a band narrowing, which pushes the system to the Mott insulator side. This effect is proposed to underlie the insulating behavior observed in the parent compounds of the newly discovered (K,Tl)yFexSe2 superconductors. © 2011 American Physical Society.

Aluie H.,Los Alamos National Laboratory
Physical Review Letters | Year: 2011

We prove that interscale transfer of kinetic energy in compressible turbulence is dominated by local interactions. In particular, our results preclude direct transfer of kinetic energy from large-scales to dissipation scales, such as into shocks, in high Reynolds number turbulence as is commonly believed. Our assumptions on the scaling of structure functions are weak and enjoy compelling empirical support. Under a stronger assumption on pressure dilatation cospectrum, we show that mean kinetic and internal energy budgets statistically decouple beyond a transitional conversion range. Our analysis establishes the existence of an ensuing inertial range over which mean subgrid scale kinetic energy flux becomes constant, independent of scale. Over this inertial range, mean kinetic energy cascades locally and in a conservative fashion despite not being an invariant. © 2011 American Physical Society.

Randrup J.,Lawrence Berkeley National Laboratory | Moller P.,Los Alamos National Laboratory
Physical Review Letters | Year: 2011

Although nuclear fission can be understood qualitatively as an evolution of the nuclear shape, a quantitative description has proven to be very elusive. In particular, until now, there existed no model with demonstrated predictive power for the fission-fragment mass yields. Exploiting the expected strongly damped character of nuclear dynamics, we treat the nuclear shape evolution in analogy with Brownian motion and perform random walks on five-dimensional fission potential-energy surfaces which were calculated previously and are the most comprehensive available. Test applications give good reproduction of highly variable experimental mass yields. This novel general approach requires only a single new global parameter, namely, the critical neck size at which the mass split is frozen in, and the results are remarkably insensitive to its specific value. © 2011 American Physical Society.

Chern G.-W.,University of Wisconsin - Madison | Batista C.D.,Los Alamos National Laboratory
Physical Review Letters | Year: 2011

We study double-exchange models with itinerant t2g electrons in spinel and pyrochlore crystals. In both cases the localized spins form a network of corner-sharing tetrahedra. We show that the strong directional dependence of t2g orbitals leads to unusual Fermi surfaces that induce spin superstructures and noncoplanar orderings for a weak coupling between itinerant electrons and localized spins. Implications of our results to ZnV 2O4 and Cd2Os2O7 are also discussed. © 2011 American Physical Society.

Vallinotto A.,Los Alamos National Laboratory
Astrophysical Journal | Year: 2012

Weak gravitational lensing is one of the key probes of cosmology. Cosmic shear surveys aimed at measuring the distribution of matter in the universe are currently being carried out (Pan-STARRS) or planned for the coming decade (DES, LSST, EUCLID, WFIRST). Crucial to the success of these surveys is the control of systematics. In this work, a new method to constrain one such family of systematics, known as multiplicative bias, is proposed. This method exploits the cross-correlation between weak-lensing measurements from galaxy surveys and the ones obtained from high-resolution cosmic microwave background experiments. This cross-correlation is shown to have the power to break the degeneracy between the normalization of the matter power spectrum and the multiplicative bias of cosmic shear and to be able to constrain the latter to a few percent. © 2012 The American Astronomical Society. All rights reserved.

Li Q.,University of Pennsylvania | Dong Y.,Purdue University | Perez D.,Los Alamos National Laboratory | Martini A.,Purdue University | Carpick R.W.,University of Pennsylvania
Physical Review Letters | Year: 2011

The atomic stick-slip behavior of a Pt tip sliding on a Au(111) surface is studied with atomic force microscopy (AFM) experiments and accelerated (i.e., reduced sliding speed) molecular dynamics (MD) simulations. The MD and AFM conditions are controlled to match, as closely as possible, the geometry and orientation, load, temperature, and compliance. We observe clear stick-slip without any damage. Comparison of both MD and AFM results with the thermally activated Prandtl-Tomlinson model shows that MD results at the highest speeds are not in the thermally activated regime. At lower speeds, within the thermally activated regime, AFM and MD provide consistent energetics, but attempt frequencies differ by orders of magnitude. Because this discrepancy lies in attempt frequencies and not energetics, atomistic details in MD simulations can be reliably used in interpreting AFM data if the MD speeds are slow enough. © 2011 American Physical Society.

Little C.M.,Princeton University | Oppenheimer M.,Princeton University | Urban N.M.,Princeton University | Urban N.M.,Los Alamos National Laboratory
Nature Climate Change | Year: 2013

Climate adaptation and flood risk assessments have incorporated sea-level rise (SLR) projections developed using semi-empirical methods (SEMs) and expert-informed mass-balance scenarios. These techniques, which do not explicitly model ice dynamics, generate upper bounds on twenty-first century SLR that are up to three times higher than Intergovernmental Panel on Climate Change estimates. However, the physical basis underlying these projections, and their likelihood of occurrence, remain unclear. Here, we develop mass-balance projections for the Antarctic ice sheet within a Bayesian probabilistic framework, integrating numerical model output and updating projections with an observational synthesis. Without abrupt, sustained, changes in ice discharge (collapse), we project a 95th percentile mass loss equivalent to ∼13 cm SLR by 2100, lower than previous upper-bound projections. Substantially higher mass loss requires regional collapse, invoking dynamics that are likely to be inconsistent with the underlying assumptions of SEMs. In this probabilistic framework, the pronounced sensitivity of upper-bound SLR projections to the poorly known likelihood of collapse is lessened with constraints on the persistence and magnitude of subsequent discharge. More realistic, fully probabilistic, estimates of the ice-sheet contribution to SLR may thus be obtained by assimilating additional observations and numerical models. © 2013 Macmillan Publishers Limited. All rights reserved.

Friedland A.,Los Alamos National Laboratory | Giannotti M.,Barry University | Wise M.,Barry University
Physical Review Letters | Year: 2013

We point out that stars in the mass window ∼8-12M⊙ can serve as sensitive probes of the axion-photon interaction, g Aγγ. Specifically, for these stars axion energy losses from the helium-burning core would shorten and eventually eliminate the blue loop phase of the evolution. This would contradict observational data, since the blue loops are required, e.g., to account for the existence of Cepheid stars. Using the MESA stellar evolution code, modified to include the extra cooling, we conservatively find gAγγ≲0.8×10-10 GeV-1, which compares favorably with the existing bounds. © 2013 American Physical Society.

Zwolak M.,Oregon State University | Riedel C.J.,IBM | Zurek W.H.,Los Alamos National Laboratory
Physical Review Letters | Year: 2014

Amplification was regarded, since the early days of quantum theory, as a mysterious ingredient that endows quantum microstates with macroscopic consequences, key to the "collapse of the wave packet," and a way to avoid embarrassing problems exemplified by Schrödinger's cat. Such a bridge between the quantum microworld and the classical world of our experience was postulated ad hoc in the Copenhagen interpretation. Quantum Darwinism views amplification as replication, in many copies, of the information about quantum states. We show that such amplification is a natural consequence of a broad class of models of decoherence, including the photon environment we use to obtain most of our information. This leads to objective reality via the presence of robust and widely accessible records of selected quantum states. The resulting redundancy (the number of copies deposited in the environment) follows from the quantum Chernoff information that quantifies the information transmitted by a typical elementary subsystem of the environment. © 2014 American Physical Society.

Kashinath A.,Massachusetts Institute of Technology | Misra A.,Los Alamos National Laboratory | Demkowicz M.J.,Massachusetts Institute of Technology
Physical Review Letters | Year: 2013

He implanted into metals precipitates into nanoscale bubbles that may later grow into voids, degrading the properties of engineering alloys. Using multiscale modeling, we show that a different class of He precipitates may form at semicoherent interfaces: nanoscale platelets. These platelets grow by wetting high-energy interface regions, remain stable under irradiation, and reduce He-induced swelling. Stable storage of He at interfaces may impart unprecedented He resistance to future structural materials. © 2013 American Physical Society.

Nishida Y.,Los Alamos National Laboratory | Moroz S.,University of Washington | Son D.T.,University of Chicago
Physical Review Letters | Year: 2013

We study a system of spinless fermions in two dimensions with a short-range interaction fine-tuned to a p-wave resonance. We show that three such fermions form an infinite tower of bound states of orbital angular momentum ℓ=±1 and their binding energies obey a universal doubly exponential scaling E3(n)a (-2e3πn/4+θ) at large n. This "super Efimov effect" is found by a renormalization group analysis and confirmed by solving the bound state problem. We also provide an indication that there are ℓ=±2 four-body resonances associated with every three-body bound state at E4(n)a (- 2e3πn/4+θ-0.188). These universal few-body states may be observed in ultracold atom experiments and should be taken into account in future many-body studies of the system. © 2013 American Physical Society.

Graesser M.L.,Los Alamos National Laboratory | Shelton J.,Harvard University
Physical Review Letters | Year: 2013

We point out that, in the irreducible natural supersymmetric spectrum, top squarks have comparable branching fractions to chargino-bottom and neutralino-top final states in the vast bulk of parameter space, provided only that both decay modes are kinematically accessible. The total top squark pair branching fractions into tt̄+MET (MET=missing transverse energy) can therefore be reduced to O(50%), whereas bb̄+X branching fractions are typically much smaller, O(10%), thus limiting the reach of traditional top squark searches. We propose a new top squark search targeting the asymmetric final state tËœtËœ*→ tχ0b̄χ-+H.c., which can restore sensitivity to natural top squarks in the 7 and 8 TeV LHC runs. In addition, we present a new variable, topness, which efficiently suppresses the dominant top backgrounds to semileptonic top partner searches. We demonstrate the utility of topness in both our asymmetric search channel and traditional tËœt Ëœ*→tt̄+MET searches and show that it matches or outperforms existing variables. © 2013 American Physical Society.

Ta Phuoc K.,ENSTA ParisTech | Corde S.,ENSTA ParisTech | Thaury C.,ENSTA ParisTech | Malka V.,ENSTA ParisTech | And 5 more authors.
Nature Photonics | Year: 2012

One of the major goals of research for laser-plasma accelerators is the realization of compact sources of femtosecond X-rays. In particular, using the modest electron energies obtained with existing laser systems, Compton scattering a photon beam off a relativistic electron bunch has been proposed as a source of high-energy and high-brightness photons. However, laser-plasma based approaches to Compton scattering have not, to date, produced X-rays above 1 keV. Here, we present a simple and compact scheme for a Compton source based on the combination of a laser-plasma accelerator and a plasma mirror. This approach is used to produce a broadband spectrum of X-rays extending up to hundreds of keV and with a 10,000-fold increase in brightness over Compton X-ray sources based on conventional accelerators. We anticipate that this technique will lead to compact, high-repetition-rate sources of ultrafast (femtosecond), tunable (X-through gamma-ray) and low-divergence (≈1/°) photons from source sizes on the order of a micrometre. © 2012 Macmillan Publishers Limited. All rights reserved.

Randrup J.,Lawrence Berkeley National Laboratory | Moller P.,Los Alamos National Laboratory
Physical Review C - Nuclear Physics | Year: 2013

The recently developed treatment of Brownian shape evolution is refined to take account of the gradual decrease in microscopic effects as the nuclear excitation energy is raised. We construct effective potential-energy surfaces by multiplying the shell-plus-pairing correction term by a suppression factor that depends on the local excitation energy. While this approach is equivalent to the modification of the Fermi-gas level density parameter suggested by Ignatyuk, we adopt a more general functional form for the suppression factor, which is adjusted to measured charge yields for 234U(E*≈11MeV). The resulting model is benchmarked by comparison with 70 measured yields. © 2013 American Physical Society.

Kim Y.S.,Los Alamos National Laboratory | Pivovar B.S.,National Renewable Energy Laboratory
Journal of the Electrochemical Society | Year: 2010

Membrane-electrode interfacial resistance and deterioration over time was investigated using a series of sulfonated poly(arylene ether) membranes and Nafion. Dimensional mismatch due to swelling/deswelling, wetting/adhesion, and water transport mismatch between the electrodes and the polymer electrolyte membrane were all investigated as potential root causes of membrane-electrode interfacial resistance and durability. The data presented from a large number of diverse polymers strongly support dimensional mismatch as the primary cause of interfacial failure. Extended direct methanol fuel cell life tests, up to 3000 h, showed high performance and good durability for membranes with low water uptake. Polymer electrolyte membranes with ∼35 vol % water uptake were the best in this study. However, a low water uptake coating layer greatly improved the performance and durability of a higher water uptake polymer electrolyte membrane. This study demonstrates the potential importance of the membrane-electrode interface on fuel cell performance and durability and provides a basis for implementing polymer electrolyte membranes effectively in high performance polymer electrolyte fuel cells (PEFCs). © 2010 The Electrochemical Society.

Sharma R.,TRIUMF Laboratory Particle and Nuclear Physics | Vitev I.,Los Alamos National Laboratory
Physical Review C - Nuclear Physics | Year: 2013

We calculate the yields of quarkonia in heavy ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC) as a function of their transverse momentum. Based upon nonrelativistic quantum chromodynamics, our results include both color-singlet and color-octet contributions and feed-down effects from excited states. In reactions with ultrarelativistic nuclei, we focus on the consistent implementation of dynamically calculated nuclear matter effects, such as coherent power corrections, cold nuclear matter energy loss, and the Cronin effect in the initial state. In the final state, we consider radiative energy loss for the color-octet state and collisional dissociation of quarkonia as they traverse through the QGP. Theoretical results are presented for J/ψ and Υ and compared to experimental data where applicable. At RHIC, a good description of the high-pT J/ψ modification observed in central Cu+Cu and Au+Au collisions can be achieved within the model uncertainties. We find that measurements of J/ψ yields in proton-nucleus reactions are needed to constrain the magnitude of cold nuclear matter effects. At the LHC, a good description of the experimental data can be achieved only in midcentral and peripheral Pb+Pb collisions. The large fivefold suppression of prompt J/ψ in the most central nuclear reactions may indicate for the first time possible thermal effects at the level of the quarkonium wave function at large transverse momenta. © 2013 American Physical Society.

Smith T.J.,U.S. Army | Hill K.K.,Los Alamos National Laboratory | Raphael B.H.,Centers for Disease Control and Prevention
Research in Microbiology | Year: 2015

For nearly one hundred years, researchers have attempted to categorize botulinum neurotoxin-producing clostridia and the toxins that they produce according to biochemical characterizations, serological comparisons, and genetic analyses. Throughout this period the bacteria and their toxins have defied such attempts at categorization. Below is a description of both historic and current Clostridium botulinum strain and neurotoxin information that illustrates how each new finding has significantly added to the knowledge of the botulinum neurotoxin-containing clostridia and their diversity. © 2014.

Roy D.,Los Alamos National Laboratory
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

We study two-photon scattering of a tightly focused weak light beam from a small atomic ensemble of two-level atoms (2LAs). This is similar to the scattering of photons from an atomic ensemble in a one-dimensional waveguide. The scaling of two-photon nonlinearity at single-photon resonance shows a nonmonotonic behavior with an increasing number of few identical 2LAs. The two-photon nonlinearity decays monotonically with an increasing number of atoms for incident photons detuned from single-photon resonance. Single-photon transport in two 2LAs is similar to that in a single V-type three-level atom (3LA). However, two-photon transport in these two systems shows very different line shapes. When single-photon transmission is zero in these systems, two transmitted photons are bunched together in a V-type 3LA, while their correlation is zero in two 2LAs. The difference in the two-photon line shape persists for few 2LAs and 3LAs. Therefore, the two-photon scattering of a tightly focused weak light beam can be used as a probe to detect atomic level structures of different atoms with similar transition energies. © 2013 American Physical Society.

McDowell N.G.,Los Alamos National Laboratory | Beerling D.J.,University of Sheffield | Breshears D.D.,University of Arizona | Fisher R.A.,U.S. National Center for Atmospheric Research | And 2 more authors.
Trends in Ecology and Evolution | Year: 2011

Climate-driven vegetation mortality is occurring globally and is predicted to increase in the near future. The expected climate feedbacks of regional-scale mortality events have intensified the need to improve the simple mortality algorithms used for future predictions, but uncertainty regarding mortality processes precludes mechanistic modeling. By integrating new evidence from a wide range of fields, we conclude that hydraulic function and carbohydrate and defense metabolism have numerous potential failure points, and that these processes are strongly interdependent, both with each other and with destructive pathogen and insect populations. Crucially, most of these mechanisms and their interdependencies are likely to become amplified under a warmer, drier climate. Here, we outline the observations and experiments needed to test this interdependence and to improve simulations of this emergent global phenomenon. © 2011 Elsevier Ltd.

Ticknor C.,Los Alamos National Laboratory | Ticknor C.,University of California at Santa Barbara
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

We present an analysis of the excitation spectrum for a two-component quasi-two-dimensional Bose-Einstein condensate (BEC). We study how excitations change character across the miscible to immiscible phase transition. We find that the bulk excitations are typical of a single-component BEC with the addition of interface bending excitations. We study how these excitations change as a function of the interaction strength. © 2013 American Physical Society.

Friar J.L.,Los Alamos National Laboratory
Physical Review C - Nuclear Physics | Year: 2013

Nuclear polarization corrections to the 2P-2S Lamb shift in μ-d atoms are developed in order (α5), and are shown to agree with a recent calculation. The nuclear physics in the resulting corrections is then evaluated in zero-range approximation. The dominant part of the correction is very simple in form and differs from a recent potential model calculation by less than 1%. It is also demonstrated how the third Zemach moment contribution largely cancels against part of the polarization correction, as it did in e-d atoms and does so exactly for pointlike nucleons. This suggests that it may be possible to reduce the uncertainty in the theory (of which nuclear polarization is the largest contributor) to less than 1%. © 2013 American Physical Society.

Das A.,Abdus Salam International Center For Theoretical Physics | Das A.,Los Alamos National Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We show that when a quantum many-body system is subjected to coherent periodic driving, the response may exhibit exotic freezing behavior in high driving frequency (ω) regime. In a periodically driven classical thermodynamic system, freezing at high ω occurs when 1/ω is much smaller than the characteristic relaxation time of the system and hence the freezing always increases there as ω is increased. Here, in the contrary, we see surprising nonmonotonic freezing behavior of the response with ω, showing curious peak-valley structure. Quite interestingly, the entire system tends to freeze almost absolutely (the freezing peaks) when driven with a certain combination of driving parameters values (amplitude and ω) due to coherent suppression of dynamics of the quantum many-body modes, which has no classical analog. We demonstrate this new freezing phenomenon analytically (supported by large-scale numerics) for a general class of integrable quantum spin systems. © 2010 The American Physical Society.

Efimov A.,Los Alamos National Laboratory
Optics Letters | Year: 2013

An image-shearing interferometer of Mach-Zehnder type with corner cubes is introduced for the purpose of measuring spatial coherence at the output of inhomogeneous optical sources, such as multimode fibers (MMFs). One arm of the interferometer is modulated in optical delay to produce dynamic interference fringes. Fringe visibility and the two individual intensities are measured nearly simultaneously to allow direct calculation of the modulus of the complex degree of coherence as a function of the lateral shear between the two interferometer arms. Spatial degree of coherence is measured for a step-index MMF pumped with monochromatic and broadband optical sources. © 2013 Optical Society of America.

Mitri F.G.,Los Alamos National Laboratory
IEEE Transactions on Antennas and Propagation | Year: 2011

This study investigates the arbitrary scattering of an unpolarized electromagnetic (EM) high-order Bessel vortex (helicoidal) beam (HOBVB) by a homogeneous water sphere in air. The radial components of the electric and magnetic scattering fields are expressed using partial wave series involving the beam-shape coefficients and the scattering coefficients of the sphere. The magnitude of the 3D electric and magnetic scattering directivity plots in the far-field region are evaluated using a numerical integration procedure for cases where the sphere is centered on the beam's axis and shifted off-axially with particular emphasis on the half-conical angle of the wave number components and the order (or helicity) of the beam. Some properties of the EM scattering of an HOBVB by the water sphere are discussed. The results are of some the scattering of importance in applications involving EM HOBVBs by a spherical object. © 2011 IEEE.

Yan G.,Los Alamos National Laboratory
Computer Networks | Year: 2013

In order to evade detection of ever-improving defense techniques, modern botnet masters are constantly looking for new communication platforms for delivering C&C (Command and Control) information. Attracting their attention is the emergence of online social networks such as Twitter, as the information dissemination mechanism provided by these networks can naturally be exploited for spreading botnet C&C information, and the enormous amount of normal communications co-existing in these networks makes it a daunting task to tease out botnet C&C messages. Against this backdrop, we explore graph-theoretic techniques that aid effective monitoring of potential botnet activities in large open online social networks. Our work is based on extensive analysis of a Twitter dataset that contains more than 40 million users and 1.4 billion following relationships, and mine patterns from the Twitter network structure that can be leveraged for improving efficiency of botnet monitoring. Our analysis reveals that the static Twitter topology contains a small-sized core sugraph, after removing which, the Twitter network breaks down into small connected components, each of which can be handily monitored for potential botnet activities. Based on this observation, we propose a method called Peri-Watchdog, which computes the core of a large online social network and derives the set of nodes that are likely to pass botnet C&C information in the periphery of online social network. We analyze the time complexity of Peri-Watchdog under its normal operations. We further apply Peri-Watchdog on the Twitter graph injected with synthetic botnet structures and investigate the effectiveness of Peri-Watchdog in detecting potential C&C information from these botnets. To verify whether patterns observed from the static Twitter graph are common to other online social networks, we analyze another online social network dataset, BrightKite, which contains evolution of social graphs formed by its users in half a year. We show not only that there exists a similarly relatively small core in the BrightKite network, but also this core remains stable over the course of BrightKite evolution. We also find that to accommodate the dynamic growth of BrightKite, the core has to be updated about every 18 days under a constrained monitoring capacity. © 2012 Elsevier B.V. All rights reserved.

Favorite J.A.,Los Alamos National Laboratory
Nuclear Science and Engineering | Year: 2013

It is often desirable to solve radiation transport problems in one-dimensional spherical geometries even if the actual object being modeled is not spherical. It may be possible to use perturbation theory to account for the difference between the real multidimensional system and the spherical approximation. This idea is tested using uncollided as well as multigroup inhomogeneous transport problems with upscattering. Asymmetric and nonuniform perturbations are made to the shielding (not the source) of spherical geometries, including transformations from a sphere to a cube (the surface transformation function is derived), and Schwinger, Roussopolos, and combined perturbation estimates are applied. For uncollided fluxes, perturbation theory, particularly the Schwinger estimate, worked very well when the response of interest was the flux measured at a symmetric spherical 4TT detector external to the geometry, but perturbation theory did not work well when the response of interest was the flux measured at a single external point (unless extra care was taken to account for geometric effects). For neutron-induced gammaray line fluxes, the Roussopolos estimate worked well when the response of interest was the flux measured at an external 4π detector or an external point detector.

Stroppa A.,CNR Institute of Neuroscience | Barone P.,CNR Institute of Neuroscience | Jain P.,Los Alamos National Laboratory | Perez-Mato J.M.,University of the Basque Country | Picozzi S.,CNR Institute of Neuroscience
Advanced Materials | Year: 2013

On the basis of first-principles calculations, we design a novel Cr-based metal-organic framework to be both multiferroic and magnetoelectric. The compound shows a "double-hybrid" nature: it is a hybrid organic-inorganic compound and it shows hybrid improper ferroelectricity. Here, the coupling of non-polar distortions, such as Jahn-Teller pseudo-rotations and tilting, pave the way to a polar behavior, with the coupling being realized through hydrogen bonds. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ticknor C.,Los Alamos National Laboratory | Wilson R.M.,University of Colorado at Boulder | Bohn J.L.,University of Colorado at Boulder
Physical Review Letters | Year: 2011

We study the superfluid character of a dipolar Bose-Einstein condensate (DBEC) in a quasi-two dimensional geometry. We consider the dipole polarization to have some nonzero projection into the plane of the condensate so that the effective interaction is anisotropic in this plane, yielding an anisotropic dispersion relation. By performing direct numerical simulations of a probe moving through the DBEC, we observe the sudden onset of drag or creation of vortex-antivortex pairs at critical velocities that depend strongly on the direction of the probe's motion. This anisotropy emerges because of the anisotropic manifestation of a rotonlike mode in the system.© 2011 American Physical Society.

Menikoff R.,Los Alamos National Laboratory
Shock Waves | Year: 2011

Heterogeneities sensitize an explosive to shock initiation. This is due to hot-spot formation and the sensitivity of chemical reaction rates to temperature. Here, we describe a numerical experiment aimed at elucidating a mechanism for hot-spot formation that occurs when a shock wave passes over a high-density impurity. The simulation performed is motivated by a physical experiment in which glass beads are added to liquid nitromethane. The impedance mismatch between the beads and the nitromethane results in shock reflections. These, in turn, give rise to transverse waves along the lead shock front. Hot spots arise on local portions of the lead front with a higher shock strength, rather than on the reflected shocks behind the beads. Moreover, the interactions generated by reflected waves from neighboring beads can significantly increase the peak hot-spot temperature when the beads are suitably spaced. © 2011 Springer-Verlag.

Mitri F.G.,Los Alamos National Laboratory
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control | Year: 2011

The production of acoustical vortices-based potential wells for particle trapping is not only restricted to the use of a Laguerre-Gaussian beam. Other useful types of vortex beams include an r-vortex beam, a non-diffracting high-order Bessel and Bessel-Gauss beam, a fractional (diffracting) highorder Bessel beam, a non-diffracting high-order Bessel beam of fractional type, and a hypergeometric beam to name a few. Representative types of vortex beams are chosen here, but the examples are not exhaustive and additional categories of vortex beams may be reported and investigated. Expressions for the incident acoustic pressure field of various vortex beams are provided. The results should assist in the development of a multitude of vortex-based potential-well models for particle entrapment and manipulation. © 2011 IEEE.

Mitri F.G.,Los Alamos National Laboratory
Journal of Sound and Vibration | Year: 2011

Mathematical expressions for the acoustic scattering, instantaneous (linear), and time-averaged (nonlinear) forces resulting from the interaction of a new type of Bessel beam, termed here a first-order non-diffracting Bessel trigonometric beam (FOBTB) with a sphere, are derived. The beam is termed trigonometric because of the dependence of its phase on the cosine function. The FOBTB is regarded as a superposition of two equi-amplitude first-order Bessel vortex (helicoidal) beams having a unit positive and negative order (known also as topological charge), respectively. The FOBTB is non-diffracting, possesses an axial null, a geometric phase, and has an azimuthal phase that depends on cos(φ±φ0), where φ0 is an initial arbitrary phase angle. Beam rotation around its wave propagation axis can be achieved by varying φ0. The 3D directivity patterns are computed, and the resulting modifications of the scattering are illustrated for a rigid sphere centered on the beams axis and immersed in water. Moreover, the backward and forward acoustic scattering by a sphere vanish for all frequencies. The present paper will shed light on the novel scattering properties of an acoustical FOBTB by a sphere that may be useful in particle manipulation and entrapment, non-destructive/medical imaging, and may be extended to other potentially useful applications in optics and electromagnetism. © 2011 Elsevier Ltd. All rights reserved.

Shoemaker I.M.,Los Alamos National Laboratory
Physics of the Dark Universe | Year: 2013

The mass of primordial dark matter (DM) protohalos remains unknown. However, the missing satellites problem may be an indication that they are quite large. In this paper, we use effective field theory to map constraints on dark matter-SM interactions into limits on the mass of DM protohalos. Given that leptons remain in the thermal bath until late times, we focus on their interactions with DM. To illustrate the method, we use the null results of LEP missing energy searches along with Fermi-LAT searches for DM annihilation in nearby dwarf galaxies, to derive limits on the protohalo mass, ≲ (10-6 to 10-1)M⊙, with the range depending on the DM mass and the operator. Thus, if DM is to remain thermally coupled until late times and account for the missing satellites, charged lepton interactions are insufficient. This motivates neutrinophilic DM, which can have protohalo masses orders of magnitude larger, with constraints arising from Planck, IceCube and unpublished Super-K data. We show that effective neutrinophilic models offer a viable solution to the missing satellites problem for sub-GeV DM masses with larger than WIMP-sized annihilation cross sections. © 2013 Ian M. Shoemaker.

Lonardoni D.,Argonne National Laboratory | Lovato A.,Argonne National Laboratory | Gandolfi S.,Los Alamos National Laboratory | Pederiva F.,University of Trento | Pederiva F.,National Institute of Nuclear Physics, Italy
Physical Review Letters | Year: 2015

The onset of hyperons in the core of neutron stars and the consequent softening of the equation of state have been questioned for a long time. Controversial theoretical predictions and recent astrophysical observations of neutron stars are the grounds for the so-called hyperon puzzle. We calculate the equation of state and the neutron star mass-radius relation of an infinite systems of neutrons and Λ particles by using the auxiliary field diffusion Monte Carlo algorithm. We find that the three-body hyperon-nucleon interaction plays a fundamental role in the softening of the equation of state and for the consequent reduction of the predicted maximum mass. We have considered two different models of three-body force that successfully describe the binding energy of medium mass hypernuclei. Our results indicate that they give dramatically different results on the maximum mass of neutron stars, not necessarily incompatible with the recent observation of very massive neutron stars. We conclude that stronger constraints on the hyperon-neutron force are necessary in order to properly assess the role of hyperons in neutron stars. © 2015 American Physical Society.

Planes A.,University of Barcelona | Castan T.,University of Barcelona | Saxena A.,Los Alamos National Laboratory
Philosophical Magazine | Year: 2014

We provide a general thermodynamic framework to study multicaloric effects in multiferroic materials. This is applied to the case of a magnetoelectric multiferroic such as BiFeO3, which is described by means of a Landau free energy with a biquadratic coupling between polarization and magnetization. We obtain a phase diagram, the isothermal entropy change and the adiabatic temperature change across different continuous and first-order phase transitions as the applied electric and magnetic fields are varied. The multicaloric effects are suitably decomposed into the corresponding electrocaloric and magnetocaloric contributions. © 2014 © 2014 Taylor & Francis.

Beresnyak A.,Los Alamos National Laboratory
Astrophysical Journal Letters | Year: 2013

Stochasticity of magnetic field lines is important for particle transport properties. Magnetic field lines separate faster than diffusively in turbulent plasma, which is called superdiffusion. We discovered that this superdiffusion is pronouncedly asymmetric, so that the separation of field lines along the magnetic field direction is different from the separation in the opposite direction. While the symmetry of the flow is broken by the so-called imbalance or cross-helicity, the difference between forward and backward diffusion is not directly due to imbalance, but a non-trivial consequence of both imbalance and non-reversibility of turbulence. The asymmetric diffusion perpendicular to the mean magnetic field entails a variety of new physical phenomena, such as the production of parallel particle streaming in the presence of perpendicular particle gradients. Such streaming and associated instabilities could be significant for particle transport in laboratory, space, and astrophysical plasmas. © 2013. The American Astronomical Society. All rights reserved..

Das T.,Northeastern University | Das T.,Los Alamos National Laboratory | Markiewicz R.S.,Northeastern University | Bansil A.,Northeastern University
Advances in Physics | Year: 2014

We review the intermediate coupling model for treating electronic correlations in the cuprates. Spectral signatures of the intermediate coupling scenario are identified and used to adduce that the cuprates fall in the intermediate rather than the weak or the strong coupling limits. A robust, beyond local-density approximation framework for obtaining wide-ranging properties of the cuprates via a GW-approximation based self-consistent self-energy correction for incorporating correlation effects is delineated. In this way, doping- and temperature-dependent spectra, from the undoped insulator to the overdoped metal, in the normal as well as the superconducting state, with features of both weak and strong coupling can be modeled in a material-specific manner with very few parameters. Efficacy of the model is shown by considering available spectroscopic data on electron- and hole-doped cuprates from angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, neutron scattering, inelastic light scattering, optical and other experiments. Generalizations to treat systems with multiple correlated bands such as the heavy-fermions, the ruthenates and the actinides are discussed. © 2014 Taylor & Francis.

Wang H.,McMaster University | Wu P.D.,McMaster University | Wang J.,Los Alamos National Laboratory
International Journal of Plasticity | Year: 2013

The inelastic behavior presenting in magnesium alloys during cyclic loading-unloading have been investigated through the finite strain elastic viscoplastic self-consistent (EVPSC) model for polycrystals (EVPSC-TDT), which has been updated by implementing the twinning and de-twinning (TDT) model. Corresponding to the existing experiments of extruded bars of Mg alloys, we constructed the extruded bars of magnesium alloys with different initial textures in our simulations to study the effects of initial textures and deformation processes (tension and compression) on inelastic behavior during cyclic loading and unloading. Taking the advantage of numerical modeling, the evolution of the instantaneous gradients, the activity of the deformation mechanisms and the evolution of twin volume fraction are characterized to interpret the inelastic behavior. We found that the alternation of deformation mechanisms corresponds to the inelastic behavior; in particular, the inelastic behavior becomes more pronounced when twinning and de-twinning are activated. Thus, a strong extrusion texture reduces the hysteresis loops of the loading-unloading cycle under uniaxial tension, while magnifies the inelastic behavior under uniaxial compression, because twinning and de-twinning are more active for extrude bars with the strong extrusion texture under compression. The simulated results are in agreement with the available experimental observations. © Published by Elsevier Ltd.

Nayyar I.H.,University of Central Florida | Masunov A.E.,University of Central Florida | Tretiak S.,Los Alamos National Laboratory
Journal of Physical Chemistry C | Year: 2013

We investigate the accuracy of different formalisms within density functional theory in prediction of two-photon absorption (2PA) spectra for substituted oligophenylvinylenes compared to the experimental measurements. The quadratic response methods are compared with the recently proposed a posteriori Tamm-Dancoff approximation (ATDA) and previously published third-order coupled electronic oscillator results. Quadratic response is found to overestimate the cross sections in all cases. We trace the reasons to unreliable excited state description above the ionization threshold. In addition, quadratic response lacks the double excitations so that their contributions to the 2PA spectra are redistributed over the nearest single character excitations. This distorts the individual contributions to the 2PA response and affects the overall picture. For this reason, we do not recommend quadratic response for the essential state analysis, while ATDA can be used both for the 2PA predictions and the structure/property correlations. As an illustration for ATDA based essential state analysis, we report the mechanism of large 2PA in symmetric donor/acceptor substituted polyphenylvinylene (PPV) oligomers. While HOMO-LUMO transition provides the only bright intermediate state, the brightness of the one-photon absorption (1PA) to 2PA transition is associated with symmetric to asymmetric linear combination of the respective donor (HOMO -1 to HOMO) or acceptor (LUMO to LUMO + 1) fragment orbitals of the donor or acceptor substituents. We also study the effect of the fraction of Hartree-Fock (HF) exchange on 2PA excitation energies and cross sections. Higher exchange (BMK and M05-2X) and range separated (CAM-B3LYP) hybrid functionals are found to yield rather inaccurate predictions both quantitatively and qualitatively. The results obtained with the long-range corrected functional LC-BLYP do not seem to be useful at all. This failure of the exchange-correlation functionals with the correct asymptotic is traced to inaccurate transition dipoles between the valence states, where only functionals with lower HF exchange succeed. A new sum over states (SOS) cutoff procedure is proposed to compensate for the collapse of the higher-lying excited states obtained with the hybrid functionals. © 2013 American Chemical Society.

Beresnyak A.,Los Alamos National Laboratory
Physical Review Letters | Year: 2011

The spectral slope of strong MHD turbulence has recently been a matter of controversy. While the Goldreich-Sridhar model predicts a -5/3 slope, shallower slopes have been observed in numerics. We argue that earlier numerics were affected by driving due to a diffuse locality of energy transfer. Our highest-resolution simulation (30722×1024) exhibited the asymptotic -5/3 scaling. We also discover that the dynamic alignment, proposed in models with -3/2 slope, saturates and cannot modify the asymptotic, high Reynolds number slope. From the observed -5/3 scaling we measure the Kolmogorov constant C KA=3.27±0.07 for Alfvénic turbulence and C K=4.2±0.2 for full MHD turbulence, which is higher than the hydrodynamic value of 1.64. This larger CK indicates inefficient energy transfer in MHD turbulence, which is in agreement with diffuse locality. © 2011 American Physical Society.

Huang J.,Los Alamos National Laboratory | Nelson A.E.,University of Washington
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

The existence of light sterile neutrinos in the eV mass range with relatively large mixing angles with the active neutrinos has been proposed for a variety of reasons, including to improve the fit to the LSND and MiniBooNE neutrino oscillation experiments, and reactor disappearance experiments. A. E. Nelson showed that neutrino mixing with a heavier sterile neutrino, in the mass range between 33 eV and several GeV, could significantly affect and improve the agreement between neutrino oscillation models with light sterile neutrinos and short baseline experimental results, allowing for a new source of charge parity violation in appearance experiments and for different apparent mixing angles in appearance and disappearance experiments. However E. Kuflik, S. D. McDermott, and K. M. Zurek, and J. Fan and P. Langacker, showed that various collider experiments, supernovae, and cosmological constraints can eliminate most of the parameter region where such a heavy sterile neutrino can have a significant effect on neutrino oscillations. In this paper we consider the effects of allowing a new light scalar in the MeV mass region, which is a potential dark matter candidate, to interact with the sterile neutrinos, and show that the resulting model is a consistent theory of neutrino oscillation anomalies and dark matter which can also potentially explain the INTEGRAL excess of 511 keV gamma rays in the central region of the galaxy. © 2013 American Physical Society.

Guo H.,James Franck Institute | Wulin D.,James Franck Institute | Chien C.-C.,Los Alamos National Laboratory | Levin K.,James Franck Institute
Physical Review Letters | Year: 2011

Recent experiments on the shear viscosity η in a unitary Fermi gas fail to see the theoretically predicted upturn in η at the lower T. In this Letter, we compute η in a fashion which is demonstrably consistent with conservation laws and, in the process, provide an understanding of recent experiments. We show that this disagreement with prior theories cannot be readily attributed to the trap, since (via edge effects) trap-averaged viscosities will be larger than their homogeneous counterparts. The small values of η we find can be simply understood; they reflect the fact that the Goldstone bosons (phonons) do not couple to transverse probes such as η, and fermionic excitations, which determine the viscosity, are necessarily absent in the ground state. © 2011 American Physical Society.

Kang D.,Massachusetts Institute of Technology | Lee C.,Los Alamos National Laboratory | Stewart I.W.,Massachusetts Institute of Technology
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We predict cross sections in deep inelastic scattering (DIS) for the production of two jets - one along the proton beam direction created by initial-state radiation (ISR) and another created by final-state radiation after the hard collision. Our results include fixed-order corrections and a summation of large logarithms up to next-to-next-to-leading logarithmic accuracy in resummed perturbation theory. We make predictions for three versions of a DIS event shape 1-jettiness, each of which constrains hadronic final states to be well collimated into two jets along the beam and final-state jet directions, but which differ in their sensitivity to the transverse momentum of the ISR from the proton beam. We use the tools of soft collinear effective theory to derive factorization theorems for these three versions of 1-jettiness. The sensitivity to the ISR gives rise to significantly different structures in the corresponding factorization theorems - for example, dependence on either the ordinary or the generalized k⊥-dependent beam function. Despite the differences among 1-jettiness definitions, we show that the leading nonperturbative correction that shifts the tail region of their distributions is given by a single universal nonperturbative parameter Ω1, even accounting for hadron mass effects. Finally, we give numerical results for Q2 and x values explored at the HERA collider, emphasizing that the target of our factorization-based analyses is to open the door for higher-precision jet phenomenology in DIS. © 2013 American Physical Society.

Mitri F.G.,Los Alamos National Laboratory
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

A class of Maxwellian beams, which is an exact solution of the vector wave equation (Helmholtz equation) and Maxwell's equations, is introduced. The solution, termed a quasi-Gaussian electromagnetic (EM) beam, is formed from a superposition of sources and sinks with complex coordinates, and is characterized by an arbitrary waist w0 and a diffraction convergence length known as the Rayleigh range zR. An attractive feature of this beam is the description of strongly focused (or strongly divergent) EM-optical wave fields for kw0≤1, where k is the wave number. A vector wave analysis is developed to determine and compute the spatial Cartesian components of the electric and magnetic fields (valid in the near field and the far field) stemming from Maxwell's vector equations and the Lorenz gauge condition, with particular emphasis on the parameter kw0 and the polarization states of the vector potentials used to derive the EM field's components. The results are potentially useful in the study of the axial and/or arbitrary wave scattering, radiation force, and torque in lasers operating with strongly focused (or strongly divergent) beams for particle manipulation in optical tweezers and imaging applications. © 2013 American Physical Society.

Sinitsyn N.A.,Los Alamos National Laboratory
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

We determine transition probabilities in two exactly solvable multistate Landau-Zener (LZ) models and discuss applications of our results to the theory of dynamic passage through a phase transition in the dissipationless quantum mechanical regime. In particular, we show that the statistics of particles in a new phase demonstrate scaling behavior. Our results also reveal a symmetry that we claim is a property of a large class of multistate LZ models, whose explicit solutions are not presently known. We support our arguments by direct numerical simulations. © 2013 American Physical Society.

Chien C.-C.,Los Alamos National Laboratory | Di Ventra M.,University of California at San Diego
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

We show that the recently developed optical lattices with Peierls substitution - which can be modeled as a lattice with a complex tunneling coefficient - may be used to induce controllable quantum transport of ultracold atoms. In particular, we show that by ramping up the phase of the complex tunneling coefficient in a spatially uniform fashion, a finite quasi-steady-state current (QSSC) ensues from the exact dynamics of noninteracting fermions. The direction and magnitude of the current can be controlled by the overall phase difference but not the details of the ramp. The entanglement entropy does not increase when the QSSC lasts. Due to different spin statistics, condensed noninteracting bosons do not support a finite QSSC under the same setup. We also find that an approximate form of the QSSC survives when perturbative effects from interactions, weak harmonic background traps, and finite temperature are present, which suggests that our findings should be observable with available experimental capabilities. © 2013 American Physical Society.

Zurek W.H.,Los Alamos National Laboratory | Zurek W.H.,Santa Fe Institute
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

An unknown quantum state of a single system cannot be discovered, as a measured system is reprepared - it jumps into an eigenstate of the measured observable. This impossibility of finding the quantum state and other symptoms usually blamed on wave-packet collapse follow (as was recently demonstrated for pure states of measured systems) from unitarity (which does not, however, allow for a literal collapse) and from the repeatability of measurements: Continuous unitary evolution and repeatability suffice to establish the discreteness that underlies quantum jumps. Here we consider mixed states of a macroscopic, open system (such as an apparatus), and we allow its microscopic state to change when, e.g., measured by an observer, provided that its salient features remain unchanged and that observers regard macroscopic state of the pointer as representing the same record. We conclude that repeatably accessible states of macroscopic systems (such as the states of the apparatus pointer) must correspond to orthogonal subspaces in the Hilbert space. The symmetry breaking we exhibit defies the egalitarian quantum superposition principle and unitary symmetry of the Hilbert space, as it singles out preferred subspaces. We conclude that the resulting discreteness (which underlies quantum jumps) emerges from the continuity of the core quantum postulates plus repeatability also in macroscopic and open - but ultimately quantum - systems such as measuring devices accessed by observers, where (in contrast with pure states of microsystems) repeatability is paramount. © 2013 American Physical Society.

Olson G.L.,Los Alamos National Laboratory
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2012

Current theories for approximating the effects of stochastic media on radiation transport assume very limited physics such as one dimension, constant grey opacities, and no material energy balance equation. When applied to more complex physical problems, the standard theory fails to match the results from direct numerical simulations. This work presents the first direct numerical simulations of multigroup radiation transport coupled to a material temperature equation in a 2D stochastic medium that are compared to closures proposed by various authors. After extending it from grey to multigroup physics, one closure that is not commonly used successfully models the results in dilute systems where one material comprises less than 5% of the total. This closure is more accurate for related grey transport problems than it is for the multigroup problem. When the specific heats are material- and temperature-dependent, it is much more difficult to fit the direct numerical solutions with an approximate closure. © 2011 Elsevier Ltd.

Bennett J.G.,Los Alamos National Laboratory
International Journal for Numerical Methods in Engineering | Year: 2015

The concept of a 'Representative Volume Model' is used in combination with 'Equivalent Mechanical Strain' or Aboudi's 'Average Strain' theorem to illustrate how a carbon nanotube reinforced composite material constitutive law for a nano-composite material can be implemented into a finite element program for modeling structural applications. Current methods of modeling each individual composite layer to build up an element composed of carbon nanotube reinforced composite material may not be the best approach for modeling structural applications of this composite. The approach presented here is based upon presentations given at the National Science Foundation-Civil and Mechanical Systems division workshop at John Hopkins University in 2004, which is referred to in this paper as the Williams-Baxter approach. This approach is also used to demonstrate that damage modeling can be included as was suggested in this workshop. © 2015 John Wiley & Sons, Ltd.

Guo F.,Los Alamos National Laboratory | Guo F.,University of Arizona | Giacalone J.,University of Arizona
Astrophysical Journal | Year: 2013

We present three-dimensional hybrid simulations of collisionless shocks that propagate parallel to the background magnetic field to study the acceleration of protons that forms a high-energy tail on the distribution. We focus on the initial acceleration of thermal protons and compare it with results from one-dimensional simulations. We find that for both one- and three-dimensional simulations, particles that end up in the high-energy tail of the distribution later in the simulation gained their initial energy right at the shock. This confirms previous results but is the first to demonstrate this using fully three-dimensional fields. The result is not consistent with the "thermal leakage" model. We also show that the gyrocenters of protons in the three-dimensional simulation can drift away from the magnetic field lines on which they started due to the removal of ignorable coordinates that exist in one- and two-dimensional simulations. Our study clarifies the injection problem for diffusive shock acceleration. © 2013. The American Astronomical Society. All rights reserved.

Devereux M.,Los Alamos National Laboratory
Journal of Physics: Conference Series | Year: 2013

Schrödinger's continuous differential equation, which, as far as we know, describes the time development of all physical systems, from microscopic to cosmological, is time-reversal invariant. It is the process of measurement of a system that von Neumann found to be irreversible, and may so account for the distinction between past and future. Developed over the last thirty years, quantum eraser theory has claimed that some quantum measurements are reversible. Holladay proposed a very simple double-slit quantum eraser experiment, hitherto never performed, which he said would refute von Neumann's measurement description and support the quantum erasure thesis. Now, that experiment can actually be implemented with inexpensive, easily accessible equipment. Remarkably, results of the experiment refute quantum eraser theory and confirm von Neumann's measurement reduction process instead.

Birn J.,Space Science Institute | Hesse M.,NASA | Nakamura R.,Austrian Academy of Sciences | Zaharia S.,Los Alamos National Laboratory
Journal of Geophysical Research: Space Physics | Year: 2013

Using the electromagnetic fields of a recent MHD simulation of magnetotail reconnection, flow bursts and dipolarization, we investigate the acceleration of test particles (protons and electrons) to suprathermal energies, confirming and extending earlier results on acceleration mechanisms and sources. (Part of the new results have been reviewed recently in Birn et al., Space Science Reviews, 167, doi:10.1007/ s11214-012-9874-4.) The test particle simulations reproduce major features of energetic particle events (injections) associated with substorms or other dipolarization events, particularly a rapid rise of energetic particle fluxes over limited ranges of energy. The major acceleration mechanisms for electrons are betatron acceleration and Fermi acceleration in the collapsing magnetic field. Ions, although non-adiabatic, undergo similar acceleration. Two major entry mechanisms into the acceleration site are identified: cross-tail drift from the inner tail plasma sheet and reconnection entry from field lines extending to the more distant plasma sheet. The former dominates early in an event and at higher energies (hundreds of keV) while the latter constitutes the main source later and at lower energies (tens of keV). Despite the fact that the injection front moves earthward in the tail, the peak of energetic particle fluxes moves to higher latitude when mapped from the near-Earth boundary to Earth in a static magnetic field model. Key Points Dipolarization electric field is identified as major acceleration mechanismCross-tail drift and reconnection constitute main entries to acceleration siteEarthward and poleward propagation of injections are found to be consistent ©2013. American Geophysical Union. All Rights Reserved.

Black-Schaffer A.M.,Uppsala University | Balatsky A.V.,NORDITA | Balatsky A.V.,Los Alamos National Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We study and classify the proximity-induced superconducting pairing in a topological insulator (TI)-superconductor (SC) hybrid structure for SCs with different symmetries. The Dirac surface state gives a coupling between spin-singlet and spin-triplet pairing amplitudes as well as pairing that is odd in frequency for p-wave SCs. We also find that all SCs induce pairing that is odd in both frequency and orbital (band) index, with oddness in frequency and orbital index being completely interchangeable. The different induced pairing amplitudes significantly modify the density of states in the TI surface layer. © 2013 American Physical Society.

Lin S.-Z.,Los Alamos National Laboratory | Koshelev A.E.,Argonne National Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

The powerful terahertz emission from intrinsic Josephson junctions in high-Tc cuprate superconductors has been detected recently. The synchronization of different junctions is enhanced by excitation of the geometrical cavity resonance. A key characteristic of the radiation is its linewidth. In this work, we study the intrinsic linewidth of the radiation near the internal cavity resonance. Surprisingly, this problem was never considered before, neither for a single Josephson junction nor for a stack of the intrinsic Josephson junctions realized in cuprate superconductors. The linewidth appears due to the slow phase diffusion, which is determined by the dissipation and amplitude of the noise. We found that both these parameters are resonantly enhanced when the cavity mode is excited but enhancement of the dissipation dominates leading to the net suppression of diffusion and dramatic narrowing of the linewidth. The line shape changes from Lorentzian to Gaussian when either the Josephson frequency is shifted away from the resonance or the temperature is increased. © 2013 American Physical Society.

Fomin N.,Los Alamos National Laboratory
AIP Conference Proceedings | Year: 2013

The NPDGamma experiment aims to measure the parity-odd correlation between the neutron spin and the direction of the emitted photon in neutron-proton capture. A parity violating asymmetry (to be measured to 10-8) from this process can be directly related to the strength of the hadronic weak interaction between nucleons, specifically the ΔI=1 contribution. As part of the commissioning runs on the Fundamental Neutron Physics beamline at the Spallation Neutron Source (SNS) at ORNL, the gamma-ray asymmetries from the parity-violating capture of cold neutrons on 35Cl and 27Al were measured, to check for systematic effects, false asymmetries, and backgrounds. Early this year, the parahydrogen target for the production run of NPDGamma was commissioned. Preliminary results for the commissioning measurements with 35Cl and 27Al will be presented as well as first results of the hydrogen run. © 2013 AIP Publishing LLC.

Ginocchio J.N.,Los Alamos National Laboratory
Journal of Physics: Conference Series | Year: 2011

The Dirac Hamiltonian with relativistic scalar and vector harmonic oscillator potentials has been solved analytically in two limits. One is the spin limit for which spin is an invariant symmetry of the Dirac Hamiltonian and the other is the pseudo-spin limit for which pseudo-spin is an invariant symmetry of the the Dirac Hamiltonian. The spin limit occurs when the scalar potential is equal to the vector potential plus a constant, and the pseudospin limit occurs when the scalar potential is equal in magnitude but opposite in sign to the vector potential plus a constant. Like the non-relativistic harmonic oscillator, each of these limits has a higher symmetry. For example, for the spherically symmetric oscillator, these limits have a U(3) and pseudo-U(3) symmetry respectively. We shall discuss the eigenfunctions and eigenvalues of these two limits and derive the relativistic generators for the U(3) and pseudo-U(3) symmetry. We also argue, that, if an anti-nucleon can be bound in a nucleus, the spectrum will have approximate spin and U(3) symmetry. © Published under licence by IOP Publishing Ltd.

Savukov I.M.,Los Alamos National Laboratory
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2016

The precision of the mixed configuration-interaction plus many-body-perturbation-theory (CI+MBPT) method is limited in multivalence atoms by the large size of valence CI space. Previously, to study this problem, the CI+MBPT method was applied to calculations of energies in a four-valence electron atom, Si i. It was found that by using a relatively small cavity of 30 a.u. and by choosing carefully the configuration space, quite accurate agreement between theory and experiment at the level of 100 cm-1 can be obtained, especially after subtraction of systematic shifts for groups of states of the same J and parity. However, other properties are also important to investigate. In this work, the CI+MBPT method is applied to studies of transition probabilities, oscillator strengths, and lifetimes. A close agreement with accurate experimental measurements and other elaborate theories is obtained. The long-term goal is to extend the CI+MBPT approach to applications in more complex atoms, such as lantanides and actinides. © 2016 American Physical Society.

Silver G.L.,Los Alamos National Laboratory
Journal of Radioanalytical and Nuclear Chemistry | Year: 2010

Six discrete formulas are used to estimate the equilibrium constant of the first hydrolysis reaction of tetravalent plutonium. They apply the pH, the oxidation number, one equilibrium constant, and fractions of two of the plutonium oxidation states. The new formulas are not restricted to the equilibrium condition. © 2010 Akadémiai Kiadó, Budapest, Hungary.

Alam U.,Los Alamos National Laboratory
Astrophysical Journal | Year: 2010

In this work, we study a class of early dark energy (EDE) models, in which, unlike in standard dark energy models, a substantial amount of dark energy exists in the matter-dominated era. We self-consistently include dark energy perturbations, and constrain these models using current observations. We consider EDE models in which the dark energy equation of state is at least wm ≳ -0.1 at early times, which could lead to an EDE density of up to . Our analysis shows that marginalizing over the non-DE parameters such as Ωm, H 0, andns , current CMB observations alone can constrain the scale factor of transition from EDE to late-time dark energy to at ≳ 0.44 and width of transition to Δt ≲ 0.37. The equation of state at present is somewhat weakly constrained to w 0 ≲ -0.6, if we allow H 0 < 60kms-1Mpc-1. Taken together with other observations, such as SNe, Hubble Space Telescope, and Sloan Digital Sky Survey luminous red galaxies, w 0 is constrained much more tightly to w 0 ≲ -0.9, while redshift of transition and width of transition are also tightly constrained to at ≲ 0.19 andΔt ≲ 0.21. The evolution of the equation of state for EDE models is thus tightly constrained to ΛCDM-like behavior at low redshifts. Incorrectly assuming dark energy perturbations to be negligible leads to different constraints on the equation of state parameters - w 0 ≲ -0.8, at ≲ 0.33, andΔt ≲ 0.31, thus highlighting the necessity of self-consistently including dark energy perturbations in the analysis. If we allow the spatial curvature to be a free parameter, then the constraints are relaxed to w 0 ≲ -0.77, at ≲ 0.35, andΔt ≲ 0.35 with -0.014 < Ωκ < 0.031 for CMB + other observations. For perturbed EDE models, the 2σ lower limit on σ8 (σ8 ≥ 0.59) is much lower than that in ΛCDM (σ8 ≥ 0.72), thus raising the interesting possibility of discriminating EDE from ΛCDM using future observations such as halo mass functions or the Sunyaev-Zeldovich power spectrum. © 2010 The American Astronomical Society. All rights reserved.

Vecchi L.,Los Alamos National Laboratory
Journal of High Energy Physics | Year: 2011

We analyze the effect of multitrace deformations in conformal field theories at leading order in a large N approximation. These theories admit a description in terms of weakly coupled gravity duals. We show how the deformations can be mapped into boundary terms of the gravity theory and how to reproduce the RG equations found in field theory. In the case of doubletrace deformations, and for bulk scalars with masses in the range ?d2/4 < m2 < ?d2/4 + 1, the deformed theory flows between two fixed points of the renormalization group, manifesting a resonant behavior at the scale characterizing the transition between the two CFT's. On the gravity side the resonance is mapped into an IR non-normalizable mode (Gamow state) whose overlap with the UV region increases as the dual operator approaches the free field limit. We argue that this resonant behavior is a generic property of large N theories in the conformal window, and associate it to a remnant of the Nambu-Goldstone mode of dilatation invariance. We emphasize the role of nonminimal couplings to gravity and establish a stability theorem for scalar/gravity systems with AdS boundary conditions in the presence of arbitrary boundary potentials and nonminimal coupling. © SISSA 2011.

Silver G.L.,Los Alamos National Laboratory
Journal of Radioanalytical and Nuclear Chemistry | Year: 2010

The first hydrolysis constant of tetravalent plutonium can be estimated by two methods that also estimate other solution parameters. The estimates of the hydrolysis constant compare favorably to the traditional values when the ancillary estimates compare favorably to the measurements. © 2010 Akadémiai Kiadó.

Ji H.,Princeton Plasma Physics Laboratory | Daughton W.,Los Alamos National Laboratory
Physics of Plasmas | Year: 2011

Recent progress in understanding the physics of magnetic reconnection is conveniently summarized in terms of a phase diagram which organizes the essential dynamics for a wide variety of applications in heliophysics, laboratory, and astrophysics. The two key dimensionless parameters are the Lundquist number and the macrosopic system size in units of the ion sound gyroradius. In addition to the conventional single X-line collisional and collisionless phases, multiple X-line reconnection phases arise due to the presence of the plasmoid instability either in collisional and collisionless current sheets. In particular, there exists a unique phase termed multiple X-line hybrid phase where a hierarchy of collisional islands or plasmoids is terminated by a collisionless current sheet, resulting in a rapid coupling between the macroscopic and kinetic scales and a mixture of collisional and collisionless dynamics. The new phases involving multiple X-lines and collisionless physics may be important for the emerging applications of magnetic reconnection to accelerate charged particles beyond their thermal speeds. A large number of heliophysical and astrophysical plasmas are surveyed and grouped in the phase diagram: Earth's magnetosphere, solar plasmas (chromosphere, corona, wind, and tachocline), galactic plasmas (molecular clouds, interstellar media, accretion disks and their coronae, Crab nebula, Sgr A, gamma ray bursts, and magnetars), and extragalactic plasmas (active galactic nuclei disks and their coronae, galaxy clusters, radio lobes, and extragalactic jets). Significance of laboratory experiments, including a next generation reconnection experiment, is also discussed. © 2011 American Institute of Physics.

Wendelberger J.R.,Los Alamos National Laboratory
Quality Engineering | Year: 2010

Statistical experiment design can be used to efficiently select experimental runs to achieve a given experimental purpose. However, uncertainty is a fact of life in experimentation. The experimenter is faced with uncertainty in inputs, uncertainty in outputs from both random variability and uncertainty in measurement processes, as well as uncertainty about the underlying model structure of the phenomenon under investigation. In the face of all this uncertainty, the experimenter must try to collect and analyze data that will address questions of scientific interest. Copyright © Taylor & Francis Group, LLC.

Weijer W.,Los Alamos National Laboratory
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2015

We study the barotropic variability in the Southeast Pacific Basin, in particular focusing on the extreme event during the fourth quarter of 2009. A 3-year integration of a barotropic shallow-water model forced with wind stress anomalies generates localized variability that is similar in spatial extent and amplitude as the observed anomalous event. An eigenmode analysis of the same model shows the presence of several free modes in the Southeast Pacific, but projection of the modal patterns on the model output shows that their amplitudes are low. Instead, the mode is interpreted as an almost-free mode. The modal excitation accounts for a considerable fraction (23% on average) of the kinetic energy input by the wind stress in the Southeast Pacific Basin, increasing to 38% for the anomalous event in 2009. Surprisingly, a similar but weaker event during the third quarter of 2008 appears to have been more significant from an energetics point of view, with almost 50% of the energy being input into the mode. Key areas of energetic dissipation appear to be the Eltanin Fracture Zone, the crest of the East Pacific Rise, and the Chile Rise/East Pacific Rise intersection. © 2012 Elsevier Ltd.

Zecevic M.,University of New Hampshire | McCabe R.J.,Los Alamos National Laboratory | Knezevic M.,University of New Hampshire
International Journal of Plasticity | Year: 2015

We present a computationally efficient, multi-scale model based on crystal plasticity theory for simulations of heterogeneous plastic deformation of metallic components in commercial finite element (FE) codes. Although the model can handle a single crystal, the primary purpose with the present model is to embed a meso-scale polycrystal homogenization at a FE integration point, with the meso-scale homogenization being a Taylor-type model. The responses of single crystals are obtained using a recently developed non-iterative solver, which is based on databases of discrete Fourier transforms allowing for fast retrieval of pre-computed crystal plasticity solutions. We have shown that, when this non-iterative solver is used in place of the Newton-Raphson iterative solver, substantial wall-clock speedups can be achieved. Additionally, the implementation presented here takes the advantage of calculations of the elastic properties based on the spectral representation. To calibrate and validate the new FE elasto-viscoplastic model, we use stress-strain curves and texture data of the cobalt-based face-centered cubic superalloy Haynes 25. For this purpose, the material was deformed monotonically in compression over a wider range of strain rates and temperatures. The model is subsequently applied to simulate the macro-scale mechanical response of the material in compression and rolling. We show that the predictions of the model compare favorably with experimental measurements in terms of the mechanical response and texture evolution. Finally, the evolution of the underlying crystallographic texture in rolling predicted by the new model is compared against the corresponding predictions from the finite element visco-plastic self-consistent (FE-VPSC) model. It is observed that both implementations produce similar predictions, but the model presented here is substantially faster. © 2015 Elsevier Ltd.All rights reserved.

Hollis K.J.,Los Alamos National Laboratory
Advanced Materials and Processes | Year: 2010

To transition high neutron-flux research reactors from the currently used highly enriched uranium fuel to the more proliferation-resistant low enriched uranium fuel, a new monolithic type uranium fuel must be developed. A zirconium diffusion barrier between the U-Mo and the aluminum cladding is required to maintain the integrity of the fuel plates during reactor service. Plasma spraying and electrospark deposition have been investigated as possible techniques for depositing Zr on the UMo fuel. Both techniques have shown promise for this application and research is continuing to fully prove their suitability.

Birn J.,Space Science Institute | Birn J.,Los Alamos National Laboratory | Hesse M.,NASA
Journal of Geophysical Research: Space Physics | Year: 2013

Using magnetohydrodynamic (MHD) simulations of magnetotail dynamics, we investigate the build-up and evolution of the substorm current wedge (SCW) and its association with plasma flows from the tail. Three different scenarios are considered: the propagation of magnetic flux ropes of artificially reduced entropy (bubbles), and the formation and propagation of bubbles resulting from magnetic reconnection in the near and far tail. The simulations confirm the important role of the entropy reduction in the earthward penetration of bubbles, as well as in the build-up of field-aligned current signatures attributed to the SCW. Low-entropy flow channels can indeed propagate close to the Earth from the distant tail, as suggested recently. However, this requires substantial entropy reduction, presumably from progression of reconnection into the lobes. The major SCW and pressure build-up occurred when the low-entropy flow channels were braked and the flow diverted azimuthally in the near-Earth region. The flows commonly exhibit multiple narrow channels, separated in space and time, whereas the associated increases in Bz (dipolarization) accumulate over a wider spatial range, spreading both azimuthally and radially. This suggests a picture of the SCW as being composed of multiple smaller "wedgelets," rather than one big wedge. Key Points Substorm current wedge may build up through multiple flow bursts. Strength of wedge currents increases with closer flow approach. Penetration of flows from distant tail requires substantial entropy loss. ©2013. American Geophysical Union. All Rights Reserved.

Taylor S.R.,Rocky Mountain Geophysics, Inc. | Patton H.J.,Los Alamos National Laboratory
Geophysical Research Letters | Year: 2013

Effects of rock damage on teleseismic mb are investigated with P wave synthetic seismograms using a moment dipole Mzz as the equivalent elastic model for damage around buried explosions. Two manifestations of late-time damage, cavity rebound and bulking from block rotations, are represented by model decompositions into compensated linear vector dipole and monopole sources, respectively. For high-velocity media, P waves from damage destructively interfere with those from the explosion. This interference reduces the rate at which mb yield scales for a pure monopole source and provides a physical basis for observed scaling in hard rock, mb~0.75 log [yield]. For over-buried explosions, such as the North Korean tests, P waves from damage are weaker, and higher scaling rates are expected than explosions conducted under standard containment conditions. Our results highlight a cautionary note of transporting the same mb-log[yield] relation between test sites to estimate yield when source phenomenology is likely to be very different. © 2013. American Geophysical Union. All Rights Reserved.

Kilina S.,North Dakota State University | Kilin D.,University of South Dakota | Tretiak S.,Los Alamos National Laboratory
Chemical Reviews | Year: 2015

Rapid advances in chemical synthesis and fabrication techniques have led to a boost in manufacturing and design of novel nanostructured materials that exhibit unique and often unforeseen properties.2 One of the greatest advantages of these nanosystems is the ability to control their electronic and optical properties through the sample size, shape, and topology utilizing a broad variety of organic and inorganic materials. While much is known about the size-dependence of the QD properties due to quantum confinement, considerably less is understood about the effects the surface morphology and passivation layer have on the photoexcited dynamics of QDs. Thus, the QD surface plays a critical role in defining the charge and energy transfer processes that govern light harvesting and conversion of light energy to electric current.

Ecke R.E.,Los Alamos National Laboratory | Niemela J.J.,International Center for Theoretical Physics
Physical Review Letters | Year: 2014

We report experimental measurements of heat transport in rotating Rayleigh-Bénard convection in a cylindrical convection cell with an aspect ratio of Γ=1/2. The fluid is helium gas with a Prandtl number Pr=0.7. The range of control parameters for Rayleigh numbers 4×109

Yu Y.,Los Alamos National Laboratory | Ridley A.J.,University of Michigan
Journal of Geophysical Research: Space Physics | Year: 2013

Heavy ions of ionospheric origin (O+) play an important role in altering global magnetospheric dynamics. While the heavy ions mainly originate from the dayside cusp and the nightside auroral region, the impact of these heavy ions on magnetospheric dynamics has not been differentiated. Controversy also remains on the role of heavy ions on tail stability and their energization mechanism in the magnetosphere. Two global MHD simulations are carried out to investigate the influence of heavy ion outflow from different source regions on reconnection rates, tail stability, and ring current energization. The local reconnection rate at the subsolar point and the total dayside reconnection rate are reduced after the outflow begins, but the decrease is more significant when the outflow comes out of the cusp region. Furthermore, the magnetotail is more disturbed when heavy ions flow out of the dayside cusp region as opposed to the nightside auroral zone. This implies that the role of O+ on tail stability is not definitively positive or negative; instead, the location of the source of heavy ions may be important in determining tail dynamics. Finally, the simulation reveals that the heavy ions originating from the dayside cusp region experience first adiabatic heating while traveling from the tail reconnection site toward the Earth and then further energization caused by flow braking near the outer boundary of the ring current. Key Points reconnection rates reduced more when outflow is originated from the cuspheavy ions energized through two steps after tail reconnectiontail stability depends on where the O+ comes out ©2012. American Geophysical Union. All Rights Reserved.

Liu C.,Los Alamos National Laboratory
Proceedings of the Society for Experimental Mechanics, Inc. | Year: 2010

Brazilian disk compression has been proposed as an alternative for measuring elastic constants of brittle solids with very low tensile strength (Hondros, Aust J Appl Sci 10:243-268, 1959). Subsequently however, the Brazilian disk geometry was mainly used for measuring fracture toughness and tensile strength of brittle materials, like rocks and concretes. In this study, we revisit the Brazilian disk specimen as a tool for determining elastic constants and for observing the deformation process up to failure. We used the optical digital image correlation (DIC) technique to obtain the displacement field on the specimen surface and proposed a scheme for determining the elastic constants from the measured displacement field and the applied load. Details of the elastic constant determination of a homogeneous material, epoxy resin, were presented. Comparison of the elastic constant measured using Brazilian disk with those obtained through more conventional means was carried out. We also present observations of the deformation evolution of the epoxy resin disk subjected to large nonlinear deformation up to failure and subjected to compressive loading and unloading. © Society for Experimental Mechanics 2009.

Silver G.L.,Los Alamos National Laboratory
Journal of Radioanalytical and Nuclear Chemistry | Year: 2010

The determination of the pH of a plutonium solution has traditionally depended on an electrode or a titration in the presence of a complexing agent. A new approach uses the equilibrium distribution of the Pu oxidation states to estimate the hydrogen ion concentration. The method is used to estimate the equilibrium constant of the first hydrolysis reaction of tetravalent plutonium. © 2010 Akadémiai Kiadó, Budapest, Hungary.

Alvermann A.,University of Greifswald | Fehske H.,University of Greifswald | Trugman S.A.,Los Alamos National Laboratory