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

具有金属导电性的过渡金属化合物通常处于混合价态。在硫族铜体系中，含有稀土元素Pr的化合物具有半导体性质，尽管它们的组成中实现了混价态。为了阐明这一原因，我们利用库仑滴定法测量了阳离子的价值，通过x射线衍射数据的里伏特分析进行了价键估计，并进行了铜反铁磁核磁共振（AFNMR）和核四极共振（NQR）。结果发现，Pr 4f与铜的3d轨道和/或氧的2p轨道之间发生杂化（混合），通过一种费米表面效应（近藤效应）使掺杂到体系中的空穴局部化。通过对稀土元素Yb与三维过渡金属Cu结合化合物的研究，发现具有立方becu_5型结构的中间体化合物体系YbCu_<5-x>Ag_x在整个组成x范围内表现出致密的近藤效应，其特征温度（近藤温度）变化较为系统。另一方面，发现同构体系YbCu_<5-x>In_x表现出从致密近道体系（x=0）到伴随价跃迁的波动体系（x=1）的交叉。此外，在中间组分中观察到近藤绝缘子样行为。在强相关电子体系中，R_2V_2O_7和R_2Ru_2O_7 （R为稀土元素）具有焦绿石型结构的化合物中也观察到典型的受挫效应现象。在仪器的改进方面，我们在去年对核磁共振仪进行了改进，引入了宽带功率放大器和数字存储系统，以便能够测量弛豫时间。今年，我们推出了高分辨率的8T超导磁体，用于测量高分辨率核磁共振。利用该装置，我们在钌酸盐体系中进行了^<17>O核磁共振，并通过测量^<17>O核自旋-晶格弛豫时间研究了钌自旋的动力学行为。结果表明，非晶型化合物SrRuO_3既不符合平均场局域矩模型，也不符合弱流动电子磁体情况下有效的自旋涨落理论，表明该体系处于中间金属电子态。此外，虽然CaRuO_3被认为具有很强的反铁磁相关，但其动力学行为表明该化合物是一个非常接近铁磁状态的泡利顺磁体，具有很强的铁磁自旋相关。CaRuO_3的比热测量结果可以用电子自旋涨落理论解释，与核磁共振结果一致。今后，我们计划从微观和动力学的角度对硫系镱化合物进行研究。少

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