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

稀土-过渡金属硫属化物的混合价态和电导率

• 批准号: 07454186
• 负责人: YOSHIMURA Kazuyoshi
• 金额: $ 4.42万
• 依托单位: KYOTO UNIVERSITY
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (B)
• 财政年份: 1995
• 资助国家: 日本
• 起止时间: 1995 至 1996
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-07454186/
• 关键词: rare-earth chalcogenide; mixed- valent state; dense Kondo effect; strongly correlated electron; nuclear magnetic resonance; transition metal compound; itinerant-electron magnetism; 混合原子価; フェルミ液体; 核磁気共鳴; 金属化合物

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

表现出金属导电性的过渡金属化合物通常已知为混合价态，在铜硫属化物系中，含有稀土元素Pr的化合物虽然具有实现混合价态的组成，但变为半导体。为了阐明这个原因，我们利用库仑滴定法测量了阳离子的价态，根据X射线衍射数据的里特维特孪晶进行了价键估计，并对铜进行了分析。反铁磁共振(AFNMR)和核四极共振(NQR)的结果发现，Pr 4f 和铜3d和/或氧2p轨道之间发生杂化(混合)，导致掺杂空穴的局域化。通过一种费米表面效应（近藤效应）对系统产生影响。我们对基于稀土元素 Yb 和 3d 过渡金属 Cu 组合的化合物的研究结果表明，具有立方BeCu_5型结构的间位化合物体系YbCu_<5-x>Ag_x被发现在整个成分x范围内表现出密集的Kondo效应，其中特征温度（Kondo温度）变化明显。另一方面，同构体系 YbCu_<5-x>In_x 被发现显示出从致密近藤体系 (x=0) 到伴随价态跃迁的波动体系的交叉(x=1) 此外，在强关联电子体系R_2V_2O_7和R_2Ru_2O_7（R为稀土元素）化合物中也观察到了由于挫败效应而产生的类似近藤绝缘体的行为。在仪器的改进方面，我们对NMR仪器进行了改进，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。去年的时候，我们引进了高分辨率的8T超导磁体，用于测量高分辨率NMR。通过使用该设备，我们在钌酸盐体系中进行了^ 17 O NMR并研究了它。通过测量^ 17 O核自旋晶格弛豫时间来研究Ru自旋的动力学行为。结果发现，殷钢型化合物SrRuO_3不遵循平均场局域矩。模型以及自旋涨落理论在弱巡回电子磁体的情况下有效，这意味着系统处于中间金属电子态。此外，尽管CaRuO_3被认为具有强反铁磁相关性，但其动力学行为表明这一点。化合物是一种泡利顺磁体，非常接近铁磁态，具有很强的铁磁自旋相关性，我们对 CaRuO_3 的比热测量结果可以用流动电子来解释。自旋涨落理论，与NMR结果一致。此外，今后我们计划从微观和动力学角度研究Yb硫系化合物。

项目成果

R.L.Corey: "^<63>Cu(2)Nuclear Quadrupole and Nuclear Magnetic Resonance Studies of YBa_2Cu_4O_8 in the Normal and Superconducing States" Phys.Rev.B. 53(9). 5907-5914 (1996)

R.L.Corey：“^<63>Cu(2)核四极杆和 YBa_2Cu_4O_8 在正常和超导状态下的核磁共振研究”Phys.Rev.B。

K. Yoshimura: "Supercondueting Properties of the La_<1+x>Ba_<2-x>Cu_3O_y System" Advances in Superconductivity VII(Springer-Verlag). 377-380 (1995)

K. Yoshimura：“La_<1x>Ba_<2-x>Cu_3O_y 系统的超导特性”Advances in Superconductivity VII（Springer-Verlag）。

M. Kato: "Coulometric Determination of Pr in the Ln-Ba-Cu-O System" Proceedings (ISS'95), Advances in Superconductivity. VIII. 359-362 (1996)

M. Kato：“Ln-Ba-Cu-O 系统中 Pr 的库仑测定”论文集 (ISS95)，超导进展。

H. Aruga-Katori: "Field-induced Valence Transition in YbIn_<1-x>Ag_xCu_4" Journal of Magnetism and Magnetic Materials. 140-144. 1245-1246 (1995)

H. Aruga-Katori：“YbIn_<1-x>Ag_xCu_4 中的场诱导价态跃迁”《磁性与磁性材料杂志》。

H. Kageyama: "Tn Dependence on Hole Concentration in the La_<2-x>Sr_xCuO_<4+8> System" Journal of Physical Society of Japan. 64. 2144-2146 (1995)

H. Kageyama：“La_<2-x>Sr_xCuO_<4 8>体系中Tn对空穴浓度的依赖性”日本物理学会杂志。