  We propose that the super-Arrhenius relaxation rates observed in glassy
materials are determined by thermally nucleated rearrangements of increasing
numbers of molecules at decreasing temperatures. In our model of this
mechanism, string-like fluctuations in the neighborhood of shear transformation
zones (STZ's) provide routes along which rearrangements can propagate, and the
entropy associated with long enough strings allows the rearrangement energy to
be distributed stably in the surrounding material. We further postulate that,
at low enough temperatures, these fluctuations are localized on the interfaces
between frustration-limited domains. Our result is a modified Vogel-Fulcher
formula at low temperatures that crosses over to a simple Arrhenius law as the
glass changes from being solid-like to liquid-like. To achieve agreement with
experiment in the crossover region, we need to make what seem to us to be
overly strong assumptions about the behavior of frustration-limited domains.
