80 research outputs found
A Photoreceptor Model that Replicates Human Light Adaptation Characteristics
Whitehall (593-24); Office of Naval Research (N00014-95-1-0409); Universidad Nacional Autonoma de Mexico (Graduate Fellowship
Heat Transport in Quantum Spin Chains: Stochastic Baths vs Quantum Trajectories
We discuss the problem of heat conduction in quantum spin chain models. To
investigate this problem it is necessary to consider the finite open system
connected to heat baths. We describe two different procedures to couple the
system with the reservoirs: a model of stochastic heat baths and the quantum
trajectories solution of the quantum master equation. The stochastic heat bath
procedure operates on the pure wave function of the isolated system, so that it
is locally and periodically collapsed to a quantum state consistent with a
boundary nonequilibrium state. In contrast, the quantum trajectories procedure
evaluates ensemble averages in terms of the reduced density matrix operator of
the system. We apply these procedures to different models of quantum spin
chains and numerically show their applicability to study the heat flow.Comment: 13 pages, 5 figures, submitted to European Physics Journal Special
Topic
Magnetically Induced Thermal Rectification
We consider far from equilibrium heat transport in chaotic billiard chains
with non-interacting charged particles in the presence of non-uniform
transverse magnetic field. If half of the chain is placed in a strong magnetic
field, or if the strength of the magnetic field has a large gradient along the
chain, heat current is shown to be asymmetric with respect to exchange of the
temperatures of the heat baths. Thermal rectification factor can be arbitrarily
large for sufficiently small temperature of one of the baths.Comment: 4 pages, 5 figure
Transport properties of a modified Lorentz gas
We present a detailed study of the first simple mechanical system that shows
fully realistic transport behavior while still being exactly solvable at the
level of equilibrium statistical mechanics. The system under consideration is a
Lorentz gas with fixed freely-rotating circular scatterers interacting with
point particles via perfectly rough collisions. Upon imposing a temperature
and/or a chemical potential gradient, a stationary state is attained for which
local thermal equilibrium holds for low values of the imposed gradients.
Transport in this system is normal, in the sense that the transport
coefficients which characterize the flow of heat and matter are finite in the
thermodynamic limit. Moreover, the two flows are non-trivially coupled,
satisfying Onsager's reciprocity relations to within numerical accuracy as well
as the Green-Kubo relations . We further show numerically that an applied
electric field causes the same currents as the corresponding chemical potential
gradient in first order of the applied field. Puzzling discrepancies in higher
order effects (Joule heating) are also observed. Finally, the role of entropy
production in this purely Hamiltonian system is shortly discussed.Comment: 16 pages, 16 figures, submitted to J. Stat. Phy
First passages for a search by a swarm of independent random searchers
In this paper we study some aspects of search for an immobile target by a
swarm of N non-communicating, randomly moving searchers (numbered by the index
k, k = 1, 2,..., N), which all start their random motion simultaneously at the
same point in space. For each realization of the search process, we record the
unordered set of time moments \{\tau_k\}, where \tau_k is the time of the first
passage of the k-th searcher to the location of the target. Clearly, \tau_k's
are independent, identically distributed random variables with the same
distribution function \Psi(\tau). We evaluate then the distribution P(\omega)
of the random variable \omega \sim \tau_1/bar{\tau}, where bar{\tau} = N^{-1}
\sum_{k=1}^N \tau_k is the ensemble-averaged realization-dependent first
passage time. We show that P(\omega) exhibits quite a non-trivial and sometimes
a counterintuitive behaviour. We demonstrate that in some well-studied cases
e.g., Brownian motion in finite d-dimensional domains) the \textit{mean} first
passage time is not a robust measure of the search efficiency, despite the fact
that \Psi(\tau) has moments of arbitrary order. This implies, in particular,
that even in this simplest case (not saying about complex systems and/or
anomalous diffusion) first passage data extracted from a single particle
tracking should be regarded with an appropriate caution because of the
significant sample-to-sample fluctuations.Comment: 35 pages, 18 figures, to appear in JSTA
Nonequlibrium particle and energy currents in quantum chains connected to mesoscopic Fermi reservoirs
We propose a model of nonequilibrium quantum transport of particles and
energy in a system connected to mesoscopic Fermi reservoirs (meso-reservoir).
The meso-reservoirs are in turn thermalized to prescribed temperatures and
chemical potentials by a simple dissipative mechanism described by the Lindblad
equation. As an example, we study transport in monoatomic and diatomic chains
of non-interacting spinless fermions. We show numerically the breakdown of the
Onsager reciprocity relation due to the dissipative terms of the model.Comment: 5pages, 4 figure
Entanglement Across a Transition to Quantum Chaos
We study the relation between entanglement and quantum chaos in one- and
two-dimensional spin-1/2 lattice models, which exhibit mixing of the
noninteracting eigenfunctions and transition from integrability to quantum
chaos. Contrary to what occurs in a quantum phase transition, the onset of
quantum chaos is not a property of the ground state but take place for any
typical many-spin quantum state. We study bipartite and pairwise entanglement
measures, namely the reduced Von Neumann entropy and the concurrence, and
discuss quantum entanglement sharing. Our results suggest that the behavior of
the entanglement is related to the mixing of the eigenfunctions rather than to
the transition to chaos.Comment: 14 pages, 14 figure
Quantum and classical echoes in scattering systems described by simple Smale horseshoes
We explore the quantum scattering of systems classically described by binary
and other low order Smale horseshoes, in a stage of development where the
stable island associated with the inner periodic orbit is large, but chaos
around this island is well developed. For short incoming pulses we find
periodic echoes modulating an exponential decay over many periods. The period
is directly related to the development stage of the horseshoe. We exemplify our
studies with a one-dimensional system periodically kicked in time and we
mention possible experiments.Comment: 7 pages with 6 reduced quality figures! Please contact the authors
([email protected]) for an original good quality pre-prin
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