Interplay Between Superconductivity and Magnetism in Cu-Doped FeSe Under Pressure

Superconductivity in iron selenide is very sensitive to any dopants at levels of only a few percent. Here, we demonstrate how iron substitution by copper destroys superconductivity in FeSe, and argue why superconductivity reemerges under pressure. Detailed Mössbauer spectroscopy studies on 57 Fe-enriched Cu 0.04 Fe 0.97 Se reveal that at low temperatures part of the iron sites is magnetically ordered, and static magnetic moments destroy the superconducting pairing. Application of pressure leads to a collapse of the static magnetism and restoration of superconductivity with the maximal T c value at pressure ∼ 8 GPa. The further pressure increase provokes a structural tetragonal-to-hexagonal phase transition and disappearance of superconductivity similarly to FeSe. The hyperfine parameters of the non-superconducting hexagonal phase of Cu 0.04 Fe 0.97 Se reveal a less covalent character of Fe–Se bonds and a noticeable higher value of the electric field gradient comparing to the tetragonal phase.