f-electrons localized and/or delocalized?
Fermi surfaces and quasiparticles in Ce- and U compounds

Gertrud Zwicknagl

Institut für Mathematische Physik, TU Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany

The central focus of the present talk are theoretical concepts and methods for the calculation of low-energy excitations in systems with partially filled f-shells. Of particular interest are the quasiparticle bands in the strongly renormalized Fermi liquid phases. The latter result from the strong local correlations and are not adequately described by standard electronic structure calculations based on the Local Density Approximation. I briefly review the Renormalized Band Method and their results for Ce-based heavy fermion compounds. For actinide compounds, increasingexperimental evidence points towards a possible coexistence of both localized and itinerant 5f-states. The "dual model" reproduces both the observed Fermi surface topology and the observed effective mass. Model calculations show how the dual character arises from the competition of intra-atomic correlations and anisotropies in the kinetic energy. The spectral functions reflect partial localization exhibiting dispersive quasiparticle peaks coexisting with an incoherent background and features related to transitions between atomic multiplets.