ATOMIC DYNAMICS THROUGH THE GLASS-TRANSITION

The results of a detailed molecular-dynamics investigation of the liquid-glass transition in a realistic model of the metal-metalloid system Ni80P20 are presented. Several static quantities, such as mean-square amplitude of vibration, volume, enthalpy, and coordination numbers, exhibit a marked change at a point we identify as the glass transition temperature, in agreement with a recent Mossbauer study of a similar system. From a dynamical viewpoint, we find, in accord with the mode-coupling theories for supercooled liquids, that relaxation proceeds in two stages: fast (or conformational) relaxation, related to local rearrangements of atoms, and slow relaxation, connected with atomic transport, i.e., diffusion; these processes are referred to as beta and alpha, respectively. Our simulations show that diffusion exists even in the glass state, where it proceeds mostly by jumps, in contrast to the liquid phase where it is continuous. This relaxation mechanism is well described by a stretched exponential (Kohlrausch) law. The fast relaxation regime, on the other hand, does not correspond to that predicted by mode-coupling theory. This may be a consequence of the presence of intense jump-diffusive motion in our system.