Dynamics of liquid and undercooled silicon: An ab initio molecular dynamics study

First-principles molecular dynamics simulations of liquid and undercooled silicon have been performed in order to study the evolution of dynamic properties across the melting point. The calculated dynamic structure factors show collective excitations at small wave vectors above the melting point, in good agreement with recent experimental data, while the transverse current correlation functions do not display propagating shear modes. Upon undercooling, our results evidence the occurrence of well-defined shear modes that result from an enhancement of local tetrahedral arrangement associated to strong covalent bonding suggesting that propagating shear waves might be sustained by a concerted motion of tetrahedra.