Transition from surface vibrations to liquidlike dynamics at an incompletely melted semiconductor surface

We present results of a first-principles molecular-dynamics simulation of the vibrational spectrum of a model semiconductor surface where, close to the bulk melting temperature T-M, the top bilayer becomes diffusive. The specific example chosen is Ge(111). In place of the ordinary surface vibrations, we obtain a spectrum more akin to that of a confined liquid. In particular, we predict a dramatic decrease in the intensity of high-frequency (bond-stretching) surface mode intensity, associated with a local melting-induced breaking of hard covalent bonds in the top bilayer.