Anomalous pressure dependence of the superconducting transition temperature of β-pyrochlore AOs2O6 oxides

High-pressure effects on the superconducting transitions of β-pyrochlore oxide superconductors AOs2O6 (A=Cs,Rb,K) are studied by measuring resistivity under high pressures up to 10 GPa. The superconducting transition temperature Tc first increases with increasing pressure in every compound and then exhibits a broad maximum at 7.6 K (6 GPa), 8.2 K (2 GPa), and 10 K (0.6 GPa) for A=Cs, Rb, and K, respectively. Finally, the superconductivity is suppressed completely at a critical pressure near 7 GPa and 6 GPa for A=Rb and K and probably above 10 GPa for A=Cs. Characteristic changes in the coefficient A of the T2 term in resistivity and residual resistivity are observed, both of which are synchronized with the corresponding change in Tc.