272 research outputs found
A local Fock-exchange potential in Kohn–Sham equations
We derive and employ a local potential to represent the Fock exchange operator in electronic single-particle equations. This local Fock-exchange (LFX) potential is very similar to the exact exchange (EXX) potential in density functional theory (DFT). The practical software implementation of the two potentials (LFX and EXX) yields robust and accurate results for a variety of systems (semiconductors, transition metal oxides) where Hartree–Fock and popular approximations of DFT typically fail. This includes examples traditionally considered qualitatively inaccessible to calculations that omit correlation
Electronic, Structural, and Mechanical Properties of SiO2 Glass at High Pressure Inferred from its Refractive Index
Topological triplon modes and bound states in a Shastry–Sutherland magnet
The twin discoveries of the quantum Hall effect, in the 1980's, and of
topological band insulators, in the 2000's, were landmarks in physics that
enriched our view of the electronic properties of solids. In a nutshell, these
discoveries have taught us that quantum mechanical wavefunctions in crystalline
solids may carry nontrivial topological invariants which have ramifications for
the observable physics. One of the side effects of the recent topological
insulator revolution has been that such physics is much more widespread than
was appreciated ten years ago. For example, while topological insulators were
originally studied in the context of electron wavefunctions, recent work has
led to proposals of topological insulators in bosonic systems: in photonic
crystals, in the vibrational modes of crystals, and in the excitations of
ordered magnets. Here we confirm the recent proposal that, in a weak magnetic
field, the dimerized quantum magnet SrCu(BO) is a bosonic
topological insulator with nonzero Chern number in the triplon bands and
topologically protected chiral edge excitations.Comment: 33 pages, 14 figures (preprint format and including supplementary
material
The MOLDY short-range molecular dynamics package
We describe a parallelised version of the MOLDY molecular dynamics program.
This Fortran code is aimed at systems which may be described by short-range
potentials and specifically those which may be addressed with the embedded atom
method. This includes a wide range of transition metals and alloys. MOLDY
provides a range of options in terms of the molecular dynamics ensemble used
and the boundary conditions which may be applied. A number of standard
potentials are provided, and the modular structure of the code allows new
potentials to be added easily. The code is parallelised using OpenMP and can
therefore be run on shared memory systems, including modern multicore
processors. Particular attention is paid to the updates required in the main
force loop, where synchronisation is often required in OpenMP implementations
of molecular dynamics. We examine the performance of the parallel code in
detail and give some examples of applications to realistic problems, including
the dynamic compression of copper and carbon migration in an iron-carbon alloy
Spin singlet formation in MgTi2O4: Evidence of a helical dimerization pattern
Published versio
Structure and spectroscopy of CuH prepared via borohydride reduction
Copper(I) hydride (cuprous hydride, CuH) was the first binary metal hydride to be discovered (in 1844) and is singular in that it is synthesized in solution, at ambient temperature. There are several synthetic paths to CuH, one of which involves reduction of an aqueous solution of CuSO(4)·5H(2)O by borohydride ions. The product from this procedure has not been extensively characterized. Using a combination of diffraction methods (X-ray and neutron) and inelastic neutron scattering spectroscopy, we show that the CuH from the borohydride route has the same bulk structure as CuH produced by other routes. Our work shows that the product consists of a core of CuH with a shell of water and that this may be largely replaced by ethanol. This offers the possibility of modifying the properties of CuH produced by aqueous routes
Quantum delocalization of molecular hydrogen in alkali-graphite intercalates
The adsorption of molecular hydrogen (H-2) in the graphite intercalation compound KC24 is studied both experimentally and theoretically. High-resolution inelastic neutron data show spectral features consistent with a strong pinning of H-2 along a single axis. First-principles calculations provide novel insight into the nature of H-2 binding in intercalates but fail to account for the symmetry of the H-2 orientational potential deduced from experiment. The above discrepancy disappears once the H-2 center of mass is allowed to delocalize in the quantum-mechanical sense across three vicinal adsorption sites, naturally leading to the well-known saturation coverage of similar to 2H(2) per metal atom in this material. Our results demonstrate that H-2 storage in metal-doped carbon substrates can be severely affected by hitherto unexplored quantum-mechanical effects
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