Irreversible and Reversible Behavior of Spin Glasses: Broken Ergodicity

Over the past decade, a great deal of effort has gone into understanding the properties of spin glasses [1,2]. However, because these are rather unique systems which show simultaneously apparent phase transition as well as metastable or glassy behavior, progress has been slow. Though it was initially believed that spin glasses could be treated as if they had a true equilibrium phase transition, we now recognize that this cannot be the whole story. Recently, it has become clear that spin glasses are very complex systems, in which irreversible and time dependent effects play an important role. We now know that one must go beyond the regime of validity of equilibrium thermodynamics. In this paper, we will discuss the mountinq evidence, both experimental and theoretical, for why nonequilibrium approaches are essential in order to understand spin glasses.