Search of tow-dimensional metallic state in a transition metal sulfide BaVS3 showing a metal-insulator transition

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In BaVS3, vanadium atoms with spin 1/2 form one-dimensional chains, which also form a triangular lattice. In spite of the simple structure, its physical properties are quite complicated, because a number of degrees of freedom such as orbital degeneracies and charge instability compete with structural one-dimensionality and spin frustration etc. The reason why the physics of BaVS3 progresses quite slowly was the unavailability of single crystals with good quality. Several yeas ago, however, the tellurium flux method was developed, leading the investigation to a new stage. The primary purpose of the present study is to set up new facilities to grow BaVS3 single crystals at Graduate School of Material Science, University of Hyogo, which was fulfilled by our recent efforts.In BaVS3, there are two 3d bands (orbitals) with different characteristics, both of which participate in the metal-insulator transition. In the present study, we noted the different sensitivity of the bands against the e … More xternal field. We expected that the sensitivity of the dx2-type band responsible for the one-dimensional conductivity and the strongly correlated dxy-type band is different, resulting in the formation of an intermediate state when the metallic state is destroyed by the field. In an early state, we called the state "a two-dimensional metallic state". The one of the purposes was to detect the state experimentally. This subject has been studied as a collaboration work with Narumi and Kindo at ISSP, University of Tokyo. The results have been summarized in a recently published paper. It was found that the notation of "the two-dimensional metallic state" is not appropriate, but should be called a "three-dimensional metallic state with poor conductivity". In the published paper, we call it "a poor conductor".Our study of BaVS3 was initiated by the observation of multiple nuclear magnetic resonance signals in a relatively low frequency range for the low-temperature insulating state. Although we interpreted them as NQR (namely no internal field), our following neutron diffraction experiments established that the ground state is antiferromagnetic, and therefore the nuclear resonances correspond to finite internal fields. This result posed new questions : (1) Why are the internal fields anomalously small? (2) Why are the magnetic sites separated. We now speculate that the metal-insulator transition of BaVS3 is a transition from a metal to a certain kind of metal cluster. Since there has been no systematic NMR investigation on magnetic metal clusters, we have therefore selected transition-metal sulfides with the GaMo4S8 type structure, in which transition-metal tetrahedral clusters are implanted, and investigated their NMR systematically to compare with that of typical metals and insulators. Less