Density of states in liquids: quadratic or linear, and what each means?

Abstract

A lot has been said about the vibrational density of states (DoS) in liquids. A more recent discussion introduces contradictions with earlier results, and here I briefly review several pieces of evidence from modeling, experiments and theory showing this. I then show that the origin of contradictions often comes from misinterpreting the nature of excitations in liquids. Classic quadratic DoS corresponds to propagating (albeit damped) phonons as in solids and applies to any medium at low frequency, whereas the linear DoS comes from overdamped modes, localised particle motions. This has implications for interpreting simulations, experiments and a theory of liquids. I also introduce a new mechanism where the exponent of quadratic DoS reduces to close to 1 due to the asymmetry of the scattering intensity, reducing the effective phonon frequency.