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Statistical physics

This theme is historically one of the first research area of the laboratory, and it actually gathers activities being conducted in several groups. It is characterized by frequent and fruitful collaborations between theoreticians and experimentalists within the laboratory.

Statistical physics is devoted to the study of the collective and macroscopic behavior of systems composed of a large number of elementary entities that interact one with the other. One speaks of "complex system" when the dynamics, or the structures that appears within the system, exhibit a rich variety of behaviors, while the microscopic entities the system is made of, and the interactions between these entities, are a priori simple. It may be noticed that the sensitivity to the initial conditions, as well as the large number of possible evolutions, play a major role in the emergence of non-trivial collective behaviors in such systems.

Fluctuations of global obervables in correlated systems

P. Holdsworth

Fluctuations of global observables, defined through integration of a dynamical field over a macroscopic volume or time, are one of the major research subjects of the laboratory. Theoretical studies conducted by our team deal with the "universal" nature of fluctuations in correlated systems, both in equilibrium and out of equilibrium, as well as on the relation between these fluctuations and the statistics of extreme values. These projects were motivated by the measurement, performed in the laboratory, of the fluctuations of the power necessary to maintain a turbulent flow.

 

 

Statistical characterization of nonequilibrium systems

A. Alastuey, P. Holdsworth

Among nonequilibrium systems, two specific classes play an important role, at least from a theoretical viewpoint: nonstationary systems that relax toward equilibrium on time scales that are much larger than the accessible experimental time scales, et systems driven into a nonequilibrium steady state by the presence of stationary fluxes. The first case corresponds to the aging phenomenon present in glassy materials. In the second case, an external forcing is necessarily present, and typical examples are granular matter under strong driving, or stationary turbulent flows, to quote only a few. To get a better understanding of the behavior of such systems, we try to determine what are the relevant macroscopic parameters to describe them; these may be for instance the mean injected or dissipated power, or nonequilibrium generalizations of the notions of temperature and chemical potential. Besides, we also try to understand the origin of rather generic properties arising in nonequilibrium systems, like the intermittency phenomenon, or the onset of long-range correlations.

 

 

Properties of systems with long-range interactions

A. Alastuey, P. Holdsworth

Significant progresses, to which we contributed, have been made recently in the understanding of the dynamics and thermodynamics of systems with long-range interactions. An interaction is said to be long-range if its energy diverges faster than the volume of the system considered: it is thus non-additive. Among numerous examples, one may quote gravity, unscreened coulombian interactions, dipolar interactions, or wave-particle interactions. Unusual behaviors may be observed in such systems, like negative specific heat, ensemble inequivalence, as well as striking dynamical effects.

 

Disordered elastic systems

A. Fedorenko

There is presently considerable effort to develop field theoretic
methods to describe glasses and complex systems beyond the mean field level and mode coupling theory. Important recent progress in that direction has been achieved for a broad class of systems with quenched disorder, known under the name of disordered elastic systems''.  It has been realized that an interplay between elasticity, pinning, thermal or quantum fluctuations and external driving leads to a wide range of new and interesting collective phenomena and complex physics, unexpected before. These systems are particularly interesting as they
share many features with glasses, and are important for numerous experimental  applications in both classical or quantum physics.  The most prominent examples include charge density waves, flux lattices in superconductors, electronic crystals,  domain walls in disordered magnets, contact-line wetting on dirty surfaces, motion of geological faults, cracks, and many more.