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索f电子化合物中非常规超导电性和外加高压引起的微小磁矩的机制。本项目包括两个样品组：(1)CeTiN_5(T=Rh，Co，Ir)和CeRh_2Si_2的压致超导体和(2)具有微小磁矩的Uru_2Si_2。本文对CeCoIn_5、CeRn_5和CeRh_n_5在1.7 Gpa以上的高压下进行了超导状态下的核磁共振测量。对于处于超导态的CeTiN_5，其核磁驰豫速率具有T^3依赖关系，自旋磁化率随冷却而减小。这些结果表明，这些体系具有d波超导电性。而正常态的CeCoIn_5正好处于反铁磁不稳定状态，而CeRh_n_5和CeRn_5在临界压力以上处于近乎反铁磁区。接下来，我们发现了来自Uru_2Si_2反铁磁区的新的^&lt；29&gt；Si核磁共振信号，以及以前报道的来自顺磁区的Si核磁共振信号。这些信号强度根据温度和施加的压力相互竞争。这一结果为特征温度以下反铁磁有序的空间不均匀发展提供了证据。U的有序磁矩约为0.4μ，B，这大约比许多中子衍射实验测得的0.03μB大一个数量级。这幅反铁磁区不均匀发展的图景将把我们带入理解隐藏有序参数的新场景，这是自85年发现这种有趣的材料以来一直没有解决的问题。

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.