Relaxation Mechanisms in Glassy Dynamics: the Arrhenius and Fragile   Regimes
  Generic glass formers exhibit at least two characteristic changes in their relaxation behavior, first to an Arrhenius-type relaxation at some characteristic temperature, and then at a lower characteristic temperature to a super-Arrhenius (fragile) behavior. We address these transitions by studying the statistics of free energy barriers for different systems at different temperatures and space dimensions. We present a clear evidence for changes in the dynamical behavior at the transition to Arrhenius and then to a super-Arrhenius behavior. A simple model is presented, based on the idea of competition between single-particle and cooperative dynamics. We argue that Arrhenius behavior can take place as long as there is enough free volume for the completion of a simple $T_1$ relaxation process. Once free volume is absent one needs a cooperative mechanism to `collect' enough free volume. We show that this model captures all the qualitative behavior observed in simulations throughout the considered temperature range. 
H. George E. Hentschel, Smarajit Karmakar, Itamar Procaccia and   Jacques Zylberg
cond-mat.dis-nn