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

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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.