The study of novel quantum phenomena caused from the competition among multiple ground states in vanadium oxides

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Inβ-vanadium bronzes, we found the competition among a rich variety of electronic states such as charge disordered metallic phases, charge ordered antiferromagnetic insulator phases, charge ordered spin-gapped insulator phases and superconducting phases, in pressure-temperature electronic phase diagrams. Metaphorically speaking, the superconductivity is in bloom in the garden with the competition among various ground states. Particularly, the superconductivity which competes with charge order is the first example in transition metal oxides. It is accompanied by the dimensional crossover and the enhancement of charge fluctuation. In hollandite vanadium oxide, the metal-insulator transition was found. It is accompanied by the charge order and the formation of spin singlet V^4+-V^4+ pairs inherent in vanadium oxides. Furthermore, a very unique metal-insulator transition remaining ferromagnetic in the insulator phase was discovered in hollandite chromium oxide.