Dynamical heterogeneity of the glassy state.

Abstract

We compare dynamical heterogeneities in equilibrated supercooled liquids and in the nonequilibrium glassy state within the framework of the random first order transition theory. Fluctuating mobility generation and transport in the glass are treated by numerically solving stochastic continuum equations for mobility and fictive temperature fields that arise from an extended mode coupling theory containing activated events. Fluctuating spatiotemporal structures in aging and rejuvenating glasses lead to dynamical heterogeneity in glasses with characteristics distinct from those found in the equilibrium supercooled liquid. The non-Gaussian distribution of activation free energies, the stretching exponent β, and the growth of characteristic lengths are studied along with the four-point dynamical correlation function. Asymmetric thermodynamic responses upon heating and cooling are predicted to be the result of the heterogeneity and the out-of-equilibrium behavior of glasses below Tg. Our numerical results agree with experimental calorimetry. We numerically confirm the prediction of Lubchenko and Wolynes in the glass that the dynamical heterogeneity can lead to noticeably bimodal distributions of local fictive temperatures during some histories of preparation which explains in a unified way recent experimental observations that have been interpreted as coming from there being two distinct equilibration mechanisms in glasses.