Theoretical study of the heavy electron state in Lanthanide and Actinide compounds

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The f electron system in most of Lanthanide and Actinide compounds is considered as a localized electron system. However, in some of the compounds such as CeCu_2Si_2 and UBe_<13> in these groups behaves as a Fermiliquid with an extremely large effective mass at low temperatures. This Fermi liquid state is called the heavy electron state. To understand the origin and nature of this state is one of the most fundamental problems in solid state physics.Our major purpose of the present project is to understand the low energy electronic state of the heavy electron system. For this purpose we calculated the density of states in the low energy region using the periodic Anderson model. The obtained result shows that a very sharp peak in the density of states curve appears near the Fermi level at low temperatures when the large renormalization effect due to the f electron spin fluctuations exists in the self-energy correction for the f and s electrons in the system. This narrow band corresponds to the well-defined one-fermion excitation with a very large effective mass. As temperature increases, our calculation shows that the narrow peak broadens and its height decreases due to a shortening of the quasi-particle lifetime. This result is consistent with the experimental result observed in the heavy electon systems which shows the crossover from the heavy electon state to the Kondo-impurity-like state.We also investigated the mechanism of superconductivity in the hight superconducting oxides because we believe that the superconducting oxides are classified into the heavy-electron superconductors. We proposed a possible mechanism for high temperature superconductivity based on the charge-fluctuation-mechanism. The anomalous temperature dependence of the nuclear relaxation time-of cu nucleus in YBCO was successfully explained.