Low-temperature specific heat of the β-pyrochlore oxide superconductors under high pressure

We report the results of the low-temperature specific heat measurements of the single crystalline β-pyrochlore oxide superconductors AOs2O6 (A=K, Rb, and Cs) under high pressure up to 13 GPa. We find that superconducting transition temperature (Tc) monotonically increases for CsOs2O6 and RbOs2O6, while the one for KOs2O6 decreases by applying the pressure. With further increasing the pressure, Tc is suddenly suppressed at the same lattice volume for all compounds, concomitant with the first-order structural phase transition.