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

点击翻译按钮获取中文摘要

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.