2,582 research outputs found
Driving particle current through narrow channels using classical pump
We study a symmetric exclusion process in which the hopping rates at two
chosen adjacent sites vary periodically in time and have a relative phase
difference. This mimics a colloidal suspension subjected to external space and
time dependent modulation of the diffusion constant. The two special sites act
as a classical pump by generating an oscillatory current with a nonzero value whose direction depends on the applied phase difference. We analyze
various features in this model through simulations and obtain an expression for
the current via a novel perturbative treatment.Comment: Revised versio
Percolation Systems away from the Critical Point
This article reviews some effects of disorder in percolation systems even
away from the critical density p_c. For densities below p_c, the statistics of
large clusters defines the animals problem. Its relation to the directed
animals problem and the Lee-Yang edge singularity problem is described. Rare
compact clusters give rise to Griffiths singuraties in the free energy of
diluted ferromagnets, and lead to a very slow relaxation of magnetization. In
biassed diffusion on percolation clusters, trapping in dead-end branches leads
to asymptotic drift velocity becoming zero for strong bias, and very slow
relaxation of velocity near the critical bias field.Comment: Minor typos fixed. Submitted to Praman
Thermal and Transport Behavior of Single Crystalline R2CoGa8 (R = Gd, Tb, Dy, Ho, Er, Tm, Lu and Y) Compounds
The anisotropy in electrical transport and thermal behavior of single
crystalline RCoGa series of compounds is presented. These compounds
crystallize in a tetragonal structure with space gropup P4/mmm. The nonmagnetic
counterparts of the series namely YCoGa and LuCoGashow
a behavior consistent with the low density of states at the fermi level. In
YCoGa, a possibility of charge density wave transition is observed
at 30 K. GdCoGa and ErCoGa show a presence of
short range correlation above the magnetic ordering temperature of the
compound. In case of GdCoGa, the magnetoresistance exhibits a
significant anisotropy for current parallel to {[}100{]} and {[}001{]}
directions. Compounds with other magnetic rare earths (R = Tb, Dy, Ho and Tm)
show the normal expected magnetic behavior whereas DyCoGa exhibits
the possibility of charge density wave (CDW) transition at approximately same
temperature as that of YCoGa. The thermal property of these
compounds is analysed on the basis of crystalline electric field (CEF)
calculations.Comment: 10 Pages 14 Figures. Submitted to PR
Heat conduction in the disordered harmonic chain revisited
A general formulation is developed to study heat conduction in disordered
harmonic chains with arbitrary heat baths that satisfy the
fluctuation-dissipation theorem. A simple formal expression for the heat
current J is obtained, from which its asymptotic system-size (N) dependence is
extracted. It is shown that the ``thermal conductivity'' depends not just on
the system itself but also on the spectral properties of the fluctuation and
noise used to model the heat baths. As special cases of our heat baths we
recover earlier results which reported that for fixed boundaries , while for free boundaries . For other choices we
find that one can get other power laws including the ``Fourier behaviour'' .Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let
Rolling tachyon solution of two-dimensional string theory
We consider a classical (string) field theory of matrix model which was
developed earlier in hep-th/9207011 and subsequent papers. This is a
noncommutative field theory where the noncommutativity parameter is the string
coupling . We construct a classical solution of this field theory and show
that it describes the complete time history of the recently found rolling
tachyon on an unstable D0 brane.Comment: 19 pages, 2 figures, minor changes in text and additional references,
correction of decay time (version to appear in JHEP.
From Gravitons to Giants
We discuss exact quantization of gravitational fluctuations in the half-BPS
sector around AdSS background, using the dual super Yang-Mills
theory. For this purpose we employ the recently developed techniques for exact
bosonization of a finite number of fermions in terms of bosonic
oscillators. An exact computation of the three-point correlation function of
gravitons for finite shows that they become strongly coupled at
sufficiently high energies, with an interaction that grows exponentially in
. We show that even at such high energies a description of the bulk physics
in terms of weakly interacting particles can be constructed. The single
particle states providing such a description are created by our bosonic
oscillators or equivalently these are the multi-graviton states corresponding
to the so-called Schur polynomials. Both represent single giant graviton states
in the bulk. Multi-particle states corresponding to multi-giant gravitons are,
however, different, since interactions among our bosons vanish identically,
while the Schur polynomials are weakly interacting at high enough energies.Comment: v2-references added, minor changes and typos corrected; 24 pages,
latex, 3 epsf figure
Correlations and scaling in one-dimensional heat conduction
We examine numerically the full spatio-temporal correlation functions for all
hydrodynamic quantities for the random collision model introduced recently. The
autocorrelation function of the heat current, through the Kubo formula, gives a
thermal conductivity exponent of 1/3 in agreement with the analytical
prediction and previous numerical work. Remarkably, this result depends
crucially on the choice of boundary conditions: for periodic boundary
conditions (as opposed to open boundary conditions with heat baths) the
exponent is approximately 1/2. This is expected to be a generic feature of
systems with singular transport coefficients. All primitive hydrodynamic
quantities scale with the dynamic critical exponent predicted analytically.Comment: 7 pages, 11 figure
Explicit characterization of the identity configuration in an Abelian Sandpile Model
Since the work of Creutz, identifying the group identities for the Abelian
Sandpile Model (ASM) on a given lattice is a puzzling issue: on rectangular
portions of Z^2 complex quasi-self-similar structures arise. We study the ASM
on the square lattice, in different geometries, and a variant with directed
edges. Cylinders, through their extra symmetry, allow an easy determination of
the identity, which is a homogeneous function. The directed variant on square
geometry shows a remarkable exact structure, asymptotically self-similar.Comment: 11 pages, 8 figure
Dynamics at a smeared phase transition
We investigate the effects of rare regions on the dynamics of Ising magnets
with planar defects, i.e., disorder perfectly correlated in two dimensions. In
these systems, the magnetic phase transition is smeared because static
long-range order can develop on isolated rare regions. We first study an
infinite-range model by numerically solving local dynamic mean-field equations.
Then we use extremal statistics and scaling arguments to discuss the dynamics
beyond mean-field theory. In the tail region of the smeared transition the
dynamics is even slower than in a conventional Griffiths phase: the spin
autocorrelation function decays like a stretched exponential at intermediate
times before approaching the exponentially small equilibrium value following a
power law at late times.Comment: 10 pages, 8eps figures included, final version as publishe
Solution of a Generalized Stieltjes Problem
We present the exact solution for a set of nonlinear algebraic equations
. These
were encountered by us in a recent study of the low energy spectrum of the
Heisenberg ferromagnetic chain \cite{dhar}. These equations are low
(density) ``degenerations'' of more complicated transcendental equation of
Bethe's Ansatz for a ferromagnet, but are interesting in themselves. They
generalize, through a single parameter, the equations of Stieltjes,
, familiar from Random Matrix theory.
It is shown that the solutions of these set of equations is given by the
zeros of generalized associated Laguerre polynomials. These zeros are
interesting, since they provide one of the few known cases where the location
is along a nontrivial curve in the complex plane that is determined in this
work.
Using a ``Green's function'' and a saddle point technique we determine the
asymptotic distribution of zeros.Comment: 19 pages, 4 figure
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