  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.
