NMR STUDY OF THE COEXISTENCE OF MAGNETISM AND SUPERCONDUCTIVITY IN STRONGLY CORRELATED ELECTRON SYSTEM

本项目利用NMR/NQR技术研究了强关联电子系统中磁性与超导电性的相互作用，在重费米子化合物Ce（Rh_<1-x>Ir_x）In_5中，所获得的结果为反铁磁有序与超导电性在0.35&lt;x&lt;0.55范围内的微观共存提供了有力的证据。当x=0.5时，在3 K观察到T_N，随后在T_c = 0.9K时出现超导电性。当x=0.45时，T_c达到最大值0.94 K，其中T_N最大（0.40 K）。结果表明，在T_N处，原子核自旋-晶格弛豫速率1/T_1出现一个宽峰，在T_c以下，原子核自旋-晶格弛豫速率随T_3变化，这表明在CeIrIn_5（T_c=0.4 K）中存在非常规的超导电性，而在有机体系（TMTSF）_2PF_6中，磁性和超导电性在宏观上是相分离的，在铜氧化物超导体Bi_2Sr_<2-x>La_xCuO_6中（x= 0.65，0.40，0.25，0.15和0），我们施加了高达43 T的强磁场来抑制超导性，并研究了低温正常态。我们发现赝能隙（PG）和超导态是共存的物质，并对Co氧化物超导体Na_xCoO_2 ~（1.3）H_2O在较宽的Na含量范围（x=0.25-0.34）进行了Co-NQR测量。我们发现T_c随x的减小而增加，但当x &lt;0.28时，T_c达到一个平台。在x=0.26的样品中，1/T_1在T_c以下和T &lt;$T_c/6以下显示T^3变化，这明确地表明超导（SC）能隙函数中存在线节点。然而，对于较大或较小的x，1/T_1偏离T^3的变化低于T^2 K，即使T_c（14.7 K）是相似的，这表明额外的激发，由于可能的竞争附近的磁或电荷秩序。

In this project we have investigated the interplay between magnetism and superconductivity in strongly correlated electron systems using the NMR/NQR technique.In the heavy fermion compounds, Ce(Rh_<1-x>Ir_x)In_5, the obtained results provide strong evidence for the microscopic coexistence of antiferromagnetic order and superconductivity in the range of 0.35<x<0.55. Specifically, for x=0.5, T_N is observed at 3 K with a subsequent onset of superconductivity at T_c =0.9 K. T_c reaches a maximum of 0.94 K at x=0.45 where T_N is found to be the highest (〜4.0 K). The nuclear spin-lattice relaxation rate 1/T_1 shows a broad peak at T_N and follows a T^3 variation below T_c, the latter property indicating unconventional SC as in CeIrIn5 (T_c=0.4 K).In contrast, in the organic system (TMTSF) _2PF_6, magnetism and superconductivity are macroscopically phase separated.In the Copper-oxide superconductors Bi_2Sr_<2-x>La_xCuO_6 (x=0.65,0.40,0.25,0.15,and 0), we applied strong magnetic fields up to 43 T to suppress superconductivity and investigated the low temperature normal state. We find that the pseudogap (PG) and the superconducting state are coexisting matters.We have also carried out Co-NQR measurements on the cobalt oxide superconductors Na_xCoO_2^*1.3H_2O over a wide Na content range x=0.25-0.34. We find that T_c increases with decreasing x but reaches to a plateau for x <0.28. In the sample with x=0.26, 1/T_1 shows a T^3 variation below T_c and down to T〜T_c/6, which unambiguously indicates the presence of line nodes in the superconducting (SC) gap function. However, for larger or smaller x, 1/T_1 deviates from the T^3 variation below T〜2 K even though the T_c (〜4.7 K) is similar, which suggests additional excitations due to possible competing magnetic or charge order nearby.