Anomalous Pressure Dependence of the Superconducting Transition Temperature in FeSe1-x Studied by DC Magnetic Measurements

The pressure dependence of superconducting transition temperature T-c has been investigated through the DC magnetic measurements for FeSe0.8 and FeSe1.0. For both samples, with increasing pressure P, the T-c-P curve exhibits a two-step increase, showing a local maximum of similar to 11 K at P similar to 1.0 GPa and a rapid increase with an extremely large pressure coefficient for P > 1.5 GPa. T-c saturates at similar to 25 K (21 K) in FeSe1.0 (FeSe0.8) for P > 3GPa. A rapid decrease in superconducting volume fraction is observed with an increase in T-c above 1.5 GPa, suggesting the presence of electronic inhomogeneity.