Stripe order and metal-insulator transition in the two-dimensional layered Ni oxide

在这项研究中，主要致力于了解在高T_c铜酸盐超导体及其等结构材料--二维镍氧化物中观察到的独特条带有序的起源。为了考察过渡金属氧化物中条带有序化的普遍性，选取了等结构二维晶系中的Co和Mn氧化物作为研究对象，用中子衍射法进行了研究。特别是，我们成功地生长了高质量的二维钴酸盐Pr_&lt；2-x&gt；Ca_xCoO_4单晶样品，详细研究了这些化合物的性质和自旋有序性。对于x&gt；1/2，形成了棋盘式电荷有序(CCO)，并且随着掺杂率的增加，观察到CCO的无序化，并有进一步金属化的趋势。另一方面，对于x&lt；1/2，CCO更改为条带顺序。这种转变伴随着Co离子的自旋态转变，详细的分析表明，条带有序是由电子形成的，而不是报道的铜酸盐和镍酸盐中的空穴条带。这一观察结果表明，在条纹有序中存在空穴-电子对称性。

In this research, main efforts were focused on understanding the origin of the unique stripe order observed in high Tc cuprate superconductor and its isostructural material, a two-dimensional Ni oxide. To examine the universality of the stripe ordering in transition metal oxides, the Co and Mn oxides in the isostructural two-dimensional crystal family were selected to study with use of neutron diffraction technique. In particular, we succeeded to grow high quality single crystal samples of the two-dimensional cobaltate Pr_<2-x>Ca_xCoO_4, the chaqrge and spin order in these compounds were investigated in detail. For x > 1/2, the checkerboard-type charge order (CCO) was formed, and as the doping rate is increased, the disordering of the CCO was observed with a tendency of further metallic behavior. On the other hand, for x < 1/2, the CCO changes over to the stripe order. This changeover is accompanied with the spin state transition of the Co ion and detailed analysis revealed that the stripe order is formed by electrons in contrast to the hole stripes reported in cuprates and nickelates. This observation demonstrates that there is a hole-electron symmetry in the stripe ordering.