166 research outputs found
Spin Glasses
This is a short review about recent methods and results, mostly for mean
field spin glasses, based on interpolation and comparison schemes. In
particular, the Parisi spontaneous replica symmetry breaking phenomenon is
described in the frame of extended variational principles, Derrida-Ruelle
probability cascades, and overlap locking.Comment: 25 page
Mathematical aspects of mean field spin glass theory
A comprehensive review will be given about the rich mathematical structure of
mean field spin glass theory, mostly developed, until now, in the frame of the
methods of theoretical physics, based on deep physical intuition and hints
coming from numerical simulation. Central to our treatment is a very simple and
yet powerful interpolation method, allowing to compare different probabilistic
schemes, by using convexity and positivity arguments. In this way we can prove
the existence of the thermodynamic limit for the free energy density of the
system, a long standing open problem. Moreover, in the frame of a generalized
variational principle, we can show the emergency of the Derrida-Ruelle random
probability cascades, leading to the form of free energy given by the
celebrated Parisi \textit {Ansatz}. All these results seem to be in full
agreement with the mechanism of spontaneous replica symmetry breaking as
developed by Giorgio Parisi.Comment: proceedings of the "4th European Congress of Mathematics", Stockholm,
2004. 17 page
Dynamical ensembles in stationary states
We propose as a generalization of an idea of Ruelle to describe turbulent
fluid flow a chaotic hypothesis for reversible dissipative many particle
systems in nonequilibrium stationary states in general. This implies an
extension of the zeroth law of thermodynamics to non equilibrium states and it
leads to the identification of a unique distribution \m describing the
asymptotic properties of the time evolution of the system for initial data
randomly chosen with respect to a uniform distribution on phase space. For
conservative systems in thermal equilibrium the chaotic hypothesis implies the
ergodic hypothesis. We outline a procedure to obtain the distribution \m: it
leads to a new unifying point of view for the phase space behavior of
dissipative and conservative systems. The chaotic hypothesis is confirmed in a
non trivial, parameter--free, way by a recent computer experiment on the
entropy production fluctuations in a shearing fluid far from equilibrium.
Similar applications to other models are proposed, in particular to a model for
the Kolmogorov--Obuchov theory for turbulent flow.Comment: 31 pages, 3 figures, compile with dvips (otherwise no pictures
Large deviations for non-uniformly expanding maps
We obtain large deviation results for non-uniformly expanding maps with
non-flat singularities or criticalities and for partially hyperbolic
non-uniformly expanding attracting sets. That is, given a continuous function
we consider its space average with respect to a physical measure and compare
this with the time averages along orbits of the map, showing that the Lebesgue
measure of the set of points whose time averages stay away from the space
average decays to zero exponentially fast with the number of iterates involved.
As easy by-products we deduce escape rates from subsets of the basins of
physical measures for these types of maps. The rates of decay are naturally
related to the metric entropy and pressure function of the system with respect
to a family of equilibrium states. The corrections added to the published
version of this text appear in bold; see last section for a list of changesComment: 36 pages, 1 figure. After many PhD students and colleagues having
pointed several errors in the statements and proofs, this is a correction to
published article answering those comments. List of main changes in a new
last sectio
Global bifurcations close to symmetry
Heteroclinic cycles involving two saddle-foci, where the saddle-foci share
both invariant manifolds, occur persistently in some symmetric differential
equations on the 3-dimensional sphere. We analyse the dynamics around this type
of cycle in the case when trajectories near the two equilibria turn in the same
direction around a 1-dimensional connection - the saddle-foci have the same
chirality. When part of the symmetry is broken, the 2-dimensional invariant
manifolds intersect transversely creating a heteroclinic network of Bykov
cycles.
We show that the proximity of symmetry creates heteroclinic tangencies that
coexist with hyperbolic dynamics. There are n-pulse heteroclinic tangencies -
trajectories that follow the original cycle n times around before they arrive
at the other node. Each n-pulse heteroclinic tangency is accumulated by a
sequence of (n+1)-pulse ones. This coexists with the suspension of horseshoes
defined on an infinite set of disjoint strips, where the first return map is
hyperbolic. We also show how, as the system approaches full symmetry, the
suspended horseshoes are destroyed, creating regions with infinitely many
attracting periodic solutions
Fluctuations of the Magnetization in Thin Films due to Conduction Electrons
A detailed analysis of damping and noise due to a {\it sd}-interaction in a
thin ferromagnetic film sandwiched between two large normal metal layers is
carried out. The magnetization is shown to obey in general a non-local equation
of motion which differs from the the Gilbert equation and is extended to the
non-adiabatic regime. To lowest order in the exchange interaction and in the
limit where the Gilbert equation applies, we show that the damping term is
enhanced due to interfacial effects but it also shows oscillations as a
function of the film thickness. The noise calculation is however carried out to
all orders in the exchange coupling constant. The ellipticity of the precession
of the magnetization is taken into account. The damping is shown to have a
Gilbert form only in the adiabatic limit while the relaxation time becomes
strongly dependent on the geometry of the thin film. It is also shown that the
induced noise characteristic of sd-exchange is inherently colored in character
and depends on the symmetry of the Hamiltonian of the magnetization in the
film. We show that the sd-noise can be represented in terms of an external
stochastic field which is white only in the adiabatic regime. The temperature
is also renormalized by the spin accumulation in the system. For large
intra-atomic exchange interactions, the Gilbert-Brown equation is no longer
valid
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