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

在本研究项目中，我们对各种强相关电子化合物进行了核磁共振和核四极共振（NQR），以研究正常状态下的超导性质和自旋动力学。特别是对双层水合物（BLH）钴酸盐超导体（Na_x(H_3O)_zCoO_<2-y>H_2O）和铜酸盐超导体（La_<2-x>Sr_xCuO_4）进行了深入研究。1)对Na_x(H_3O)_zCoO_<2y>H_2OWe的Co NMR/NQR研究发现，非超导钴酸BLH样品具有弱磁性。该样品在T_M ~ 5.5 K处发散为1/T_1 T， Co- nqr谱在此以下由于Co位点出现了内部场而展宽。我们还通过对10多个不同T_c和T_M的BLH样品的NQR光谱和1/TlT的测量发现，NQR频率v_〓与T_c以上的磁波动相关。利用T_c和T_M与v_〓的关系，建立了BLH钴酸盐超导体的相图。结果表明，在磁相附近可以观察到T_c最高的超导性，表明磁波动在非常规超导性的发生中起重要作用。2) La_<1.93>Sr_<0.07>CuO_4 （T_c ~ 17 K）的横场(TF) μSR研究了La_<2-x>， sr_xcuo_4中静磁性和超导性的关系，发现其接近两相边界。我们发现在远低于T_c的T_g ~ 4K处存在静磁性。磁体积分数估计在90%以上。我们得出了交错相关的静态磁性与超导性共存的结论。

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.