Anomalous dispersion of the acoustic mode in liquid Bi

Abstract

Inelastic x-ray scattering measurements on liquid Bi were carried out. Prominent acoustic mode excitations were observed in the dynamic structure factor to beyond $12{\mathrm{nm}}^{\ensuremath{-}1}$, which resolves previously conflicting results as to their presence beyond $6{\mathrm{nm}}^{\ensuremath{-}1}$. We find the dispersion curve of the excitation energy with momentum transfer is nearly flat from 7 to $15{\mathrm{nm}}^{\ensuremath{-}1}$ consistent with ab initio calculations of liquid Bi [J. Souto et al., Phys. Rev. B 81, 134201 (2010)]. Our ab initio and classical molecular dynamics simulations suggest that a long-range force is needed to reproduce the flatness of the dispersion curve, and the long-range force is correlated with a local structure consisting of shorter and longer bonds in the liquid.