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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 Tc from zero-resistivity temperatures revealed that Tc shows a slightly distorted dome-shaped curve, with maximum Tc (30 K) at 6 GPa, which is lower than a previously reported Tc value (~37 K). With the application of pressure, the temperature dependence of resistivity above Tc changes dramatically to a linear dependence; a non-Fermi-liquid-like "high-Tc" phase appears above 3 GPa. We found a striking correlation between Tc and the Se height: the lower the Se height, the more enhanced is Tc. Moreover, this relation is broadly applicable to other iron pnictides, strongly indicating that high-temperature superconductivity can appear only around the optimum anion height (~1.38A). On the basis of these results, we suggest that the anion height should be considered as a key determining factor of Tc of iron-based superconductors containing various anions.