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

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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.