High-pressure studies on Tc and crystal structure of iron chalcogenide superconductors.

The superconducting transition temperature, Tc, in iron-based solids can be enhanced by applied pressure: Tc increases from 8 to 37 K for the 11-type FeSe when the pressure is raised from 0 to 4 GPa. High-pressure studies can elucidate the mechanism of superconductivity in such novel materials. In this paper, we present a high-pressure study of Fe(Se1-x Te x ) and Fe(Se1-x S x ). In the case of Fe(Se1-x Te x ), the maximum Tc under high pressure did not exceed the Tc of FeSe, which can be attributed to the structural transition to the monoclinic phase. For Fe(Se1-x S x ) (0 < x < 0.3), Tc exhibited a significant increase with pressure; however, the maximum Tc under high pressure did not exceed the Tc of FeSe. This may be due to the disorder induced by substituting S for Se, which is similar to the pressure effect on Tc for the 1111-type superconductor Ca(Fe1-x Co x )AsF. The Tc of Fe(Se1-x S x ) showed a complex behavior below 1 GPa, first decreasing and then increasing with increasing pressure. From high-pressure x-ray diffraction measurements, the Tc (P) curve was correlated with the local structural parameter.