Large, growing length scale controls the melting mechanism of stable glasses

Exceptionally stable vapor-deposited glass films melt via a constant velocity front initiated at the surface, whereas ordinary liquid cooled glass films melt homogeneously. The melting time for stable glass films increase linearly with film thickness until a crossover thickness l_c, which can be as large as a micrometer. For films thicker than l_c the melting time is constant, suggesting that a bulk transformation mechanism is then prominent. Here we use the swap Monte Carlo algorithm to prepare stable glass films that we subsequently melt films using molecular dynamics simulations, in order to study the microscopic differences between the two melting mechanisms of glass films. Several of our stable films completely melt via a front initiated at the surface for lower melting temperatures, but melt via a bulk mechanism at higher melting temperatures. For intermediate melting temperature we directly determine l_c, and then we use bulk melting simulations to approximate
l_c to lower melting temperatures. We find that l_c grows with increasing stability and decreasing melting temperature. Using parameters corresponding to toluene, we determine that the crossover length for our most stable glass is comparable to that measured in experiments.