17,854 research outputs found
"The numerical accuracy of truncated Ewald sums for periodic systems with long-range Coulomb interactions"
Ewald summation is widely used to calculate electrostatic interactions in
computer simulations of condensed-matter systems. We present an analysis of the
errors arising from truncating the infinite real- and Fourier-space lattice
sums in the Ewald formulation. We derive an optimal choice for the
Fourier-space cutoff given a screening parameter . We find that the
number of vectors in Fourier space required to achieve a given accuracy scales
with . The proposed method can be used to determine computationally
efficient parameters for Ewald sums, to assess the quality of Ewald-sum
implementations, and to compare different implementations.Comment: 6 pages, 3 figures (Encapsulated PostScript), LaTe
Topological invariants from non-restricted quantum groups
We introduce the notion of a relative spherical category. We prove that such
a category gives rise to the generalized Kashaev and Turaev-Viro-type
3-manifold invariants defined in arXiv:1008.3103 and arXiv:0910.1624,
respectively. In this case we show that these invariants are equal and extend
to what we call a relative Homotopy Quantum Field Theory which is a branch of
the Topological Quantum Field Theory founded by E. Witten and M. Atiyah. Our
main examples of relative spherical categories are the categories of finite
dimensional weight modules over non-restricted quantum groups considered by C.
De Concini, V. Kac, C. Procesi, N. Reshetikhin and M. Rosso. These categories
are not semi-simple and have an infinite number of non-isomorphic irreducible
modules all having vanishing quantum dimensions. We also show that these
categories have associated ribbon categories which gives rise to re-normalized
link invariants. In the case of sl(2) these link invariants are the
Alexander-type multivariable invariants defined by Y. Akutsu, T. Deguchi, and
T. Ohtsuki.Comment: 37 pages, 16 figure
Multiple jumps and vacancy diffusion in a face-centered cubic metal
The diffusion of monovacancies in gold has been studied by computer
simulation. Multiple jumps have been found to play a central role in the atomic
dynamics at high temperature, and have been shown to be responsible for an
upward curvature in the Arrhenius plot of the diffusion coefficient.
Appropriate saddle points on the potential energy surface have been found,
supporting the interpretation of vacancy multiple jumps as distinct migration
mechanisms.Comment: 16 page
Molecular Clouds as the Origin of the Fermi Gamma-Ray GeV-Excess
The so-called "GeV-excess" of the diffuse Galactic gamma-ray emission is
studied with a spectral template fit based on energy spectra. The spectral
templates can be obtained in a data-driven way from the gamma-ray data, which
avoids the use of emissivity models to subtract the standardbackground
processes from the data. Instead, one can determine these backgrounds
simultaneously with any "signals" in any sky direction, including the Galactic
disk and the Galactic center. Using the spectral template fit two hypothesis of
the "GeV-excess" were tested: the dark matter (DM) hypothesis assuming the
excess is caused by DM annihilation and the molecular cloud (MC) hypothesis
assuming the "GeV-excess" is related to a depletion of gamma-rays below 2 GeV,
as is directly observed in the Central Molecular Zone (CMZ). Both hypotheses
provide acceptable fits, if one considers a limited field-of-view centered
within 20 around the Galactic center and applies cuts on the energy
range and/or excludes low latitudes, cuts typically applied by the proponents
of the DM hypothesis. However, if one considers the whole gamma-ray sky and
includes gamma-ray energies up to 100 GeV we find that the MC hypothesis is
preferred over the DM hypothesis for several reasons: i) The MC hypothesis
provides significantly better fits; ii) The morphology of the "GeV-excess"
follows the morphology of the CO-maps, a tracer of MCs, i.e. there exists a
strong "GeV-excess" in the Galactic disk also at large longitudes; iii) The
massive CMZ with a rectangular field-of-view of shows the maximum of the energy flux per log bin in the
diffuse gamma-ray spectrum at 2 GeV, i.e. the "GeV-excess", already in the raw
data without any analysis. The rectangular profile contradicts the spherical
morphology expected for DM annihilation.Comment: 53 pages, 8 figures (+ 42 figures in Appendices), extended version of
arXiv:1610.08926 accepted for publication in PR
First-principles phase diagram calculations for the HfC–TiC, ZrC–TiC, and HfC–ZrC solid solutions
We report first-principles phase diagram calculations for the binary systems HfC–TiC, TiC–ZrC, and HfC–ZrC. Formation energies for superstructures of various bulk compositions were computed with a plane-wave pseudopotential method. They in turn were used as a basis for fitting cluster expansion Hamiltonians, both with and without approximations for excess vibrational free energies. Significant miscibility gaps are predicted for the systems TiC–ZrC and HfC–TiC, with consolute temperatures in excess of 2000 K. The HfC–ZrC system is predicted to be completely miscibile down to 185 K. Reductions in consolute temperature due to excess vibrational free energy are estimated to be ~7%, ~20%, and ~0%, for HfC–TiC, TiC–ZrC, and HfC–ZrC, respectively. Predicted miscibility gaps are symmetric for HfC–ZrC, almost symmetric for HfC–TiC and asymmetric for TiC–ZrC
Broken Time Translation Symmetry as a model for Quantum State Reduction
The symmetries that govern the laws of nature can be spontaneously broken,
enabling the occurrence of ordered states. Crystals arise from the breaking of
translation symmetry, magnets from broken spin rotation symmetry and massive
particles break a phase rotation symmetry. Time translation symmetry can be
spontaneously broken in exactly the same way. The order associated with this
form of spontaneous symmetry breaking is characterised by the emergence of
quantum state reduction: systems which spontaneously break time translation
symmetry act as ideal measurement machines. In this review the breaking of time
translation symmetry is first compared to that of other symmetries such as
spatial translations and rotations. It is then discussed how broken time
translation symmetry gives rise to the process of quantum state reduction and
how it generates a pointer basis, Born's rule, etc. After a comparison between
this model and alternative approaches to the problem of quantum state
reduction, the experimental implications and possible tests of broken time
translation symmetry in realistic experimental settings are discussed.Comment: 15 pages, 5 figure
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