A Microscopic View of Stable Glasses via Surface-Initiated Melting

Melting fronts have been observed in experimental vapor-deposited glasses, but are absent for ordinary glasses created by cooling a liquid. In order to gain insight into the different melting mechanisms, here we study the melting of an ultrastable glass film into a supercooled liquid using molecular simulations. We identify two mechanisms for this transformation: (i) the uniform melting of the film, and (ii) the initiation of the melting front from its surface. The former takes place in poorly thermalized materials, while the latter only takes place in ultrastable samples. We thus demonstrate that the melting front results from the (ultra-)stability of the glass and not from the vapor deposition process itself. Interestingly, for glass films of intermediate stability, the two mechanisms compete and the growth front propogates a distance l before the glass melts. Experimentally measuring how deep the melting front propogates into the film should be able to validate this proposal.