Origin of the metal-insulator transition of orbital-ordered spin-singlet system BaVS3

三角晶格(六方)过渡金属硫化物BaVS3在70K时表现出金属-绝缘体的转变，这种转变的起源与自旋、电荷和轨道自由度以及/或晶体结构的低维和自旋受挫有关，这引起了许多研究者的关注。因此，对其起源的阐明是值得探讨的。晶体结构为六方钙钛矿型，钒原子沿c轴形成一维链，在c面上形成三角形晶格。钒的形式价态是4+，因此处于自旋1/2状态。本项目的目的是生长高质量的BaVS3晶体，并利用各种复杂的实验方法弄清楚金属绝缘体相变的起源。在早期阶段，基于文献报道的实验结果和我们的核磁共振结果，我们提出了一个模型，即本项目标题中出现的轨道有序自旋-单重态(由自旋受挫驱动)。然而，经过一些实验，我们得到了与模型明显不一致的新结果。于是，没想到的是，我们不得不一步一步地核对报道的实验结果。因此，我们发现许多已报告的数据和他们的分析不充分。由新的实验结果得出如下结论：BaVSS是一种三维各向异性导体，其中d个轨道在c方向和c平面上都形成了带。这些带具有相当不同的特征，但都对金属绝缘体的转变做出了贡献。换句话说，BaVSS是一种非常独特的各向异性金属，其中类一维和类二维相互作用产生了复杂的电子性质。

A triangular-lattice (hexagonal) transition-metal sulfide BaVS3 shows a metal-insulator transition at 70 K. The origin of this transition would be related to spin, charge and orbital degrees of freedom, and/or low-dimensionality of the crystal structure and spin frustration, which has been attracting much attention of many researchers. Therefore the elucidation of the origin is worth investigating. The crystal structure is the hexagonal perovskite type, where vanadium atoms form one-dimensional chains along the c axis and a triangular lattice in the c plane. The formal valence of vanadium is 4+, hence in the state of spin 1/2. The purpose of this project is to grow high-quality crystals of BaVS3 and to make clear the origin of the metalinsulator transition with use of various sophisticated experimental methods. In an early stage, based on experimental results reported in literature and our NMR results, we proposed a model, which is the orbital-ordered spin-singlet state (driven by spin frustration) as appeared in the title of this project. After some experiments, however, we obtained new results which are apparently inconsistent with the model. Thus, unexpectedly, we had to check the reported experimental results step by step. As a result we found many reported data and their analyses insufficient. The conclusion obtained from new experimental results are as follows: BaVSS is a three-dimensional but anisotropic conductor, in which d orbitals form bands both along the c direction and in the c plane. These bands have considerably different characters but both contribute to the metalinsulator transition. In other words, BaVSS is a very unique anisotropic metal, where the interplay between one-dimensional-like and two-dimensional-like interactions gives rise to complicated electronic properties.