Understanding the problem of glass transition on the basis of elastic waves in a liquid

Abstract

We propose that the properties of glass transition can be understood on the basis of elastic waves. Elastic waves originating from atomic jumps in a liquid propagate local expansion due to the anharmonicity of interatomic potential. This creates dynamic compressive stress, which increases the activation barrier for other events in a liquid. The non-trivial point is that the range of propagation of high-frequency elastic waves, $d_{\rm el}$, increases with liquid relaxation time $τ$. A self-consistent calculation shows that this increase gives the Vogel-Fulcher-Tammann (VFT) law. In the proposed theory, we discuss the origin of two dynamic crossovers in a liquid: 1) the crossover from exponential to non-exponential and from Arrhenius to VFT relaxation at high temperature and 2) the crossover from the VFT to a more Arrhenius-like relaxation at low temperature. The corresponding values of $τ$ at the two crossovers are in quantitative parameter-free agreement with experiments. The origin of the second crossover allows us to reconcile the ongoing controversy surrounding the possible divergence of $τ$. The crossover to Arrhenius relaxation universally takes place when $d_{\rm el}$ reaches system size, thus avoiding divergence and associated theoretical complications such as identifying the nature of the phase transition and the second phase itself. Finally, we discuss the effect of volume on $τ$ and the origin of liquid fragility.