Oak Ridge, TN, United States

Oak Ridge National Laboratory

Oak Ridge, TN, United States

Oak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the largest science andenergy national laboratory in the Department of Energysystem by acreage. ORNL is located in Oak Ridge, Tennessee, near Knoxville. ORNL's scientific programs focus on materials,neutron science, energy, high-performance computing,systems biology and national security.ORNL partners with the state of Tennessee, universities and industries to solve challenges in energy, advanced materials, manufacturing, security and physics.The laboratory is home to several of the world's top supercomputers including the world's second most powerful supercomputer ranked by the TOP500, Titan, and is a leading neutron science and nuclear energy research facility that includes the Spallation Neutron Source and High Flux Isotope Reactor. ORNL hosts the Center for Nanophase Materials science, the BioEnergy Science Center, and the Consortium for Advanced Simulation of Light-Water Reactors. Wikipedia.

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Dagotto E.,University of Tennessee at Knoxville | Dagotto E.,Oak Ridge National Laboratory
Reviews of Modern Physics | Year: 2013

The iron-based superconductors that contain FeAs layers as the fundamental building block in the crystal structures have been rationalized in the past using ideas based on the Fermi surface nesting of hole and electron pockets when in the presence of weak Hubbard U interactions. This approach seemed appropriate considering the small values of the magnetic moments in the parent compounds and the clear evidence based on photoemission experiments of the required electron and hole pockets. However, recent results in the context of alkali metal iron selenides, with generic chemical composition A xFe2-ySe2 (A=alkali metal element), have challenged those previous ideas since at particular compositions y the low-temperature ground states are insulating and display antiferromagnetic order with large iron magnetic moments. Moreover, angle-resolved photoemission studies have revealed the absence of hole pockets at the Fermi level in these materials. The present status of this exciting area of research, with the potential to alter conceptually our understanding of the iron-based superconductors, is here reviewed, covering both experimental and theoretical investigations. Other recent related developments are also briefly reviewed, such as the study of selenide two-leg ladders and the discovery of superconductivity in a single layer of FeSe. The conceptual issues considered established for the alkali metal iron selenides, as well as several issues that still require further work, are discussed. © 2013 American Physical Society.

Xiao D.,Oak Ridge National Laboratory | Chang M.-C.,National Taiwan Normal University | Niu Q.,University of Texas at Austin
Reviews of Modern Physics | Year: 2010

Ever since its discovery the notion of Berry phase has permeated through all branches of physics. Over the past three decades it was gradually realized that the Berry phase of the electronic wave function can have a profound effect on material properties and is responsible for a spectrum of phenomena, such as polarization, orbital magnetism, various (quantum, anomalous, or spin) Hall effects, and quantum charge pumping. This progress is summarized in a pedagogical manner in this review. A brief summary of necessary background is given and a detailed discussion of the Berry phase effect in a variety of solid-state applications. A common thread of the review is the semiclassical formulation of electron dynamics, which is a versatile tool in the study of electron dynamics in the presence of electromagnetic fields and more general perturbations. Finally, a requantization method is demonstrated that converts a semiclassical theory to an effective quantum theory. It is clear that the Berry phase should be added as an essential ingredient to our understanding of basic material properties. © 2010 The American Physical Society.

Okamoto S.,Oak Ridge National Laboratory
Physical Review Letters | Year: 2013

The electronic properties of Mott insulators realized in (111) bilayers of perovskite transition-metal oxides are studied. The low-energy effective Hamiltonians for such Mott insulators are derived in the presence of a strong spin-orbit coupling. These models are characterized by the antiferromagnetic Heisenberg interaction and the anisotropic interaction whose form depends on the d orbital occupancy. From exact diagonalization analyses on finite clusters, the ground state phase diagrams are derived, including a Kitaev spin liquid phase in a narrow parameter regime for t2g systems. Slave-boson mean-field analyses indicate the possibility of novel superconducting states induced by carrier doping into the Mott-insulating parent systems, suggesting the present model systems as unique playgrounds for studying correlation-induced novel phenomena. Possible experimental realizations are also discussed. © 2013 American Physical Society.

Custelcean R.,Oak Ridge National Laboratory
Chemical Communications | Year: 2013

The persistent ability of tripodal TREN-based tris-urea receptors (TREN = tris(2-aminoethyl)amine) to self-assemble with a variety of oxoanions into dimeric capsules upon crystallization is reviewed. The capsule crystallization allows for charge-, shape-, and size-selective encapsulation of tetrahedral XO4 n- anions (n = 2,3), and provides an effective way to separate these anions from competitive aqueous environments. © 2013 The Royal Society of Chemistry.

Custelcean R.,Oak Ridge National Laboratory
Chemical Society Reviews | Year: 2014

The ability of cationic coordination cages to act as anion receptors is reviewed, with an emphasis on the anion encapsulation chemistry and the dynamics of cage assembly, anion exchange, and other anion-induced structural transformations. The first part of the review describes various examples of anion-encapsulating coordination cages, categorized on the basis of their M xLy stoichiometry (M = metal cation; L = organic ligand). The second part deals with the dynamic aspects of anion encapsulation, including the kinetics and mechanism of anion binding, release, and exchange, as well as the structural evolution of the coordination complexes involved. © The Royal Society of Chemistry.

Hay B.P.,Oak Ridge National Laboratory
Chemical Society Reviews | Year: 2010

This tutorial review presents an account of how de novo structure-based design methods have been used to facilitate the discovery of novel anion receptors formed by combination of hydrogen bond donor groups. Topics include the development of criteria needed for the design, how the input structures for each design were obtained, and subsequent use of molecular modeling to more accurately rank the initial list of candidates produced by the builder. The effectiveness of the design approach is illustrated in several cases where host molecules identified on the computer were subsequently synthesized and shown to function as efficient anion hosts. © 2010 The Royal Society of Chemistry.

Custelcean R.,Oak Ridge National Laboratory
Chemical Society Reviews | Year: 2010

This tutorial review presents a current account of anions in crystal engineering, organized around two main questions: (i) how do anions influence and control crystal structures, and (ii) how do crystal environments recognize and select anions? The first part pertains to deliberate assembly of new crystalline materials using anionic components, by taking advantage of the strong and directional interactions of anions in the solid state. Along this line, the various structural roles of anions in crystals are examined. The second question is related to selective separation of anions by crystallization, by exploiting chemical recognition phenomena in the well-defined and highly structured environment inside crystals. © 2010 The Royal Society of Chemistry.

Jiang D.-E.,Oak Ridge National Laboratory
Nanoscale | Year: 2013

Thiolated gold nanoclusters form a universe of their own. Researchers in this field are constantly pushing the boundary of this universe by identifying new compositions and in a few "lucky" cases, solving their structures. Such solved structures, even if there are only few, provide important hints for predicting the many identified compositions that are yet to be crystallized or structure determined. Structure prediction is the most pressing issue for a computational chemist in this field. The success of the density functional theory method in gauging the energetic ordering of isomers for thiolated gold clusters has been truly remarkable, but to predict the most stable structure for a given composition remains a great challenge. In this feature article from a computational chemist's point of view, the author shows how one understands and predicts structures for thiolated gold nanoclusters based on his old and new results. To further entertain the reader, the author also offers several "imaginative" structures, claims, and challenges for this field. © 2013 The Royal Society of Chemistry.

Sefat A.S.,Oak Ridge National Laboratory
Reports on Progress in Physics | Year: 2011

Insight into the mechanism of high-temperature superconductivity can be gained by pressure-dependent studies of structural, thermodynamics and transport data. The role of pressure may be complicated by the level of hydrostaticity. High-pressure studies on two iron-based families of RFeAsO (R = rare-earth metals) and AFe2As2 (A = alkaline-earth metals) are reviewed here. © 2011 IOP Publishing Ltd.

Singh D.J.,Oak Ridge National Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The thermopower of PbTe as a function of temperature and doping level is reported based on Boltzmann transport calculations using the first principles relativistic electronic structure as obtained with the Engel-Vosko generalized gradient approximation. The results are discussed in relation to experimental data. For p -type material there is an enhancement at high-doping levels due to the onset of an increased density of states starting ∼0.2 eV below the valence band edge. This leads to agreement between the calculated thermopower and recent results on PbTe with heavy Tl doping. © 2010 The American Physical Society.

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