Mixed-Valent State and Conductivity of Rare Earth-Transition Metal Chalcogenide

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Transition metal compounds, which show metallic conductivity, are generally known to be in mixed-valent states. Among copper chalcogenide system, the compounds containing the rare earth element Pr becomes semiconducting, although they have a composition in which the mixed-valent states are realized. In order to elucidate this reason, we have measured the values of valences of cations utilizing the Coulometric titration method, performed valence bond estimations from rietvert analyzes of X-ray diffraction data, done the copper antiferromagnetic nuclear resonance (AFNMR) and nuclear quadrupole resonance (NQR). As their results, it has been found that there occurs a hybridization (mixing) between Pr 4f and copper 3d and/or oxygen 2p orbitals, resulting in the localization of holes doped to the system through a kind of Fermi surface effect (Kondo effect).As a result of our investigation of compounds based on the combination of rare earth element Yb and 3d transition metal Cu, the intermeta … More llic compound system YbCu_<5-x>Ag_x with the cubic BeCu_5-type structure has been found to show a dense Kondo effect in the whole composition x-range, where the characteristic temperature (Kondo temperature) varies systematically. On the other hand, isostructural system YbCu_<5-x>In_x has been found to show a crossover from a dense Kondo system (x=0) to a fluctuating system accompanied with the valence transition (x=1). Furthermore, the Kondo insulator-like behavior was observed in the intermediate composition. Typical phenomena due to the frustration effect have also been observed in the strongly correlated electron system, R_2V_2O_7 and R_2Ru_2O_7 (R being rare earth element) compounds with the pyrochlore-type structure.As for improving the apparatus, we made a improvement of NMR apparatus by introducing the wide-band power amplifier and the digital memory system in order to be able to measure the relaxation time in the last year. In this year, we have introduced the 8T superconducting magnet with high resolution in order to measure high resolution NMR.By using this apparatus, we have performed the ^<17>O NMR in the ruthenate system and investigated the dynamical behavior of Ru spins by measuring ^<17>O nuclear spin-lattice relaxation time. As a result, it has been found that the invar-type compound SrRuO_3 does not follow the mean-field localized moment model as well as the spin fluctuation theory which is valid in the case of weak itinerant electron magnets, implying that the system is in the intermediate metallic electron state. Furthermore, although CaRuO_3 was thought to have strong antiferromagnetic correlation, its dynamical behavior indicates that this compound is a Pauli paramagnet being very close to the ferromagnetic state with a strong ferromagnetic spin correlation. Our result of the specific heat measurement on CaRuO_3 can be explained by the itinerant electron spin fluctuation theory, which is consistent with the NMR result.Furthermore, from now on, we have a plan to investigate the Yb chalcogenide compounds from the microscopic and dynamical view points. Less