Metal-insulator transitions induced by pressure for perovskite cobalt oxides

具有CoO_6八面体网状结构的钴氧化物由于具有各种显著的物理特性而引起了人们的关注。本研究的主要课题是寻找控制钴氧化物电子态的方法。我们研究了(1)钙钛矿型氧化钴Pr_<1-x>Ca_xCoO_3的金属-绝缘体（M-I）跃迁机理和(2)氧化钴超导体Na_xCoO_<2-y>H_2O超导序参量的对称性。(1)在常压和高压条件下对（Pr_<1- Y >R_y）_<1-x>Ca_xCoO_3（R=稀土元素和Y）进行了结构研究和磁输运测量。讨论了Co离子周围的局部结构与Co^<3+ b>离子的电子（或自旋）态之间的密切关系。我们发现Pr和Co原子对于M-I跃迁的发生都是必需的，这意味着Pr 4f-O 2ppπ杂化可能与M-I跃迁的发生有关。(2)进行了^<59>Co-NMR Knight shift K和核磁弛豫率1/T_1的测量。研究了杂质对Na_xCoO_<2-y>H_2O超导转变温度T_c的影响。对于磁场方向，H‖和CoO_2平面，随着T的减小，K在T_c(H)以下减小，表明超导电子对处于单重态。1/T_1T-T曲线的相干峰小得可以忽略不计，乍一看，超导序参量有一个节点。然而，非磁性杂质对T_c的影响很小，不能简单地用这种各向异性的序参量来解释。现在，重要的是要始终如一地理解这些结果。我们推测，一种可能的方法是考虑s波超导体中准粒子展宽的影响，因为展宽带来了可以忽略不计的小相干峰。

Cobalt oxides with network of CoO_6 octahedra have attracted interest, because they exhibit various notable physical characteristics. The main subject of this research is the search of the method to control the electronic state of the cobalt oxides. We have studied (1)the mechanisms of the metal-insulator(M-I) transition of the perovskite cobalt oxide Pr_<1-x>Ca_xCoO_3 and (2)the symmetry of the superconducting order parameter of a cobalt oxide superconductor Na_xCoO_<2-y>H_2O.(1)The structural studies and the magnetic and transport measurements have been carried out on (Pr_<1-y>R_y)_<1-x>Ca_xCoO_3(R=rare earth elements and Y) at ambient pressure and high pressure. The intimate relationship between the local structure around the Co ions and the electronic (or spin) state of Co^<3+> ions has been discussed. We have found that both the Pr and Co atoms are necessary for the occurrence of the M-I transition, which implies that the Pr 4f-O 2pπ hybridization may be relevant to the occurrence of the transition.(2)Measurements of ^<59>Co-NMR Knight shift K and nuclear magnetic relaxation rate 1/T_1 have been carried out. Studies of the impurity effect on the superconducting transition temperature T_c have also been carried out on Na_xCoO_<2-y>H_2O. For the magnetic field directions, H‖ and ⊥ CoO_2 plane, K decreases below the T_c(H) with decreasing T, indicating that the superconducting electron pairs are in the singlet state. The negligibly small coherence peak of the 1/T_1T-T curve indicates, at first sight, that the superconducting order parameter has a node. However, the small effect of nonmagnetic impurities on T_c cannot simpty be explained by such kinds of anisotropic order parameter. Now, it is important to understand these results consistently. One possible way is, we speculate, to consider effects of quasi-particle broadening in s-wave superconductors because the broadening brings about the negligibly small coherence peak.