Soft spots in amorphous thin films: a structural signature of free surfaces

While it is known that the dynamics in thin films strongly depend strongly on the distance from a free surface, standard measures of the static structure in these systems (e.g., the density, the radial distribution function, or the distribution of under-coordinated particles) typically find at most a monolayer of particles at the surface that differ from those in the bulk. We investigate energy-minimized, thin-film configurations of Lennard-Jones particles and find that the presence of a free surface leads to low-energy vibrational surface modes with properties very different from those in the bulk. By analyzing the structure of these modes, we find that the density of ``soft spots,'' local regions of high mode amplitude, is higher near the surface. These soft spots have well-defined length scales characterizing both how far they penetrate into the bulk and how extended along the surface each one is. Furthermore, these soft spots have a high correlation to particle rearrangements or enhanced mobility. We discuss the implications of surface soft spots for existing results on glassy thin films.