Relationship between unconventional superconducting phase and magnetic phase in strongly correlated electron compounds

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During this research project, we have performed NMR and nuclear quadrupole resonance (NQR) on various strongly correlated electron compounds in order to investigate superconducting properties and spin dynamics in the normal state. Especially, bilayered hydrate (BLH) cobaltate superconductors (Na_x(H_3O)_zCoO_<2-y>H_2O) and cuprate superconductors (La_<2-x>Sr_xCuO_4) are intensively studied.1) Co NMR/NQR Studies on Na_x(H_3O)_zCoO_<2y>H_2OWe found a weak magnetism in a nonsuperconducting BLH cobaltate sample with Co-NQR measurements. This sample shows the divergence of 1/T_1 T at T_M〜5.5 K, below which the Co-NQR spectrum is broadened due to the appearance of the internal field at the Co site. We also found that the NQR frequency v_〓 correlates with magnetic fluctuations just above T_c from measurements of NQR spectra and 1/TlT in more than 10 BLH samples with various T_c and T_M. Phase diagram of BLH cobaltate superconductors was developed by plotting T_c and T_M against v_〓. It was shown that superconductivity with the highest T_c is observed in the vicinity of a magnetic phase, suggesting that magnetic fluctuations play an important role in the occurrence of unconventional superconductivity.2) Static magnetism and superconductivity in La_<2-x>,Sr_xCuO_4Relationship between magnetism and superconductivity was investigated with transverse-field (TF) μSR in La_<1.93>Sr_<0.07>CuO_4 (T_c〜17 K), which is close to the border of two phases. We found the occurrence of the static magnetism at T_g〜4K far below T_c. The magnetic volume fraction was estimated to be more than 90 %. We conclude that the static magnetism with staggered correlation coexists with superconductivity.