Zero-sound-like modes in simple liquid metals

Abstract

Coherent neutron-scattering experiments on liquid cesium and rubidium near the melting point reveal an anomalous dispersion of collective modes for frequencies \ensuremath{\omega}${\mathrm{\ensuremath{\tau}}}_{\mathit{M}}$\ensuremath{\gg}1 with ${\mathrm{\ensuremath{\tau}}}_{\mathit{M}}$ the Maxwell relaxation time for shear stress in the liquid. Within the framework of generalized hydrodynamics, these modes are shown to propagate similar to zero sound in a restricted range of wave numbers around Q=0.4 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$. Such a behavior is known from glass-forming systems, but here simple liquid metals near the melting point are shown to exhibit such modes too. The feature of the dispersion relation in this Q region yields quantities known from the theory of elasticity of solids such as the shear modulus ${\mathit{G}}_{\mathrm{\ensuremath{\infty}}}$ and the Poisson ratio \ensuremath{\nu}. Some evidence is found for the existence of small clusters in the liquid on the picosecond time scale.