5,618 research outputs found
On chaos in mean field spin glasses
We study the correlations between two equilibrium states of SK spin glasses
at different temperatures or magnetic fields. The question, presiously
investigated by Kondor and Kondor and V\'egs\"o, is approached here
constraining two copies of the same system at different external parameters to
have a fixed overlap. We find that imposing an overlap different from the
minimal one implies an extensive cost in free energy. This confirms by a
different method the Kondor's finding that equilibrium states corresponding to
different values of the external parameters are completely uncorrelated. We
also consider the Generalized Random Energy Model of Derrida as an example of
system with strong correlations among states at different temperatures.Comment: 19 pages, Late
The liquid-glass transition of silica
We studied the liquid-glass transition of by means of replica theory,
utilizing an effective pair potential which was proved to reproduce a few
experimental features of silica. We found a finite critical temperature ,
where the system undergoes a phase transition related to replica symmetry
breaking, in a region where experiments do not show any transition. The
possible sources of this discrepancy are discussed.Comment: 14 pages, 6 postscript figures. Revised version accepted for
pubblication on J.Chem.Phy
Chaos in temperature in the Sherrington-Kirkpatrick model
We prove the existence of chaos in temperature in the
Sherringhton-Kirkpatrick model. The effect is exceedingly small, namely of the
ninth order in perturbation theory. The equations describing two systems at
different temperatures constrained to have a fixed overlap are studied
analytically and numerically, yielding information about the behaviour of the
overlap distribution function in finite-size systems.Comment: REVTEX, 6 pages, 2 figure
Temperature evolution and bifurcations of metastable states in mean-field spin glasses, with connections with structural glasses
The correlations of the free-energy landscape of mean-field spin glasses at
different temperatures are investigated, concentrating on models with a first
order freezing transition. Using a ``potential function'' we follow the
metastable states of the model in temperature, and discuss the possibility of
level crossing (which we do not find) and multifurcation (which we find). The
dynamics at a given temperature starting from an equilibrium configuration at a
different temperature is also discussed. In presence of multifurcation, we find
that the equilibrium is never achieved, leading to aging behaviour at slower
energy levels than usual aging. The relevance of the observed mechanisms for
real structural glasses is discussed, and some numerical simulations of a soft
sphere model of glass are presented.Comment: 16 pages, LaTeX, 10 figures (12 postscript files
Error threshold in simple landscapes
We consider the quasispecies description of a population evolving in both the
"master sequence" landscape (where a single sequence is evolutionarily
preferred over all others) and the REM landscape (where the fitness of
different sequences is an independent, identically distributed, random
variable). We show that, in both cases, the error threshold is analogous to a
first order thermodynamical transition, where the overlap between the average
genotype and the optimal one drops discontinuously to zero.Comment: 10 pages and 2 figures, Plain LaTe
Effective "Penetration Depth" in the Vortex State of a d-wave Superconductor
The temperature and field dependence of the effective magnetic penetration
depth in the vortex state of a d-wave superconductor, as measured by muon spin
rotation experiments, is calculated using a nonlocal London model. We show that
at temperatures below T^* \propto \sqrt{B}, the linear T-dependence of the
effective penetration depth crosses over to a T^3-dependence. This could
provide an explanation for the low temperature flattening of the effective
penetration depth curves observed in a recent muon spin rotation experiment.Comment: 4 pages, RevTex, 3 Postscript figure
A relativistic quark model for the Omega- electromagnetic form factors
We compute the Omega- electromagnetic form factors and the decuplet baryon
magnetic moments using a quark model application of the Covariant Spectator
Theory. Our predictions for the Omega- electromagnetic form factors can be
tested in the future by lattice QCD simulations at the physical strange quark
mass.Comment: 13 pages, 5 figure
omega --> pi0 gamma* and phi --> pi0 gamma* Transition form factors in dispersion theory
We calculate the omega --> pi0 gamma* and phi --> pi0 gamma* electromagnetic
transition form factors based on dispersion theory, relying solely on a
previous dispersive analysis of the corresponding three-pion decays and the
pion vector form factor. We compare our findings to recent measurements of the
omega --> pi0 mu+ mu- decay spectrum by the NA60 collaboration, and strongly
encourage experimental investigation of the Okubo-Zweig-Iizuka-forbidden phi
--> pi0 l+ l- decays in order to understand the strong deviations from
vector-meson dominance found in these transition form factors.Comment: 11 pages, 8 figures; references updated, version published in Phys.
Rev.
Nonequilibrium dynamics of a simple stochastic model
We investigate the low-temperature dynamics of a simple stochastic model,
introduced recently in the context of the physics of glasses. The slowest
characteristic time at equilibrium diverges exponentially at low temperature.
On smaller time scales, the nonequilibrium dynamics of the system exhibits an
aging regime. We present an analytical study of the scaling behaviour of the
mean energy, of its local correlation and response functions, and of the
associated fluctuation-dissipation ratio throughout the regime of low
temperature and long times. This analysis includes the aging regime, the
convergence to equilibrium, and the crossover behaviour between them.Comment: 36 pages, plain tex, 7 figures, to be published by Journal of Physics
Evolution of a bosonic mode across the superconducting dome in the high-Tc cuprate Pr(2-x)Ce(x)CuO(4-{\delta})
We report a detailed spectroscopic study of the electron doped cuprate
superconductor Pr(2-x)Ce(x)CuO(4-{\delta}) using point contact junctions for
x=0.125(underdoped), x=0.15(optimally doped) and x=0.17(overdoped). From our
conductance measurements we are able to identify bosonic resonances for each
doping. These excitations disappear above the critical temperature, and above
the critical magnetic field. We find that the energy of the bosonic excitations
decreases with doping, which excludes lattice vibrations as the paring glue. We
conclude that the bosonic mediator for these cuprates is more likely to be spin
excitations.Comment: 4 page
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