Dynamics near free surfaces of molecular dynamics simulated Ni0.5Zr0.5 metallic glass films

Results are reported from molecular dynamics simulations of glass forming films with Ni0.5Zr0.5 adapted potentials, which show a dramatic enhancement of the dynamics near the surface. After relaxation of the film for 10(-8)-10(-7) s, the dynamics is investigated by a spatially resolved effective diffusion constant and by the decay rates of Ni-Zr next-neighbor bonds. At the simulated bulk glass transition temperature, the atomic mobility varies across the film by more than two orders of magnitude from that of a liquid at the film surface to bulk values of an arrested solid in the interior. This variation of the dynamics is also observed slightly above and below the glass transition. Despite the large variation in the atomic mobility, an analysis of the van Hove correlation functions indicates a global dynamic glass transition, common to the surface and core of the film. The decrease of the mobility with depth is exponential, with a smooth transition between surface and bulk behavior. A Landau analysis is applied to interpret the spatial dependence of the mobility, using a hidden parameter, possibly related to the time scale of the fluctuations in the system. The decay length and surface excess mobility, corresponding to the exponential decrease, are only slightly temperature dependent, without a singularity at the glass transition. Analogies of this phenomenon to the dynamics of confined glass forming liquids and to the surface melting of metallic crystals are discussed.