NMR Study of Pressure-induced Superconductivity and Pressure-induced Weak Magnetism in f-electron Systems

本课题的主要目的是探讨f电子化合物在高压作用下产生非常规超导性和微小磁矩的机制。本项目由两个样本组组成；(1)具有微小磁矩的CeTIn_5 （T= Rh, Co, Ir）和CeRh_2Si_2的压力感应超导体和(2)Uru_2Si_2。在高于临界压力1.7GPa的高压下，对CeCoIn_5、Celrln_5和CeRhln_5进行了超导态的^<115>NQR测量。对于处于超导态的CeTIn_5，其核磁弛豫率与T^3相关，自旋磁化率随冷却而降低。这些结果表明这些体系表现出d波超导性。而正常状态下的CeCoIn_5刚好处于反铁磁不稳定区，临界压力以上的CeRhln_5和Celrln_5则处于近反铁磁区。接下来，我们在Uru_2Si_2的反铁磁区发现了新的^<29>Si核磁共振信号，并与之前报道的顺磁区Si核磁共振信号相结合。这些信号强度根据温度和施加的压力而相互竞争。这一结果为反铁磁有序在特征温度以下的空间非均匀发展提供了证据。U的有序磁矩约为0.4 μ， B，比许多中子衍射实验测得的0.03 μ B大一个数量级。这幅反铁磁区域的非均匀发展图景将把我们带入理解隐藏有序参数的新场景，这是自1985年发现这种有趣的材料以来仍未解决的问题。

The main purpose of this subject is for exploring the mechanisms of unconventional superconductivity and a tiny magnetic moment induced by applied high pressure in f-electron compounds. This project consists of two-sample groups; (1) pressure induced superconductors of CeTIn_5 (T= Rh, Co, Ir) and CeRh_2Si_2 and (2) Uru_2Si_2 with tiny magnetic moments. Present ^<115>NQR measurements in the super conducting state were performed on CeCoIn_5, Celrln_5 and CeRhln_5 under high pressure above the critical pressure of 1.7GPa. As for CeTIn_5 in the super conducting state, the nuclear magnetic relaxation rate has T^3 -dependence and the spin susceptibility decreases upon cooling. These results indicate that these systems exhibit d-wave superconductivity. On the other hand, CeCoIn_5 in the normal state is located just at antiferromagnetic instability and CeRhln_5 above the critical pressure and Celrln_5 are in the nearly antiferromagnetic region. Next, we have found new ^<29>Si NMR signals coming from the antiferromagnetic region in Uru_2Si_2 together with previously reported Si NMR signals from the paramagnetic region. These signal-intensities compete with each other depending on temperature and applied pressure. This result gives evidence for spatially inhomogeneous development of antiferromagnetic ordering below characteristic temperature. The ordered magnetic moments of U are expected as about 0.4 μ, B, which is of about one order of magnitude larger than 0.03 μ B measured by many neutron diffraction experiments. This picture of inhomogeneous development of antiferromagnetic region will bring us to the new scenario of understanding of hidden order parameters, which is still unsolved since the discovery of this interesting material in '85.