Martin Luc Rosinberg（Universite Pierre et Marie Curie）
Hysteresis and metastability in disordered spin systems: the exemple of the random field Ising model
The dynamical properties of random and frustrated systems are dominated at low temperature by the presence of a multitude of metastable states. This complex energy landscape is responsible for the irreversible, hysteretic response to an external field observed in a variety of systems (randommagnets, superconductors, fluids in porous media...). This response is also discontinuous and charaterized by a series of jumps (avalanches) which often obey a power-law distribution. A prototype of this behavior is the nonequilibrium random field Ising model (RFIM) at zero temperature which has been extensively studied in recent years. This model has also been used in several socio-economics contexts to simulate collective effects induced by imitation and social pressure. We shall review the main properties of the model and focus on the relationship between the hysteretic response and the distribution of the metastable states in the field-magnetization plane, obtained by computing analytically or numerically their entropy density (or quenched complexity).