Electronic states and the Fermi surface in f-electron compounds

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A nature of the f electrons in the heavy-electron compound has recently been clarified well by band theory. In particular, on the basis of an itinerant-electron model for the 4f electrons, the shape of the Fermi surface of CeSn_3, apart from the cyclotron effective mass, is well explained by a relativistic band theory. It means that in paramagnetic Ce compounds they may be itinerant in the ground state and contribute directly to the formation of the Fermi surface. It is a main purpose of this project to confirm an energy band picture for the f electrons in the Ce compounds and the U compounds. Firstly, the Fermi surface is calculated for the two paramagnetic compounds CeNi and CeRu_2Si_2 and compared with the recent de Haas-van Alphen(dHvA) efeect experiments. We find a reasonably good agreement between theory and experiment and thus make the validity of an energy band picture for the 4f electrons more sound. Secondly, the Fermi surface is calculated for LaRu_2Ge_2 and compared with the dHvA effect experiment for the ferromagnetic compound CeRu_2Ge_2. The experimental result is explained almost perfectly by using the Fermi surface of LaRu_2Ge_2 without any modifications. This result implies that the 4f electrons are localized and scarcely influence the Fermi surface in CeRu_2Ge_2. Thirdly, the Fermi surface is calculated for UB_<12> and compared with the dHvA effect experiment. In UB_<12>, both the magnitude and the angle dependence of the dHvA frequencies is explained reasonably well. In addition, the cyclotron masses as well as the electronic specific heat coefficient can be explained quantitatively well. In this compound, the 5f electrons are itinerant through a strong hybridization with the B 2p electrons. Preliminary calculations are carried out for CeRh_2, UGe_2 and PuX, where X means S, Se and Te.