Contribution of transverse modes to the dynamics of density fluctuations

The transverse and longitudinal current correlation functions are evaluated in liquid and amorphous sodium by computer simulation. The study of the corresponding spectra as a function of the wavevector $k$ allows the evaluation of a dispersion curve for their peak position. The results are compared with recent experimental findings [PNAS 107, 21985 (2010)], obtained by a new analysis of $S(k,{\omega})$ measured by inelastic X-ray scattering in liquid and polycrystalline sodium. A substantial agreement between experimental and computer simulation results is found. The analysis of the line widths supports the Vibration-Transit theory picture of the dynamics in liquids.