Physical aging and heterogeneous dynamics.

Physical aging appears consistent with homogeneous models that are based upon a single "inner clock", but incompatible with the established heterogeneous nature of relaxation in glass-forming materials and the concomitant dispersion of aging rates. This work demonstrates that aging follows the conceptually simpler model of heterogeneous dynamics that differs from the homogeneous case in the rate at which equilibrium is approached. However, the very fast modes within the relaxation time dispersion age according to a common inner clock, because their fictive temperatures are slaved to macroscopic softening. Evidence is provided for such a transition to homogeneous aging as the frequency is increased into the excess wing. The results explain why aging of glasses appears homogeneous and consistent with time aging-time superposition in cases where observations are based on the high frequency behavior.