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

在这项研究中，主要的努力集中在理解的独特条纹秩序观察到的高Tc铜氧化物超导体和它的同构材料，一个二维的镍氧化物的起源。为了考察过渡金属氧化物条纹有序的普遍性，选择了同质二维晶体族中的Co和Mn氧化物，利用中子衍射技术进行了研究。特别是，我们成功地生长了二维钴酸盐Pr_ Ca_xCoO_4的高质量单晶样品<2-x>，并详细研究了这些化合物中的电荷和自旋序。当x &gt; 1/2时，形成了棋盘型电荷有序（CCO），随着掺杂率的增加，CCO的无序化趋势进一步向金属化方向发展。另一方面，对于x &lt; 1/2，CCO转换为条带顺序。这种转变是伴随着自旋状态的过渡的Co离子和详细的分析表明，条纹秩序形成的电子在铜酸盐和镍酸盐中报道的孔条纹。这一观察结果表明，有一个空穴-电子对称的条纹有序。

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.