Charge order and superconductivity in vanadium oxides

摘要

We have performed high-pressure experiments on beta-vanadium bronzes, beta-A_(0.33)V_2O_5 (A = Li~+, Na~+, Ag~+, Ca~(2+), Sr~(2+) and Pb~(2+)) which are quasi-one-dimensional conductors. All of compounds except beta-Pb_(0.33)V_2O_5 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 beta-A_(0.33)V_2O_5. The presence/absence of superconductivity in beta-A_(0.33)~+V_2O_5/beta-A_(0.33)~(2+)V_2O_5 suggests an optimum doping level (V~(4+)/V~(5+) = 1/5) for the superconductivity in beta-vanadium bronzes.