1,221 research outputs found
Green Function Simulation of Hamiltonian Lattice Models with Stochastic Reconfiguration
We apply a recently proposed Green Function Monte Carlo to the study of
Hamiltonian lattice gauge theories. This class of algorithms computes quantum
vacuum expectation values by averaging over a set of suitable weighted random
walkers. By means of a procedure called Stochastic Reconfiguration the long
standing problem of keeping fixed the walker population without a priori
knowledge on the ground state is completely solved. In the model,
which we choose as our theoretical laboratory, we evaluate the mean plaquette
and the vacuum energy per plaquette. We find good agreement with previous works
using model dependent guiding functions for the random walkers.Comment: 14 pages, 5 PostScript Figures, RevTeX, two references adde
Historical and interpretative aspects of quantum mechanics: a physicists' naive approach
Many theoretical predictions derived from quantum mechanics have been
confirmed experimentally during the last 80 years. However, interpretative
aspects have long been subject to debate. Among them, the question of the
existence of hidden variables is still open. We review these questions, paying
special attention to historical aspects, and argue that one may definitively
exclude local realism on the basis of present experimental outcomes. Other
interpretations of Quantum Mechanics are nevertheless not excluded.Comment: 30 page
Temperature-extended Jarzynski relation: Application to the numerical calculation of the surface tension
We consider a generalization of the Jarzynski relation to the case where the
system interacts with a bath for which the temperature is not kept constant but
can vary during the transformation. We suggest to use this relation as a
replacement to the thermodynamic perturbation method or the Bennett method for
the estimation of the order-order surface tension by Monte Carlo simulations.
To demonstrate the feasibility of the method, we present some numerical data
for the 3D Ising model
Dynamic critical behaviour in Ising spin glasses
The critical dynamics of Ising spin glasses with Bimodal, Gaussian, and
Laplacian interaction distributions are studied numerically in dimensions 3 and
4. The data demonstrate that in both dimensions the critical dynamic exponent
, the non-equilibrium autocorrelation decay exponent
, and the critical fluctuation-dissipation ratio
all vary strongly and systematically with the form of the
interaction distribution.Comment: 8 pages, 4 figures, version to appear in Phys. Rev.
Aging phenomena in critical semi-infinite systems
Nonequilibrium surface autocorrelation and autoresponse functions are studied
numerically in semi-infinite critical systems in the dynamical scaling regime.
Dynamical critical behaviour is examined for a nonconserved order parameter in
semi-infinite two- and three-dimensional Ising models as well as in the
Hilhorst-van Leeuwen model. The latter model permits a systematic study of
surface aging phenomena, as the surface critical exponents change continuously
as function of a model parameter. The scaling behaviour of surface two-time
quantities is investigated and scaling functions are confronted with
predictions coming from the theory of local scale invariance. Furthermore,
surface fluctuation-dissipation ratios are computed and their asymptotic values
are shown to depend on the values of surface critical exponents.Comment: 12 pages, figures included, version to appear in Phys. Rev.
Critical Behavior and Lack of Self Averaging in the Dynamics of the Random Potts Model in Two Dimensions
We study the dynamics of the q-state random bond Potts ferromagnet on the
square lattice at its critical point by Monte Carlo simulations with single
spin-flip dynamics. We concentrate on q=3 and q=24 and find, in both cases,
conventional, rather than activated, dynamics. We also look at the distribution
of relaxation times among different samples, finding different results for the
two q values. For q=3 the relative variance of the relaxation time tau at the
critical point is finite. However, for q=24 this appears to diverge in the
thermodynamic limit and it is ln(tau) which has a finite relative variance. We
speculate that this difference occurs because the transition of the
corresponding pure system is second order for q=3 but first order for q=24.Comment: 9 pages, 13 figures, final published versio
Work fluctuations in quantum spin chains
We study the work fluctuations of two types of finite quantum spin chains
under the application of a time-dependent magnetic field in the context of the
fluctuation relation and Jarzynski equality. The two types of quantum chains
correspond to the integrable Ising quantum chain and the nonintegrable XX
quantum chain in a longitudinal magnetic field. For several magnetic field
protocols, the quantum Crooks and Jarzynski relations are numerically tested
and fulfilled. As a more interesting situation, we consider the forcing regime
where a periodic magnetic field is applied. In the Ising case we give an exact
solution in terms of double-confluent Heun functions. We show that the
fluctuations of the work performed by the external periodic drift are maximum
at a frequency proportional to the amplitude of the field. In the nonintegrable
case, we show that depending on the field frequency a sharp transition is
observed between a Poisson-limit work distribution at high frequencies toward a
normal work distribution at low frequencies.Comment: 10 pages, 13 figure
Scaling and universality in the aging kinetics of the two-dimensional clock model
We study numerically the aging dynamics of the two-dimensional p-state clock
model after a quench from an infinite temperature to the ferromagnetic phase or
to the Kosterlitz-Thouless phase. The system exhibits the general scaling
behavior characteristic of non-disordered coarsening systems. For quenches to
the ferromagnetic phase, the value of the dynamical exponents, suggests that
the model belongs to the Ising-type universality class. Specifically, for the
integrated response function , we find
consistent with the value found in the two-dimensional
Ising model.Comment: 16 pages, 14 figures (please contact the authors for figures
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