Grimvall G.,Albanova University Center |
Magyari-Kope B.,Stanford University |
OzolinENM,University of California at Los Angeles |
Persson K.A.,Lawrence Berkeley National Laboratory
Reviews of Modern Physics | Year: 2012
Most metallic elements have a crystal structure that is either body-centered cubic (bcc), face-centered close packed, or hexagonal close packed. If the bcc lattice is the thermodynamically most stable structure, the close-packed structures usually are dynamically unstable, i.e., have elastic constants violating the Born stability conditions or, more generally, have phonons with imaginary frequencies. Conversely, the bcc lattice tends to be dynamically unstable if the equilibrium structure is close packed. This striking regularity essentially went unnoticed until ab initio total-energy calculations in the 1990s became accurate enough to model dynamical properties of solids in hypothetical lattice structures. After a review of stability criteria, thermodynamic functions in the vicinity of an instability, Bain paths, and how instabilities may arise or disappear when pressure, temperature, and/or chemical composition is varied are discussed. The role of dynamical instabilities in the ideal strength of solids and in metallurgical phase diagrams is then considered, and comments are made on amorphization, melting, and low-dimensional systems. The review concludes with extensive references to theoretical work on the stability properties of metallic elements. © 2012 American Physical Society.
Larsson M.,Albanova University Center |
Geppert W.D.,Albanova University Center |
Nyman G.,Gothenburg University
Reports on Progress in Physics | Year: 2012
We review the gas-phase chemistry in extraterrestrial space that is driven by reactions with atomic and molecular ions. Ions are ubiquitous in space and are potentially responsible for the formation of increasingly complex interstellar molecules. Until recently, positively charged atoms and molecules were the only ions known in space; however, this situation has changed with the discovery of various molecular anions. This review covers not only the observation, distribution and reactions of ions in space, but also laboratory-based experimental and theoretical methods for studying these ions. Recent results from space-based instruments, such as those on the CassiniHuygens space mission and the Herschel Space Observatory, are highlighted. © 2012 IOP Publishing Ltd.
Hermanns M.,Albanova University Center
Physical Review Letters | Year: 2010
A most interesting feature of certain fractional quantum Hall states is that their quasiparticles obey non-Abelian fractional statistics. So far, candidate non-Abelian wave functions have been constructed from conformal blocks in cleverly chosen conformal field theories. In this work we present a hierarchy scheme by which we can construct daughter states by condensing non-Abelian quasiparticles (as opposed to quasiholes) in a parent state, and show that the daughters have a non-Abelian statistics that differs from the parent. In particular, we discuss the daughter of the bosonic, spin-polarized Moore-Read state at ν=4/3 as an explicit example. © 2010 The American Physical Society.
Geppert W.D.,Albanova University Center |
Larsson M.,Albanova University Center
Chemical Reviews | Year: 2013
Ion chemistry also plays a large role in formation of hydrogen, which is crucial for cooling of the denser clumps formed in the primeval clouds, which were formation sites of the first stars in the early universe. Diffuse clouds can condense and further evolve into dark clouds. During this process the chemical composition of these objects changes substantially. Atomic hydrogen is turned into molecular hydrogen by reactions on grain surfaces, and atomic carbon and CO become the main carbon reservoirs. In the outer regions with lower opacity (density) UV photoionization by the interstellar radiation field is the dominating ionization process. Nevertheless, the inhomogeneous, clumpy structure of dark clouds might make it possible for some UV radiation to penetrate into their inner parts via less dense regions.
Brandenburg A.,Albanova University Center
Monthly Notices of the Royal Astronomical Society | Year: 2010
Using two- and three-dimensional hydromagnetic simulations for a range of different flows, including laminar and turbulent ones, it is shown that solutions expressing the field in terms of Euler potentials (EP) are in general incorrect if the EP are evolved with an artificial diffusion term. In three dimensions, standard methods using the magnetic vector potential are found to permit dynamo action when the EP give decaying solutions. With an imposed field, the EP method yields excessive power at small scales. This effect is more exaggerated in the dynamic case, suggesting an unrealistically reduced feedback from the Lorentz force. The EP approach agrees with standard methods only at early times when magnetic diffusivity did not have time to act. It is demonstrated that the usage of EP with even a small artificial magnetic diffusivity does not converge to a proper solution of hydromagnetic turbulence. The source of this disagreement is not connected with magnetic helicity or the three-dimensionality of the magnetic field, but is simply due to the fact that the non-linear representation of the magnetic field in terms of EP that depend on the same coordinates is incompatible with the linear diffusion operator in the induction equation. © 2009 RAS.
Blennow M.,Albanova University Center
Journal of High Energy Physics | Year: 2014
We study the framework of Bayesian statistics for analyzing the capabilities and results of future experiments looking to solve the issue of the neutrino mass ordering. Starting from the general scenario, we then give examples of the procedure for experiments with Gaussian and non-Gaussian distributions for the indicator. We describe in detail what can and cannot be said about the neutrino mass ordering and a future experiment's capabilities to determine it. Finally, we briefly comment on the application to other binary measurements, such as the determination of the octant of θ 23. © 2014 The Author(s).
Siegbahn P.E.M.,Albanova University Center |
Blomberg M.R.A.,Albanova University Center
Chemical Reviews | Year: 2010
In the present review, quantum chemical descriptions of PCET in metalloenzymes have been discussed. The most common type of PCET occurring in enzymes is of the twostep type, where the electron and proton move in separate steps and where the electron and proton have different donors and acceptors. The enzymes discussed here of that type are characterized by the uptake (or release) of electrons and protons from (to) the outside of the enzyme, and this type of long-range electron and proton transfer typically occurs in several steps of PCET. It has been shown how quantum chemical methods can be used to increase the understanding of the mechanisms for these quite complicated types of processes. An important aspect has been how a combination of calculated relative energies and experimental redox potentials can be used to obtain reliable energies for the intermediates. The two enzymes most thoroughly discussed here are cytochrome c oxidase (CcO) and photosystem II (PSII), involving the reduction of molecular oxygen to water, or the reverse reaction of water oxidation with the formation of molecular oxygen. Both these reactions occur in four steps, each comprising the uptake (or release) of one electron and one proton. In the case of CcO, each of the reduction steps is coupled to the translocation of another proton across the entire membrane (in which the enzyme is located). The fact that this pumping occurs without major structural changes or involvement of ATP but is only governed by the electrostatic effects of moving the electrons may make it unique in biology. It was discussed how quantum chemical calculations could be used to elucidate the mechanisms for this complicated coupling of the electron and proton transfer leading to a gating of the protons to the desired place in each step. Another important enzyme discussed is ribonucleotide reductase (RNR), where quantum chemical studies have shown that several different types of PCET occur. In particular, RNR seems to be the most clear case for which hydrogen-atom transfer (HAT) is involved in long-range radical transfer, with the electron and proton being transferred between the same donor and acceptor molecule (two tyrosines or a cysteine and a tyrosine). The QM models used for quantum chemical studies of PCET processes have grown over the past two decades from about 20 atoms to the present 250 atoms. Increasing the size of the models leads to additional difficulties, such as the presence of many local minima, which is at the present stage a major challenge in the modeling. It can be expected that the growing experience with large models will lead to still better treatments of this local minima problem in the future. However, it should be stressed that very large models are not needed, not even desired, for all types of problems. It can be predicted that also in the future the main features of most PCET mechanisms will be elucidated using rather small models. Extended models can then be used to make the mechanisms found with the small models more convincing. © 2010 American Chemical Society.
Ohlsson T.,Albanova University Center
Reports on Progress in Physics | Year: 2013
The phenomenon of neutrino oscillations has been established as the leading mechanism behind neutrino flavor transitions, providing solid experimental evidence that neutrinos are massive and lepton flavors are mixed. Here we review sub-leading effects in neutrino flavor transitions known as non-standard neutrino interactions (NSIs), which is currently the most explored description for effects beyond the standard paradigm of neutrino oscillations. In particular, we report on the phenomenology of NSIs and their experimental and phenomenological bounds as well as an outlook for future sensitivity and discovery reach. © 2013 IOP Publishing Ltd.
Krasnov V.M.,Albanova University Center
Physical Review B - Condensed Matter and Materials Physics | Year: 2011
I propose a new mechanism of intense high-frequency electromagnetic wave generation by spatially uniform stacked Josephson junctions at zero magnetic field. The ac-Josephson effect converts the dc-bias voltage into ac supercurrent; however, in the absence of spatial variation of the Josephson phase difference it does not provide dc-to-ac power conversion, needed for emission of electromagnetic waves. Here I demonstrate that at geometrical resonance conditions, spatial homogeneity of the phase can be spontaneously broken by the appearance of breathers (bound fluxon-antifluxon pairs), facilitating effective dc-to-ac power conversion. This leads to self-oscillations at cavity-mode frequencies, accompanied by the emission of radiation. The proposed mechanism explains all major features of recently observed THz radiation from large-area Bi2Sr2CaCu 2O8+x mesa structures. © 2011 American Physical Society.
Krasnov V.M.,Albanova University Center
Physical Review B - Condensed Matter and Materials Physics | Year: 2010
I derive simple nonlocal dynamic boundary conditions, suitable for modeling of radiation emission from stacked Josephson junctions in an arbitrary dynamic state. Coherent flux-flow emission from intrinsic Josephson junctions in high- Tc superconductors is analyzed. It is shown that due to the lack of Lorentz contraction of fluxons in stacked junctions, high-quality geometrical resonances are prerequisite for high power flux-flow emission from the stack. This leads to a dual role of the radiative impedance: on one hand, small impedance increases the efficiency of emission from the stack, on the other hand, enhanced radiative losses reduce the quality factor of geometrical resonances, which may decrease the total emission power. Optimal conditions are achieved when radiative losses are comparable to resistive losses inside the stack. © 2010 The American Physical Society.