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 hightemperature superconductivity can appear only around the optimum anion height (∼1.38Å). 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.
High-pressure resistivity experiments were performed on FeSe 0:5 Te 0:5 . The onset temperature of T c (T onset c ) increases rapidly from 13.5 to 26.2 K upon applying pressure up to 2 GPa. Above 2 GPa, T c decreases linearly and the metallic phase was observed at P ¼ 14 GPa. The same relationship between normalized T c and pressure in both FeSe 0:5 Te 0:5 and FeSe suggests a universal pressure dependence in this system. This report presents the first observation of a phase transition from the superconducting to the metallic phase on the P-T c phase diagram.
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