4,982 research outputs found
Mean-value identities as an opportunity for Monte Carlo error reduction
In the Monte Carlo simulation of both Lattice field-theories and of models of
Statistical Mechanics, identities verified by exact mean-values such as
Schwinger-Dyson equations, Guerra relations, Callen identities, etc., provide
well known and sensitive tests of thermalization bias as well as checks of
pseudo random number generators. We point out that they can be further
exploited as "control variates" to reduce statistical errors. The strategy is
general, very simple, and almost costless in CPU time. The method is
demonstrated in the two dimensional Ising model at criticality, where the CPU
gain factor lies between 2 and 4.Comment: 10 pages, 2 tables. References updated and typos correcte
Lattice-Spin Mechanism in Colossal Magnetoresistant Manganites
We present a single-orbital double-exchange model, coupled with cooperative
phonons (the so called breathing-modes of the oxygen octahedra in manganites).
The model is studied with Monte Carlo simulations. For a finite range of doping
and coupling constants, a first-order Metal-Insulator phase transition is
found, that coincides with the Paramagnetic-Ferromagnetic phase transition. The
insulating state is due to the self-trapping of every carrier within an oxygen
octahedron distortion.Comment: 4 pages, 5 figures, ReVTeX macro, accepted for publication in PR
Optimized Monte Carlo Method for glasses
A new Monte Carlo algorithm is introduced for the simulation of supercooled
liquids and glass formers, and tested in two model glasses. The algorithm is
shown to thermalize well below the Mode Coupling temperature and to outperform
other optimized Monte Carlo methods. Using the algorithm, we obtain finite size
effects in the specific heat. This effect points to the existence of a large
correlation length measurable in equal time correlation functions.Comment: Proceedings of "X International workshop on Disordered Systems" held
in Molveno (Italy), March 200
Finite size effects in the specific heat of glass-formers
We report clear finite size effects in the specific heat and in the
relaxation times of a model glass former at temperatures considerably smaller
than the Mode Coupling transition. A crucial ingredient to reach this result is
a new Monte Carlo algorithm which allows us to reduce the relaxation time by
two order of magnitudes. These effects signal the existence of a large
correlation length in static quantities.Comment: Proceeding of "3rd International Workshop on Complex Systems". Sendai
(Japan). To appear on AIP Conference serie
On the critical behavior of the specific heat in glass-formers
We show numeric evidence that, at low enough temperatures, the potential
energy density of a glass-forming liquid fluctuates over length scales much
larger than the interaction range. We focus on the behavior of translationally
invariant quantities. The growing correlation length is unveiled by studying
the Finite Size effects. In the thermodynamic limit, the specific heat and the
relaxation time diverge as a power law. Both features point towards the
existence of a critical point in the metastable supercooled liquid phase.Comment: Version to be published in Phys. Rev.
What is the temperature of a granular medium?
In this paper we discuss whether thermodynamical concepts and in particular
the notion of temperature could be relevant for the dynamics of granular
systems. We briefly review how a temperature-like quantity can be defined and
measured in granular media in very different regimes, namely the glassy-like,
the liquid-like and the granular gas. The common denominator will be given by
the Fluctuation-Dissipation Theorem, whose validity is explored by means of
both numerical and experimental techniques. It turns out that, although a
definition of a temperature is possible in all cases, its interpretation is far
from being obvious. We discuss the possible perspectives both from the
theoretical and, more importantly, from the experimental point of view
The cumulative overlap distribution function in realistic spin glasses
We use a sample-dependent analysis, based on medians and quantiles, to
analyze the behavior of the overlap probability distribution of the
Sherrington-Kirkpatrick and 3D Edwards-Anderson models of Ising spin glasses.
We find that this approach is an effective tool to distinguish between RSB-like
and droplet-like behavior of the spin-glass phase. Our results are in agreement
with a RSB-like behavior for the 3D Edwards-Anderson model.Comment: Version accepted in PRB. 12 pages, 10 figure
A CORAVEL radial-velocity monitoring of giant Ba and S stars: spectroscopic orbits and intrinsic variations
This paper provides orbital parameters for 38 barium stars and 10 extrinsic S
stars derived from a decade-long CORAVEL monitoring. Lower bounds on the
orbital period (generally exceeding 10 y) have been obtained for 10 more
systems. Mira S, SC and (Tc-poor) C stars have also been monitored and show
intrinsic radial-velocity variations due to atmospheric phenomena. Tentative
orbital solutions are proposed for 3 stars (S UMa, X Cnc, BD-08:1900) where the
velocity and photometric periods are different. Three stars (RZ Peg, SS Vir and
R CMi) exhibit radial-velocity variations synchronous with the light
variations. Pseudo-orbital solutions have been derived for those stars. In the
case of RZ Peg, a line-doubling phenomenon is observed near maximum light, and
probably reflects the shock wave propagating through the photosphere.Comment: Astronomy & Astrophysics Supplements, 20 pages, 8 figures, 8 tables
(LaTeX). Also available at:
http://obswww.unige.ch/~udry/cine/barium/barium.htm
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