Charge order and superconductivity in vanadium oxides

We have performed high-pressure experiments on β-vanadium bronzes, β-A0.33V2O5 (A=Li+, Na+, Ag+, Ca2+, Sr2+ and Pb2+) which are quasi-one-dimensional conductors. All of compounds except β-Pb0.33V2O5 show metal-insulator transitions accompanied by charge ordering. Under hydrostatic high pressure the charge ordered phase is suppressed and the superconducting phase appears. The superconducting phase adjacent to the charge ordered phase implies an important role for charge fluctuation in the superconductivity of β-A0.33V2O5. The presence/absence of superconductivity in β-A0.33+V2O5/β-A0.332+V2O5 suggests an optimum doping level (V4+/V5+=1/5) for the superconductivity in β-vanadium bronzes.