17 research outputs found
Tempering simulations in the four dimensional +-J Ising spin glass in a magnetic field
We study the four dimensional (4D) Ising spin glass in a magnetic
field by using the simulated tempering method recently introduced by Marinari
and Parisi. We compute numerically the first four moments of the order
parameter probability distribution . We find a finite cusp in the
spin-glass susceptibility and strong tendency to paramagnetic ordering at low
temperatures. Assuming a well defined transition we are able to bound its
critical temperature.Comment: 6 Pages including 5 figures, Revte
Griffiths singularities in the two dimensional diluted Ising model
We study numerically the probability distribution of the Yang-Lee zeroes
inside the Griffiths phase for the two dimensional site diluted Ising model and
we check that the shape of this distribution is that predicted in previous
analytical works. By studying the finite size scaling of the averaged smallest
zero at the phase transition we extract, for two values of the dilution, the
anomalous dimension, , which agrees very well with the previous estimated
values.Comment: 11 pages and 4 figures, some minor changes in Fig. 4, available at
http://chimera.roma1.infn.it/index_papers_complex.htm
Dynamical Behaviour of Low Autocorrelation Models
We have investigated the nature of the dynamical behaviour in low
autocorrelation binary sequences. These models do have a glass transition
of a purely dynamical nature. Above the glass transition the dynamics is not
fully ergodic and relaxation times diverge like a power law with close to . Approaching the glass transition
the relaxation slows down in agreement with the first order nature of the
dynamical transition. Below the glass transition the system exhibits aging
phenomena like in disordered spin glasses. We propose the aging phenomena as a
precise method to determine the glass transition and its first order nature.Comment: 19 pages + 14 figures, LateX, figures uuencoded at the end of the
fil
Continuous phase transition in a spin-glass model without time-reversal symmetry
We investigate the phase transition in a strongly disordered short-range
three-spin interaction model characterized by the absence of time reversal
symmetry in the Hamiltonian. In the mean-field limit the model is well
described by the Adam-Gibbs-DiMarzio scenario for the glass transition; however
in the short-range case this picture turns out to be modified. The model
presents a finite temperature continuous phase transition characterized by a
divergent spin-glass susceptibility and a negative specific heat exponent. We
expect the nature of the transition in this 3-spin model to be the same as the
transition in the Edwards-Anderson model in a magnetic field, with the
advantage that the strong crossover effects present in the latter case are
absent.Comment: 19 pages, Latex, 16 ps figure
Static chaos and scaling behaviour in the spin-glass phase
We discuss the problem of static chaos in spin glasses. In the case of
magnetic field perturbations, we propose a scaling theory for the spin-glass
phase. Using the mean-field approach we argue that some pure states are
suppressed by the magnetic field and their free energy cost is determined by
the finite-temperature fixed point exponents. In this framework, numerical
results suggest that mean-field chaos exponents are probably exact in finite
dimensions. If we use the droplet approach, numerical results suggest that the
zero-temperature fixed point exponent is very close to
. In both approaches is the lower critical dimension in
agreement with recent numerical simulations.Comment: 28 pages + 6 figures, LateX, figures uuencoded at the end of fil
Energy-Decreasing Dynamics in Mean-Field Spin Models
We perform a statistical analysis of deterministic energy-decreasing
algorithms on mean-field spin models with complex energy landscape like the
Sine model and the Sherrington Kirkpatrick model. We specifically address the
following question: in the search of low energy configurations is it convenient
(and in which sense) a quick decrease along the gradient (greedy dynamics) or a
slow decrease close to the level curves (reluctant dynamics)? Average time and
wideness of the attraction basins are introduced for each algorithm together
with an interpolation among the two and experimental results are presented for
different system sizes. We found that while the reluctant algorithm performs
better for a fixed number of trials, the two algorithms become basically
equivalent for a given elapsed time due to the fact that the greedy has a
shorter relaxation time which scales linearly with the system size compared to
a quadratic dependence for the reluctant.Comment: 20 pages, 6 figures. New version, to appear on J.Phys.
Quantum Fields and Extended Objects in Space-Times with Constant Curvature Spatial Section
The heat-kernel expansion and -regularization techniques for quantum
field theory and extended objects on curved space-times are reviewed. In
particular, ultrastatic space-times with spatial section consisting in manifold
with constant curvature are discussed in detail. Several mathematical results,
relevant to physical applications are presented, including exact solutions of
the heat-kernel equation, a simple exposition of hyperbolic geometry and an
elementary derivation of the Selberg trace formula. With regards to the
physical applications, the vacuum energy for scalar fields, the one-loop
renormalization of a self-interacting scalar field theory on a hyperbolic
space-time, with a discussion on the topological symmetry breaking, the finite
temperature effects and the Bose-Einstein condensation, are considered. Some
attempts to generalize the results to extended objects are also presented,
including some remarks on path integral quantization, asymptotic properties of
extended objects and a novel representation for the one-loop (super)string free
energy.Comment: Latex file, 122 page