102 research outputs found
Plane waves in noncommutative fluids
We study the dynamics of the noncommutative fuid in the Snyder space
perturbatively at the first order in powers of the noncommutative parameter.
The linearized noncommutative fluid dynamics is described by a system of
coupled linear partial differential equations in which the variables are the
fluid density and the fluid potentials. We show that these equations admit a
set of solutions that are monocromatic plane waves for the fluid density and
two of the potentials and a linear function for the third potential. The
energy-momentum tensor of the plane waves is calculated.Comment: 11 pages. Version published as a Lette
Fluidos em Espaços Não-Comutativos.
Neste trabalho apresentaremos os resultados da nossa pesquisa sobre a generalização dos fluidos relativísticos na parametrização de Kähler para espaços não-comutativos aplicando métodos de teoria de campo. A nossa proposta se aplica a uma grande classe de fluidos parame-trizados por duas funções arbitrárias suaves: a primeira que generaliza o potencial de Kähler de-nido na superfície dos potenciais complexos do
fluido e a segunda que parametriza a equação de estado. Determinamos, também, os vínculos que os graus de liberdade dos
fluidos devem satisfazer para que a teoria tenha as simetrias funda-mentais da geometria não-comutativa e calcularemos as grandezas físicas do fluido não-comutativo
Noncommutative fluid dynamics in the K\"{a}hler parametrization
In this paper, we propose a first order action functional for a large class
of systems that generalize the relativistic perfect fluids in the K\"{a}hler
parametrization to noncommutative spacetimes. We calculate the equations of
motion for the fluid potentials and the energy-momentum tensor in the first
order in the noncommutative parameter. The density current does not receive any
noncommutative corrections and it is conserved under the action of the
commutative generators but the energy-momentum tensor is not.
Therefore, we determine the set of constraints under which the energy-momentum
tensor is divergenceless. Another set of constraints on the fluid potentials is
obtained from the requirement of the invariance of the action under the
generalization of the volume preserving transformations of the noncommutative
spacetime. We show that the proposed action describes noncommutative fluid
models by casting the energy-momentum tensor in the familiar fluid form and
identifying the corresponding energy and momentum densities. In the commutative
limit, they are identical to the corresponding quantities of the relativistic
perfect fluids. The energy-momentum tensor contains a dissipative term that is
due to the noncommutative spacetime and vanishes in the commutative limit.
Finally, we particularize the theory to the case when the complex fluid
potentials are characterized by a function that is a deformation
of the complex plane and show that this model has important common features
with the commutative fluid such as infinitely many conserved currents and a
conserved axial current that in the commutative case is associated to the
topologically conserved linking number.Comment: References fixed. Typos corrected. 12 page
Noncommutative fluid dynamics in the Snyder space-time
In this paper, we construct for the first time the non-commutative fluid with
the deformed Poincare invariance. To this end, the realization formalism of the
noncommutative spaces is employed and the results are particularized to the
Snyder space. The non-commutative fluid generalizes the fluid model in the
action functional formulation to the noncommutative space. The fluid equations
of motion and the conserved energy-momentum tensor are obtained.Comment: 12 pages. Version published by Phys. Rev.
Quantum fluids in the Kaehler parametrization
In this paper we address the problem of the quantization of the perfect
relativistic fluids formulated in terms of the K\"{a}hler parametrization. This
fluid model describes a large set of interesting systems such as the power law
energy density fluids, Chaplygin gas, etc. In order to maintain the generality
of the model, we apply the BRST method in the reduced phase space in which the
fluid degrees of freedom are just the fluid potentials and the fluid current is
classically resolved in terms of them. We determine the physical states in this
setting, the time evolution and the path integral formulation.Comment: 12 pages. Minor typos correcte
The Structure, Dynamics and Electronic Structure of Liquid Ag-Se Alloys Investigated by Ab Initio Simulation
Ab initio molecular-dynamics simulations have been used to investigate the
structure, dynamics and electronic properties of the liquid alloy Ag(1-x)Se(x)
at 1350 K and at the three compositions x=0.33, 0.42 and 0.65. The calculations
are based on density-functional theory in the local density approximation and
on the pseudopotential plane-wave method. The reliability of the simulations is
confirmed by detailed comparisons with very recent neutron diffraction results
for the partial structure factors and radial distribution functions (RDF) of
the stoichiometric liquid Ag2Se. The simulations show a dramatic change of the
Se-Se RDF with increasing Se content. This change is due to the formation of Se
clusters bound by covalent bonds, the Se-Se bond length being almost the same
as in pure c-Se and l-Se. The clusters are predominantly chain-like, but for
higher x a large fraction of 3-fold coordinated Se atoms is also found. It is
shown that the equilibrium fractions of Se present as isolated atoms and in
clusters can be understood on a simple charge-balance model based on an ionic
interpretation. The Ag and Se diffusion coefficients both increase with Se
content, in spite of the Se clustering. An analysis of the Se-Se bond dynamics
reveals surprisingly short bond lifetimes of less than 1 ps. The changes in the
density of states with composition arise directly from the formation of Se-Se
covalent bonds. Results for the electronic conductivity obtained using the
Kubo-Greenwood approximation are in adequate agreement with experiment for
l-Ag2Se, but not for the high Se contents. Possible reasons for this are
discussed.Comment: 14 pages, Revtex, 14 Postscript figures embedded in the tex
Ensemble density-functional theory for ab-initio molecular dynamics of metals and finite-temperature insulators
A new method is presented for performing first-principles molecular-dynamics
simulations of systems with variable occupancies. We adopt a matrix
representation for the one-particle statistical operator Gamma, to introduce a
``projected'' free energy functional G that depends on the Kohn-Sham orbitals
only and that is invariant under their unitary transformations. The Liouville
equation [ Gamma , H ] = 0 is always satisfied, guaranteeing a very efficient
and stable variational minimization algorithm that can be extended to
non-conventional entropic formulations or fictitious thermal distributions.Comment: 5 pages, two-column style with 2 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#nm_meta
Reverse Monte Carlo modeling of amorphous silicon
An implementation of the Reverse Monte Carlo algorithm is presented for the
study of amorphous tetrahedral semiconductors. By taking into account a number
of constraints that describe the tetrahedral bonding geometry along with the
radial distribution function, we construct a model of amorphous silicon using
the reverse monte carlo technique. Starting from a completely random
configuration, we generate a model of amorphous silicon containing 500 atoms
closely reproducing the experimental static structure factor and bond angle
distribution and in improved agreement with electronic properties. Comparison
is made to existing Reverse Monte Carlo models, and the importance of suitable
constraints beside experimental data is stressed.Comment: 6 pages, 4 PostScript figure
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