397 research outputs found
Assessing the Performance of Recent Density Functionals for Bulk Solids
We assess the performance of recent density functionals for the
exchange-correlation energy of a nonmolecular solid, by applying accurate
calculations with the GAUSSIAN, BAND, and VASP codes to a test set of 24 solid
metals and non-metals. The functionals tested are the modified
Perdew-Burke-Ernzerhof generalized gradient approximation (PBEsol GGA), the
second-order GGA (SOGGA), and the Armiento-Mattsson 2005 (AM05) GGA. For
completeness, we also test more-standard functionals: the local density
approximation, the original PBE GGA, and the Tao-Perdew-Staroverov-Scuseria
(TPSS) meta-GGA. We find that the recent density functionals for solids reach a
high accuracy for bulk properties (lattice constant and bulk modulus). For the
cohesive energy, PBE is better than PBEsol overall, as expected, but PBEsol is
actually better for the alkali metals and alkali halides. For fair comparison
of calculated and experimental results, we consider the zero-point phonon and
finite-temperature effects ignored by many workers. We show how Gaussian basis
sets and inaccurate experimental reference data may affect the rating of the
quality of the functionals. The results show that PBEsol and AM05 perform
somewhat differently from each other for alkali metal, alkaline earth metal and
alkali halide crystals (where the maximum value of the reduced density gradient
is about 2), but perform very similarly for most of the other solids (where it
is often about 1). Our explanation for this is consistent with the importance
of exchange-correlation nonlocality in regions of core-valence overlap.Comment: 32 pages, single pdf fil
Coil optimization for electromagnetic levitation using a genetic like algorithm
he technique of electromagnetic levitation (EML) provides a means for thermally processing an electrically conductive specimen in a containerless manner. For the investigation of metallicliquids and related melting or freezing transformations, the elimination of substrate-induced nucleation affords access to much higher undercooling than otherwise attainable. With heating and levitation both arising from the currents induced by the coil, the performance of any EML system depends on controlling the balance between lifting forces and heating effects, as influenced by the levitation coil geometry. In this work, a genetic algorithm is developed and utilized to optimize the design of electromagnetic levitation coils. The optimization is targeted specifically to reduce the steady-state temperature of the stably levitated metallic specimen. Reductions in temperature of nominally 70 K relative to that obtained with the initial design are achieved through coil optimization, and the results are compared with experiments foraluminum. Additionally, the optimization method is shown to be robust, generating a small range of converged results from a variety of initial starting conditions. While our optimizationcriterion was set to achieve the lowest possible sample temperature, the method is general and can be used to optimize for other criteria as well
Force-matched embedded-atom method potential for niobium
Large-scale simulations of plastic deformation and phase transformations in
alloys require reliable classical interatomic potentials. We construct an
embedded-atom method potential for niobium as the first step in alloy potential
development. Optimization of the potential parameters to a well-converged set
of density-functional theory (DFT) forces, energies, and stresses produces a
reliable and transferable potential for molecular dynamics simulations. The
potential accurately describes properties related to the fitting data, and also
produces excellent results for quantities outside the fitting range. Structural
and elastic properties, defect energetics, and thermal behavior compare well
with DFT results and experimental data, e.g., DFT surface energies are
reproduced with less than 4% error, generalized stacking-fault energies differ
from DFT values by less than 15%, and the melting temperature is within 2% of
the experimental value.Comment: 17 pages, 13 figures, 7 table
Thermoelastic dissipation in inhomogeneous media: loss measurements and displacement noise in coated test masses for interferometric gravitational wave detectors
The displacement noise in the test mass mirrors of interferometric
gravitational wave detectors is proportional to their elastic dissipation at
the observation frequencies. In this paper, we analyze one fundamental source
of dissipation in thin coatings, thermoelastic damping associated with the
dissimilar thermal and elastic properties of the film and the substrate. We
obtain expressions for the thermoelastic dissipation factor necessary to
interpret resonant loss measurements, and for the spectral density of
displacement noise imposed on a Gaussian beam reflected from the face of a
coated mass. The predicted size of these effects is large enough to affect the
interpretation of loss measurements, and to influence design choices in
advanced gravitational wave detectors.Comment: 42 pages, 7 figures, uses REVTeX
Thermodynamic properties and structural stability of thorium dioxide
Using density functional theory (DFT) calculations, we have systematically
investigated the thermodynamic properties and structural stabilities of thorium
dioxide (ThO). Based on the calculated phonon dispersion curves, we
calculate the thermal expansion coefficient, bulk modulus, and heat capacities
at different temperatures for ThO under the quasi-harmonic approximation.
All the results are in good agreement with corresponding experiments proving
the validity of our methods. Our theoretical studies can help people more
clearly understand the thermodynamic behaviors of ThO at different
temperatures. In addition, we have also studied possible defect formations and
diffusion behaviors of helium in ThO, to discuss its structural stability.
It is found that in intrinsic ThO without any Fermi energy shifts, the
interstitial Th defect other than oxygen or thorium vacancies,
interstitial oxygen, and any kinds of Frenkel pairs, is most probable to form
with an energy release of 1.74 eV. However, after upshifting the Fermi energy,
the formation of the other defects also becomes possible. For helium diffusion,
we find that only through the thorium vacancy can it happen with the small
energy barrier of 0.52 eV. Otherwise, helium atoms can hardly incorporate or
diffuse in ThO. Our results indicate that people should prevent upshifts of
the Fermi energy of ThO to avoid the formation of thorium vacancies and so
as to prevent helium caused damages.Comment: 11 pages, 11 figure
Dynamical properties of Au from tight-binding molecular-dynamics simulations
We studied the dynamical properties of Au using our previously developed
tight-binding method. Phonon-dispersion and density-of-states curves at T=0 K
were determined by computing the dynamical-matrix using a supercell approach.
In addition, we performed molecular-dynamics simulations at various
temperatures to obtain the temperature dependence of the lattice constant and
of the atomic mean-square-displacement, as well as the phonon density-of-states
and phonon-dispersion curves at finite temperature. We further tested the
transferability of the model to different atomic environments by simulating
liquid gold. Whenever possible we compared these results to experimental
values.Comment: 7 pages, 9 encapsulated Postscript figures, submitted to Physical
Review
Melting of tantalum at high pressure determined by angle dispersive x-ray diffraction in a double-sided laser-heated diamond-anvil cell
The high pressure and high temperature phase diagram of Ta has been studied
in a laser-heated diamond-anvil cell (DAC) using x-ray diffraction measurements
up to 52 GPa and 3800 K. The melting was observed at nine different pressures,
being the melting temperature in good agreement with previous laser-heated DAC
experiments, but in contradiction with several theoretical calculations and
previous piston-cylinder apparatus experiments. A small slope for the melting
curve of Ta is estimated (dTm/dP = 24 K/GPa at 1 bar) and a possible
explanation for this behaviour is given. Finally, a P-V-T equation of states is
obtained, being the temperature dependence of the thermal expansion coefficient
and the bulk modulus estimated.Comment: 31 pages, 8 figures, to appear in J.Phys.:Cond.Matte
Self-diffusion of adatoms, dimers, and vacancies on Cu(100)
We use ab initio static relaxation methods and semi-empirical
molecular-dynamics simulations to investigate the energetics and dynamics of
the diffusion of adatoms, dimers, and vacancies on Cu(100). It is found that
the dynamical energy barriers for diffusion are well approximated by the
static, 0 K barriers and that prefactors do not depend sensitively on the
species undergoing diffusion. The ab initio barriers are observed to be
significantly lower when calculated within the generalized-gradient
approximation (GGA) rather than in the local-density approximation (LDA). Our
calculations predict that surface diffusion should proceed primarily via the
diffusion of vacancies. Adatoms are found to migrate most easily via a jump
mechanism. This is the case, also, of dimers, even though the corresponding
barrier is slightly larger than it is for adatoms. We observe, further, that
dimers diffuse more readily than they can dissociate. Our results are discussed
in the context of recent submonolayer growth experiments of Cu(100).Comment: Submitted to the Physical Review B; 15 pages including postscript
figures; see also http://www.centrcn.umontreal.ca/~lewi
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