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

In this study, we performed high-pressure electrical resistivity measurements of polycrystalline FeSe in the pressure range of 1-16.0 GPa at temperatures of 4-300 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(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 (similar to 1.38 angstrom). 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.