The rejuvenation effect in the two-state random energy model
  Theoretical analyses of the random energy model with only two states and its extension with a hierarchy of only two levels show that these models reproduce out-of-equilibrium phenomena observed in experiments of glassy materials; the rejuvenation effect (the chaos effect), i.e. the abrupt jump and subsequent relaxation of the out-of-phase susceptibility as if the system rejuvenates when the temperature is lowered, and the power-law relaxation of the two-time correlation function. Our results suggest that also in an assembly of small systems with relaxation times distributed broadly some of these interesting out-of-equilibrium phenomena can be observed. 
Mitsuhiro Kawasaki