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

在这个项目中，我们用核磁共振/核磁共振技术研究了强关联电子系统中磁性和超导电性之间的相互作用。在重费米子化合物Ce(Rh_&lt；1-x&gt；Ir_x)In_5中，得到的结果为0.35&lt；x&lt；0.55范围内反铁磁有序和超导的微观共存提供了强有力的证据。当x=0.5时，在3K观察到T_N，在T_c=0.9K时超导开始，T_c在x=0.45时达到最大值0.94K，其中T_N最高(~4.0K)。核自旋-晶格驰豫速率1/T_1在T_N处有一个宽峰，并在T_c以下呈T^3变化，后者表明了与CeIrIn5(T_c=0.4K)不同的Sc。相反，在有机体系(TMTSF)_2PF_6中，磁性和超导电性在宏观上是分离的。在铜氧化物超导体Bi2Sr_&lt；2-x&gt；La_xCuO_6(x=0.65，0.40，0.25，0.15和0)中，我们施加了高达43T的强磁场来抑制超导电性，并研究了低温正常态。我们还对钴氧化物超导体NaxCoO2^*1.3H2O在较宽的Na含量x=0.25-0.34范围内进行了Co-NQR测量。我们发现T_c随x的减小而增大，但在x&lt；0.28处达到一个平台值。在x=0.26的样品中，1/T_1在T_c以下有T^3的变化，直到T~T_c/6，这明确地表明在超导(SC)能隙函数中存在线节。然而，对于较大或较小的x，尽管T_c(~4.7K)相似，但1/T_1偏离T~2K以下的T^3变化，这表明附近可能存在竞争磁序或电荷序的额外激发。

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.