An inelastic X-ray scattering study on the collective dynamics in the 'non-simple' liquid metal Si

The dynamic structure factor S(Q,ω) of liquid silicon has been measured using inelastic X-ray scattering. The data show that distinct longitudinal collective modes exist in this 'non-simple' liquid metal similar as in the so-called 'simple' metallic fluids as, e.g. the liquid alkalis. However, the Si-modes are considerably more strongly damped and in S(Q,ω) the corresponding excitation peaks cease to exist for momentum transfers of about half the structure factor maximum position. The degree of positive dispersion, in the Q-ω-relation of the modes is, however, similar as in the liquid alkalis, indicating that the microscopic viscosity is similar in l-Si and that the stronger damping may rather be related to the higher thermal diffusivity. Additionally, it is found that the shape of the quasielastic line close to the structure factor maximum deviates considerably from the shape found in simple liquid metals. This observation is related to the onset of short time correlations between colliding particles on the sub picosecond scale which is in accord with findings from ab initio computer simulations under similar conditions.