Fluctuating pseudoatoms in metallic fluids

Abstract

The nature of long-range many-body interactions in metallic fluids is examined with emphasis on their possible role in the unique features of these systems observed near the liquid–vapor critical point. A reexamination of recent theoretical results demonstrating the existence of van der Waals forces between ‘‘pseudoatoms’’ (ion cores and associated screening electrons) reveals a direct correspondence with dispersion forces in insulating systems. In the limit of high conduction electron number densities ρ, pseudoatoms have an effective frequency-dependent polarizability α(ω)=α(0)ω2p/(ω2p−ω2), where α(0)=z/4πρ, with z the ion valence, and ωp is the classical electron gas plasma frequency (4πρe2/m)1/2. It is the dynamic nature of the interactions (arising from fluctuations of the pseudoatoms) that permits such a long-range interaction to exist. The dimensionless parameter α(0)ρ which in insulating fluids characterizes the relative importance of triplet (Axilrod–Teller) to pair dispersion interactions is thus system independent and significantly larger than in nonmetallic fluids. The nature of this dynamic polarizability is further examined in the context of a transport theory for a classical plasma based on the Boltzmann equation. The statistical mechanics of fluctuating pseudoatoms at finite temperature is studied both for the metallic fluid and in the Wigner crystal. These various approaches suggest that the pseudoatom interaction may be viewed as a potential mediated by the exchange of plasmons, just as conventional van der Waals forces arise from the exchange of virtual excitations of atomic levels. A description of the critical point in terms of pseudoatom interactions appears to explain qualitatively the extreme liquid–vapor asymmetry of the coexistence curves of cesium and rubidium as arising from the magnitude of three-body interactions. Additionally, it suggests that the thermal energy at the critical point scales with the plasmon energy, consistent with experiment.