74 research outputs found
Expansion of a Bose-Einstein Condensate in the Presence of Disorder
Expansion of a Bose-Einstein condensate (BEC) is studied, in the presence of
a random potential. The expansion is controlled by a single parameter,
, where is the chemical potential, prior to the
release of the BEC from the trap, and is a transport relaxation
time which characterizes the strength of the disorder. Repulsive interactions
(nonlinearity) facilitate transport and can lead to diffusive spreading of the
condensate which, in the absence of interactions, would have remained localized
in the vicinity of its initial location
Resonances in one-dimensional Disordered Chain
We study the average density of resonances, is
defined in the complex energy plane and the distance from the real axes
determines the resonance width. We concentrate on strong disorder and derive
the asymptotic behavior of in the limit of small .Comment: latex, 1 eps figure, 9 pages; v2 - final version, published in the
JPhysA Special Issue Dedicated to the Physics of Non-Hermitian Operator
Comment on ``Critical Behavior in Disordered Quantum Systems Modified by Broken Time--Reversal Symmetry''
In a recent Letter [Phys. Rev. Lett. 80, 1003 (1998)] Hussein and Pato
employed the maximum entropy principle (MEP) in order to derive interpolating
ensembles between any pair of universality classes in random matrix theory.
They apply their formalism also to the transition from random matrix to Poisson
statistics of spectra that is observed for the case of the Anderson-type
metal-insulator transition. We point out the problems with the latter
procedure.Comment: 1 page in PS, to appear in PRL Sept. 2
Perturbation Theory for the Rosenzweig-Porter Matrix Model
We study an ensemble of random matrices (the Rosenzweig-Porter model) which,
in contrast to the standard Gaussian ensemble, is not invariant under changes
of basis. We show that a rather complete understanding of its level
correlations can be obtained within the standard framework of diagrammatic
perturbation theory. The structure of the perturbation expansion allows for an
interpretation of the level structure on simple physical grounds, an aspect
that is missing in the exact analysis (T. Guhr, Phys. Rev. Lett. 76, 2258
(1996), T. Guhr and A. M\"uller-Groeling, cond-mat/9702113).Comment: to appear in PRE, 5 pages, REVTeX, 2 figures, postscrip
Statistics of Rare Events in Disordered Conductors
Asymptotic behavior of distribution functions of local quantities in
disordered conductors is studied in the weak disorder limit by means of an
optimal fluctuation method. It is argued that this method is more appropriate
for the study of seldom occurring events than the approaches based on nonlinear
-models because it is capable of correctly handling fluctuations of the
random potential with large amplitude as well as the short-scale structure of
the corresponding solutions of the Schr\"{o}dinger equation. For two- and
three-dimensional conductors new asymptotics of the distribution functions are
obtained which in some cases differ significantly from previously established
results.Comment: 17 pages, REVTeX 3.0 and 1 Postscript figur
Deviations from the Gaussian distribution of mesoscopic conductance fluctuations
The conductance distribution of metallic mesoscopic systems is considered.
The variance of this distribution describes the universal conductance
fluctuations, yielding a Gaussian distribution of the conductance. We calculate
diagrammatically the third cumulant of this distribution, the leading deviation
from the Gaussian. We confirm random matrix theory calculations that the
leading contribution in quasi-one dimension vanishes. However, in quasi two
dimensions the third cumulant is negative, whereas in three dimensions it is
positive.Comment: 9 pages, Revtex, with eps figures,to appear in Phys Rev
Economic Ideas and Institutional Change: Evidence from Soviet Economic Discourse 1987-1991
Correlated Evolution of Nearby Residues in Drosophilid Proteins
Here we investigate the correlations between coding sequence substitutions as a function of their separation along the protein sequence. We consider both substitutions between the reference genomes of several Drosophilids as well as polymorphisms in a population sample of Zimbabwean Drosophila melanogaster. We find that amino acid substitutions are âclusteredâ along the protein sequence, that is, the frequency of additional substitutions is strongly enhanced within â10 residues of a first such substitution. No such clustering is observed for synonymous substitutions, supporting a âcorrelation lengthâ associated with selection on proteins as the causative mechanism. Clustering is stronger between substitutions that arose in the same lineage than it is between substitutions that arose in different lineages. We consider several possible origins of clustering, concluding that epistasis (interactions between amino acids within a protein that affect function) and positional heterogeneity in the strength of purifying selection are primarily responsible. The role of epistasis is directly supported by the tendency of nearby substitutions that arose on the same lineage to preserve the total charge of the residues within the correlation length and by the preferential cosegregation of neighboring derived alleles in our population sample. We interpret the observed length scale of clustering as a statistical reflection of the functional locality (or modularity) of proteins: amino acids that are near each other on the protein backbone are more likely to contribute to, and collaborate toward, a common subfunction
Numerical hydrodynamics in general relativity
The current status of numerical solutions for the equations of ideal general
relativistic hydrodynamics is reviewed. With respect to an earlier version of
the article the present update provides additional information on numerical
schemes and extends the discussion of astrophysical simulations in general
relativistic hydrodynamics. Different formulations of the equations are
presented, with special mention of conservative and hyperbolic formulations
well-adapted to advanced numerical methods. A large sample of available
numerical schemes is discussed, paying particular attention to solution
procedures based on schemes exploiting the characteristic structure of the
equations through linearized Riemann solvers. A comprehensive summary of
astrophysical simulations in strong gravitational fields is presented. These
include gravitational collapse, accretion onto black holes and hydrodynamical
evolutions of neutron stars. The material contained in these sections
highlights the numerical challenges of various representative simulations. It
also follows, to some extent, the chronological development of the field,
concerning advances on the formulation of the gravitational field and
hydrodynamic equations and the numerical methodology designed to solve them.Comment: 105 pages, 12 figures. The full online-readable version of this
article, including several animations, will be published in Living Reviews in
Relativity at http://www.livingreviews.or
- âŠ