NMR Study on Metal-Insulator Transition and High T_C Superconductivity

在本项目中，苏黎世大学和北海道大学通过核磁共振测量研究了强相关体系（如季化合物YNi_2B_2C和RPt_2B_2C）的异常电子性质。^<11 b> B在低温下T_1T和Knight位移的增强不是由于超导YNi_2B_2C族中三维自旋涨落的影响，而是由于其能带结构的特殊性。我们还制备了一些CuIr_2S_4的Cu尖晶石化合物，这些化合物表现出金属-绝缘体（MI）转变。与俄罗斯课程组合作制备了富同位素^<33>S的CuIr_2S_4样品。从Cu-NMR谱线宽和Knight位移的结果，我们证实了CuIr_2S_4在绝缘状态下不存在磁矩。我们还得到了CuIr_2 (S_<1-x>Se_x)_4的MI跃迁相图。在北京举行的国际高tc超导材料与机制会议上，讨论了金属-绝缘体跃迁中的磁态和局域电子态。

In this project, unusual electronic properties of the strongly correlated systems such as quaternary compound YNi_2B_2C and RPt_2B_2C have been investigated by NMR measurement at both Zurich university and Hokkaido university. The enhancements of T_1T and Knight shift of ^<11>B at low temperature are not attributed to the effect of 3d spin fluctuations in the superconducting YNi_2B_2C families, but are attributed to peculiarities of their band structure.We have also prepared some Cu spinel compound of CuIr_2S_4 which shows metal-insulator (MI) transition. Sample of CuIr_2S_4 with enriched isotope of ^<33>S are prepared with collaboration of Russian group. From the results of line width and Knight shift of Cu-NMR,we confirmed no magnetic moment in the insulating state of CuIr_2S_4. We obtained also phase diagram of the MI transition of CuIr_2 (S_<1-x>Se_x)_4. Magnetic and local electronic states at the Metal-Insulator transition have been discussed and presented at the International Conference of Materials and Mechanism of High Tc-Superconductivity at Beijing.