Zero-sound-like modes in simple liquid metals

Coherent neutron-scattering experiments on liquid cesium and rubidium near the melting point reveal an anomalous dispersion of collective modes for frequencies {ω}τM{}1 with τM 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^-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 G_∞ and the Poisson ratio {ν}. Some evidence is found for the existence of small clusters in the liquid on the picosecond time scale.