ABSTRACT
This chapter considers the interactions between the constituents of say a metal, i.e., electrons and ions. By adopting a mean field approach, the dynamics of the electrons can be obtained by perturbation theory from the properties of the non-interacting electrons, and the effective electron-electron interaction in good conductors is considered. Electron diffraction or X-ray experiments reveal the grainy character of a metal. Charge is separated spatially into two pieces: the nuclei and the tightly bound electrons, and the conduction electrons, which have their density spread throughout the solid. The mean field Hamiltonian is specified in terms of the mean field, which in turn is specified in terms of the average charge density. The diagrammatic interpretation of the degenerate free Fermi gas density response function allows us also to get a diagrammatic interpretation of the interacting Fermi gas density response function.
