5,751 research outputs found
Simulation of inhomogeneous distributions of ultracold atoms in an optical lattice via a massively parallel implementation of nonequilibrium strong-coupling perturbation theory
We present a nonequilibrium strong-coupling approach to inhomogeneous systems
of ultracold atoms in optical lattices. We demonstrate its application to the
Mott-insulating phase of a two-dimensional Fermi-Hubbard model in the presence
of a trap potential. Since the theory is formulated self-consistently, the
numerical implementation relies on a massively parallel evaluation of the
self-energy and the Green's function at each lattice site, employing thousands
of CPUs. While the computation of the self-energy is straightforward to
parallelize, the evaluation of the Green's function requires the inversion of a
large sparse matrix, with . As a crucial ingredient,
our solution heavily relies on the smallness of the hopping as compared to the
interaction strength and yields a widely scalable realization of a rapidly
converging iterative algorithm which evaluates all elements of the Green's
function. Results are validated by comparing with the homogeneous case via the
local-density approximation. These calculations also show that the
local-density approximation is valid in non-equilibrium setups without mass
transport.Comment: 14 pages, 9 figure
The Exotic Barium Bismuthates
We review the remarkable properties, including superconductivity,
charge-density-wave ordering, and metal-insulator transitions, of lead- and
potassium-doped barium bismuthate. We discuss some of the early theoretical
studies of these systems. Our recent theoretical work, on the negative-U\/,
extended-Hubbard model for these systems, is also described. Both the large-
and intermediate-U\/ regimes of this model are examined, using mean-field and
random-phase approximations, particularly with a view to fitting various
experimental properties of these bismuthates. On the basis of our studies, we
point out possibilities for exotic physics in these systems. We also emphasize
the different consequences of electronic and phonon-mediated mechanisms for the
negative U.\/ We show that, for an electronic mechanism, the \secin
\,\,phases of these bismuthates must be unique, with their transport properties
{\it dominated by charge Cooperon bound states}. This can explain the
observed difference between the optical and transport gaps. We propose other
experimental tests for this novel mechanism of charge transport and comment on
the effects of disorder.Comment: UUencoded LaTex file, 122 pages, figures available on request To
appear in Int. J. Mod. Phys. B as a review articl
Spectral properties in the charge density wave phase of the half-filled Falicov-Kimball Model
We study the spectral properties of charge density wave (CDW) phase of the
half-filled spinless Falicov-Kimball model within the framework of the
Dynamical Mean Field Theory. We present detailed results for the spectral
function in the CDW phase as function of temperature and . We show how the
proximity of the non-fermi liquid phase affects the CDW phase, and show that
there is a region in the phase diagram where we get a CDW phase without a gap
in the spectral function. This is a radical deviation from the mean-field
prediction where the gap is proportional to the order parameter
Equilibrium glassy phase in a polydisperse hard sphere system
The phase diagram of a polydisperse hard sphere system is examined by
numerical minimization of a discretized form of the Ramakrishnan-Yussouff free
energy functional. Crystalline and glassy local minima of the free energy are
located and the phase diagram in the density-polydispersity plane is mapped out
by comparing the free energies of different local minima. The crystalline phase
disappears and the glass becomes the equilibrium phase beyond a "terminal"
value of the polydispersity. A crystal to glass transition is also observed as
the density is increased at high polydispersity. The phase diagram obtained in
our study is qualitatively similar to that of hard spheres in a quenched random
potential.Comment: 4 pages, 4 figure
Laser induced reentrant freezing in two-dimensional attractive colloidal systems
The effects of an externally applied one-dimensional periodic potential on
the freezing/melting behaviour of two-dimensional systems of colloidal
particles with a short-range attractive interaction are studied using Monte
Carlo simulations. In such systems, incommensuration results when the
periodicity of the external potential does not match the length-scale at which
the minimum of the attractive potential occurs. To study the effects of this
incommensuration, we consider two different models for the system. Our
simulations for both these models show the phenomenon of reentrant freezing as
the strength of the periodic potential is varied. Our simulations also show
that different exotic phases can form when the strength of the periodic
potential is high, depending on the length-scale at which the minimum of the
attractive pair-potential occurs.Comment: 24 pages (including figures) in preprint forma
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