Probing the ultimate density and stability of disordered films

The density and stability of experimental vapor deposited films increase as they are equilibrated to lower temperatures; equilibration become difficult below the glass transition temperature, Tg. Creating such films in silico, using size-dispersed spheres to avoid crystallization and atom swapping in a near-surface region to mimic the experimental surface mobility allows probing further. With this system, surface mobility is independent of temperature, and full equilibration can be obtained to ~0.75 Tg, with ρ(T=0) increasing to ~ 0.78, stability ~1000 times. We find that such equilibration creates icosahedral neighborhoods (densest local sphere packing), whose concentration increase linearly with log(swap intensity) to c_icos(T=0) ~5 %. The density rise slows substantially as the icosahedron concentration increases, suggesting close approach to limiting density and structural values.