Dielectric response and quasiparticle energies in nickel

The utility of traditional many-body perturbation theory in highly correlated d-electron systems is investigated taking full account of the underlying one-particle structure. Starting from a zeroth order Greens function based on a self-consistent local-density calculation, the dielectric matrix of ferromagnetic nickel is computed within the RPA, thus demonstrating the feasibility of many-body calculations in realistic systems. From the results, the optical absorption, the optical conductivity, and the electron-energy-loss spectra are obtained. A deviation from experiment in our optical absorption spectrum is easily traced back to our local-density starting point. Using instead quasiparticle energies in the zeroth order Green's function results in much better agreement with experiment. The particle-hole interaction is relatively unimportant in optical absorption and the local-field effects (LFE) are small, especially at lower energies and small momentum transfers. From the dielectric matrix, the GW approximation for the electronic self-energy is obtained and from there, the quasi-particle bandstructure which compares well with experiment. On the other hand, no satellite 6 eV below the Fermi energy is obtained and our spin-splitting is too large, thus demonstrating the need for a theory beyond the GW approximation.