Front-Mediated Melting of Isotropic Ultrastable Glasses

Ultrastable vapor-deposited glasses display uncommon material properties. Most remarkably, upon heating they are believed to melt via a liquid front that originates at the free surface and propagates over a mesoscopic crossover length, before crossing over to bulk melting. We combine swap Monte Carlo with molecular dynamics simulations to prepare and melt isotropic amorphous films of unprecedendtly high kinetic stability. We are able to directly observe both bulk and front melting, and the crossover between them. We measure the front velocity over a broad range of conditions, and a crossover length scale that grows to nearly 400 particle diameters in the regime accessible to simulations. Our results disentangle the relative roles of kinetic stability and vapor deposition in the physical properties of stable glasses.

Figure 1

Structural relaxation time ${\tau }_{\alpha }$ for the equilibrated bulk system (circles) fitted with a parabolic law (solid line; see text). The estimated experimental $T_g$ is shown with a dashed line. The blue box denotes the range of $T_i$ over which bulk systems are prepared as film equilibrium precursors. The red box denotes the range of $T_m$ over which films were melted. The inset presents the same information as a function of $1/T$ for reference.

Figure 2

Examples of glass film melting: Left: a pure melting front initiated at the surface. Center: a front competing with bulk melting. Right: pure bulk melting. (a),(d),(g) Representative snapshots of the three regimes are presented using transparent blue surfaces around regions of at least three mobile particles, opaque regions are glassy, and the dark green layer is the substrate. (b),(e),(h) Overlap functions color coded based on the substrate distance: blue curves denote $z$ closest to the substrate and red curves those closest to the surface. (c),(f),(i) Melting times ${\tau }_m$ defined from the decay of the overlap function to 0.2 (dashed lines in middle row) for several waiting times $t_w$. A front propagation velocity (red dashed line) can be extracted in (c) and (f).

Figure 3

Melting front velocity versus (a) $1/T_m$ and (b) ${\tau }_{\alpha }(T_m)$ with, respectively, Arrhenius fits $v=v(T_i)e^{-E_a/T_m}$ with $E_a\approx 2.0$ for all $T_i$, and power-law fits $v=v_0(T_i){\tau }_{\alpha }^{-\gamma }$ with $\gamma =1$ for all $T_i$.

Figure 4

The crossover length ${\ell }_c$ characterizes the maximum film thickness for where front melting dominates. The brown region denotes the range of ${\ell }_c$ smaller than the film thickness considered here. In this regime, front-mediated melting competes with bulk melting.