3,862 research outputs found
Evaluation of advanced optimisation methods for estimating Mixed Logit models
The performances of different simulation-based estimation techniques for mixed logit modeling are evaluated. A quasi-Monte Carlo method (modified Latin hypercube sampling) is compared with a Monte Carlo algorithm with dynamic accuracy. The classic Broyden-Fletcher-Goldfarb-Shanno (BFGS) optimization algorithm line-search approach and trust region methods, which have proved to be extremely powerful in nonlinear programming, are also compared. Numerical tests are performed on two real data sets: stated preference data for parking type collected in the United Kingdom, and revealed preference data for mode choice collected as part of a German travel diary survey. Several criteria are used to evaluate the approximation quality of the log likelihood function and the accuracy of the results and the associated estimation runtime. Results suggest that the trust region approach outperforms the BFGS approach and that Monte Carlo methods remain competitive with quasi-Monte Carlo methods in high-dimensional problems, especially when an adaptive optimization algorithm is used
Monte Carlo study of the growth of striped domains
We analyze the dynamical scaling behavior in a two-dimensional spin model
with competing interactions after a quench to a striped phase. We measure the
growth exponents studying the scaling of the interfaces and the scaling of the
shrinking time of a ball of one phase plunged into the sea of another phase.
Our results confirm the predictions found in previous papers. The correlation
functions measured in the direction parallel and transversal to the stripes are
different as suggested by the existence of different interface energies between
the ground states of the model. Our simulations show anisotropic features for
the correlations both in the case of single-spin-flip and spin-exchange
dynamics.Comment: 15 pages, ReVTe
Metastability for reversible probabilistic cellular automata with self--interaction
The problem of metastability for a stochastic dynamics with a parallel
updating rule is addressed in the Freidlin--Wentzel regime, namely, finite
volume, small magnetic field, and small temperature. The model is characterized
by the existence of many fixed points and cyclic pairs of the zero temperature
dynamics, in which the system can be trapped in its way to the stable phase.
%The characterization of the metastable behavior %of a system in the context of
parallel dynamics is a very difficult task, %since all the jumps in the
configuration space are allowed. Our strategy is based on recent powerful
approaches, not needing a complete description of the fixed points of the
dynamics, but relying on few model dependent results. We compute the exit time,
in the sense of logarithmic equivalence, and characterize the critical droplet
that is necessarily visited by the system during its excursion from the
metastable to the stable state. We need to supply two model dependent inputs:
(1) the communication energy, that is the minimal energy barrier that the
system must overcome to reach the stable state starting from the metastable
one; (2) a recurrence property stating that for any configuration different
from the metastable state there exists a path, starting from such a
configuration and reaching a lower energy state, such that its maximal energy
is lower than the communication energy
Persistence exponent in a superantiferromagnetic quenching
We measure the persistence exponent in a phase separating two-dimensional
spin system with non-conserved dynamics quenched in a region with four
coexisting stripe phases. The system is an Ising model with nearest neighbor,
next-to-the-nearest neighbor and plaquette interactions. Due the particular
nature of the ground states, the order parameter is defined in terms of blocks
of spins. Our estimate of the persistence exponent, , differs from
those of the two-dimensional Ising and four state Potts models. Our procedure
allows the study of persistence properties also at finite temperature : our
results are compatible with the hypothesis that does not depend on
below the critical point.Comment: LaTeX file with postscript figure
New aspects of microwave properties of Nb in the mixed state
We present a study of the frequency dependence of the vortex dynamics in a
conventional superconductor. We have employed a swept-frequency, Corbino-disk
technique to investigate the temperature (3.6K-Tc) and high-field (from Hc2/2
to Hc2) microwave complex resistivity in Nb thin (20-40 nm) films as a function
of the frequency (1-20 GHz). We have found several previously unnoticed
features: (i) a field-dependent depinning frequency in the GHz range; (ii)
deviations from the accepted frequency dependence, that can be ascribed to some
kind of vortex creep; (iii) the presence of switching phenomena, reminiscent of
vortex instabilities. We discuss the possible origin of the features here
reported.Comment: 5 pages, 3 figures, presented at VORTEX VI Conference, to appear on
Physica
Relaxation Height in Energy Landscapes: an Application to Multiple Metastable States
The study of systems with multiple (not necessarily degenerate) metastable
states presents subtle difficulties from the mathematical point of view related
to the variational problem that has to be solved in these cases. We introduce
the notion of relaxation height in a general energy landscape and we prove
sufficient conditions which are valid even in presence of multiple metastable
states. We show how these results can be used to approach the problem of
multiple metastable states via the use of the modern theories of metastability.
We finally apply these general results to the Blume--Capel model for a
particular choice of the parameters ensuring the existence of two multiple, and
not degenerate in energy, metastable states
Superconducting nanowire quantum interference device based on Nb ultrathin films deposited on self-assembled porous Si templates
Magnetoresistance oscillations were observed on networks of superconducting
ultrathin Nb nanowires presenting evidences of either thermal or quantum
activated phase slips. The magnetic transport data, discussed in the framework
of different scenarios, reveal that the system behaves coherently in the
temperature range where the contribution of the fluctuations is important.Comment: accepted for publication on Nanotechnolog
Interface Transparency of Nb/Pd Layered Systems
We have investigated, in the framework of proximity effect theory, the
interface transparency T of superconducting/normal metal layered systems which
consist of Nb and high paramagnetic Pd deposited by dc magnetron sputtering.
The obtained T value is relatively high, as expected by theoretical arguments.
This leads to a large value of the ratio although Pd does
not exhibit any magnetic ordering.Comment: To be published on Eur. Phys. J.
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