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

The pressure dependence of superconducting transition temperature $T_{rm c}$ has been investigated through the DC magnetic measurements for FeSe$_{0.8}$ and FeSe$_{1.0}$. For both samples, with increasing pressure $P$, the $T_{rm c}$$-$$P$ curve exhibits a two-step increase, showing a local maximum of $sim$11 K at $P$$sim$1.0 GPa and a rapid increase with an extremely large pressure coefficient for $P$$>$1.5 GPa. $T_{rm c}$ saturates at $sim$25 K (21 K) in FeSe$_{1.0}$ (FeSe$_{0.8}$) for $P$$>$3 GPa. A rapid decrease in superconducting volume fraction is observed with an increase in $T_{rm c}$ above 1.5 GPa, suggesting the presence of electronic inhomogeneity.