Pressure-induced high- T c superconducting phase in FeSe: Correlation between anion height and T c

In this study, we performed high-pressure electrical resistivity measurements of polycrystalline FeSe in the pressure range of 1\char21{}16.0 GPa at temperatures of 4\char21{}300 K. A precise evaluation of ${T}_{c}$ from zero-resistivity temperatures revealed that ${T}_{c}$ shows a slightly distorted dome-shaped curve with maximum ${T}_{c}^{\text{offset}}$ (30 K) at 6 GPa. With the application of pressure, the temperature dependence of resistivity above ${T}_{c}$ changes dramatically to a linear dependence, that the high-${T}_{c}$ phase appears above 3 GPa. We found a striking correlation between ${T}_{c}$ and the Se height: the lower the Se height, the more enhanced is ${T}_{c}$. Moreover, this relation is broadly applicable to other iron pnictides, strongly indicating that high-temperature superconductivity can appear only around the optimum anion height $(\ensuremath{\sim}1.38\text{ }\text{\AA{}})$. On the basis of these results, we suggest that the anion height should be considered as a key determining factor of ${T}_{c}$ of iron-based superconductors containing various anions.