2,131 research outputs found
Physical Wigner functions
In spite of their potential usefulness, the characterizations of Wigner
functions for Bose and Fermi statistics given by O'Connell and Wigner himself
almost thirty years ago has drawn little attention. With an eye towards
applications in quantum chemistry, we revisit and reformulate them in a more
convenient way.Comment: Latex, 10 page
Langevin theory of absorbing phase transitions with a conserved magnitude
The recently proposed Langevin equation, aimed to capture the relevant
critical features of stochastic sandpiles, and other self-organizing systems is
studied numerically. This equation is similar to the Reggeon field theory,
describing generic systems with absorbing states, but it is coupled linearly to
a second conserved and static (non-diffusive) field. It has been claimed to
represent a new universality class, including different discrete models: the
Manna as well as other sandpiles, reaction-diffusion systems, etc. In order to
integrate the equation, and surpass the difficulties associated with its
singular noise, we follow a numerical technique introduced by Dickman. Our
results coincide remarkably well with those of discrete models claimed to
belong to this universality class, in one, two, and three dimensions. This
provides a strong backing for the Langevin theory of stochastic sandpiles, and
to the very existence of this new, yet meagerly understood, universality class.Comment: 4 pages, 3 eps figs, submitted to PR
Sticky grains do not change the universality class of isotropic sandpiles
We revisit the sandpile model with ``sticky'' grains introduced by Mohanty
and Dhar [Phys. Rev. Lett. {\bf 89}, 104303 (2002)] whose scaling properties
were claimed to be in the universality class of directed percolation for both
isotropic and directed models. Simulations in the so-called fixed-energy
ensemble show that this conclusion is not valid for isotropic sandpiles and
that this model shares the same critical properties of other stochastic
sandpiles, such as the Manna model. %as expected from the existence of an extra
%conservation-law, absent in directed percolation. These results are
strengthened by the analysis of the Langevin equations proposed by the same
authors to account for this problem which we show to converge, upon
coarse-graining, to the well-established set of Langevin equations for the
Manna class. Therefore, the presence of a conservation law keeps isotropic
sandpiles, with or without stickiness, away from the directed percolation
class.Comment: 4 pages. 3 Figures. Subm. to PR
Classical and Quantum Chaos in a quantum dot in time-periodic magnetic fields
We investigate the classical and quantum dynamics of an electron confined to
a circular quantum dot in the presence of homogeneous magnetic
fields. The classical motion shows a transition to chaotic behavior depending
on the ratio of field magnitudes and the cyclotron
frequency in units of the drive frequency. We determine a
phase boundary between regular and chaotic classical behavior in the
vs plane. In the quantum regime we evaluate the quasi-energy
spectrum of the time-evolution operator. We show that the nearest neighbor
quasi-energy eigenvalues show a transition from level clustering to level
repulsion as one moves from the regular to chaotic regime in the
plane. The statistic confirms this
transition. In the chaotic regime, the eigenfunction statistics coincides with
the Porter-Thomas prediction. Finally, we explicitly establish the phase space
correspondence between the classical and quantum solutions via the Husimi phase
space distributions of the model. Possible experimentally feasible conditions
to see these effects are discussed.Comment: 26 pages and 17 PstScript figures, two large ones can be obtained
from the Author
Monte Carlo calculation of the current-voltage characteristics of a two dimensional lattice Coulomb gas
We have studied the nonlinear current-voltage characteristic of a two
dimensional lattice Coulomb gas by Monte Carlo simulation. We present three
different determinations of the power-law exponent of the nonlinear
current-voltage characteristic, . The determinations rely on
both equilibrium and non-equilibrium simulations. We find good agreement
between the different determinations, and our results also agree closely with
experimental results for Hg-Xe thin film superconductors and for certain single
crystal thin-film high temperature superconductors.Comment: late
Phase synchronization in tilted deterministic ratchets
We study phase synchronization for a ratchet system. We consider the
deterministic dynamics of a particle in a tilted ratchet potential with an
external periodic forcing, in the overdamped case. The ratchet potential has to
be tilted in order to obtain a rotator or self-sustained nonlinear oscillator
in the absence of external periodic forcing. This oscillator has an intrinsic
frequency that can be entrained with the frequency of the external driving. We
introduced a linear phase through a set of discrete time events and the
associated average frequency, and show that this frequency can be synchronized
with the frequency of the external driving. In this way, we can properly
characterize the phenomenon of synchronization through Arnold tongues, which
represent regions of synchronization in parameter space, and discuss their
implications for transport in ratchets.Comment: 14 pages, 6 figures, Physica A, in pres
Multifractal properties of return time statistics
Fluctuations in the return time statistics of a dynamical system can be
described by a new spectrum of dimensions. Comparison with the usual
multifractal analysis of measures is presented, and difference between the two
corresponding sets of dimensions is established. Theoretical analysis and
numerical examples of dynamical systems in the class of Iterated Functions are
presented.Comment: 4 pages, 3 figure
Transformation of spin information into large electrical signals via carbon nanotubes
Spin electronics (spintronics) exploits the magnetic nature of the electron,
and is commercially exploited in the spin valves of disc-drive read heads.
There is currently widespread interest in using industrially relevant
semiconductors in new types of spintronic devices based on the manipulation of
spins injected into a semiconducting channel between a spin-polarized source
and drain. However, the transformation of spin information into large
electrical signals is limited by spin relaxation such that the magnetoresistive
signals are below 1%. We overcome this long standing problem in spintronics by
demonstrating large magnetoresistance effects of 61% at 5 K in devices where
the non-magnetic channel is a multiwall carbon nanotube that spans a 1.5 micron
gap between epitaxial electrodes of the highly spin polarized manganite
La0.7Sr0.3MnO3. This improvement arises because the spin lifetime in nanotubes
is long due the small spin-orbit coupling of carbon, because the high nanotube
Fermi velocity permits the carrier dwell time to not significantly exceed this
spin lifetime, because the manganite remains highly spin polarized up to the
manganite-nanotube interface, and because the interfacial barrier is of an
appropriate height. We support these latter statements regarding the interface
using density functional theory calculations. The success of our experiments
with such chemically and geometrically different materials should inspire
adventure in materials selection for some future spintronicsComment: Content highly modified. New title, text, conclusions, figures and
references. New author include
Flux front penetration in disordered superconductors
We investigate flux front penetration in a disordered type II superconductor
by molecular dynamics (MD) simulations of interacting vortices and find scaling
laws for the front position and the density profile. The scaling can be
understood performing a coarse graining of the system and writing a disordered
non-linear diffusion equation. Integrating numerically the equation, we observe
a crossover from flat to fractal front penetration as the system parameters are
varied. The value of the fractal dimension indicates that the invasion process
is described by gradient percolation.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let
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