Strongly correlated electronic ground states in metals near the metal-insulator transition

Abstract

The model in which positive ions in a metal are smeared into a uniform neutralizing background in which interacting electrons move is first considered at some length, especially rather near to the metal-insulator (Wiper) transition. Properties considered carefully are (a) the electronic momentum distribution as a function of background density, and (b) the pair correlation function. Features connected with both the long-range Coulombic repulsion and the transition from delocalized to localized behaviour will be highlighted in terms of (a) and (b) above. The possible use of this model as a reference state against which to consider the alkali metals, in both normal and expanded form, will then be discussed. In Na, the importance of 3s–3p hybridization will be emphasized, the Heisenberg model providing a surprisingly useful account of some bulk properties. Diffraction evidence on the degenerate electron assembly in molten Na and K will be considered in support of pronounced metallic bonding. Expanded Cs along the coexistence curve will next be treated, especially the observed magnetic susceptibility which changes from Fermi liquid behaviour to Curie-Weiss form as the critical point is approached. The crossover point is discussed in terms of heavy fermion theory and is shown to contain information about the discontinuity of the electronic momentum distribution at the Fermi surface. This discontinuity is much smaller than that in jellium (the smeared ion model above) and testifies to the importance of the electron-ion interaction at this density, which cannot be treated perturbatively. Finally, the possibility of the co-existence of molecules in metallic phases is considered, with particular reference to metallic hydrogen and metallic iodine near the metal-insulator transition. For the latter, experimental evidence can leave little doubt that there is at least a limited range of pressures over which the metallic ground state of iodine contains I₂ molecules, and some discussion of this strongly correlated state will be included.