Superconducting state properties of Cu-substituted Fe0.99Te0.66Se0.34 exhibiting superconductivity recovered under hydrostatic pressure

Magnetic measurements under hydrostatic pressure (P) up to 10 kbar and x-ray diffraction measurements up to 10.7 kbar were performed for a single crystal of Fe_0.975Cu_0.025Te_0.66Se_0.34, which is non-superconducting at ambient pressure. In this compound, we have found pressure-induced recovery of the superconducting state at T_c equal to 13.0 K for P = 10 kbar. We determined the parameters characterizing the superconducting state, including the lower and the upper critical fields, coherence length, and penetration depth, and compared them with those for Fe_0.99Te_0.66Se_0.34. We found that the lower critical field for Fe_0.975Cu_0.025Te_0.66Se_0.34 at 0 K and 10 kbar is comparable to the lower critical field for Fe_0.99Te_0.66Se_0.34 at ambient pressure, while the upper critical field is significantly higher than that for Fe_0.99Te_0.66Se_0.34 at ambient pressure. The estimated increase in superconducting carrier density and effective mass under pressure can be explained if one assumes that applied pressure leads to an increase in structural disorder in the studied material. At 10 kbar, the zero-field critical current density for Fe_0.975Cu_0.025Te_0.66Se_0.34 is four times larger than that for Fe_0.99Te_0.66Se_0.34 at ambient pressure. The x-ray diffraction results indicate that under pressure crystal quality apparently degrades. Comprehensive studies of the impact of pressure on the crystal structure indicate an increasing mosaicity evolution with pressure, suggesting that the pressure-induced superconductivity of Fe_0.975Cu_0.025Te_0.66Se_0.34 originates from inhomogeneities, associated also with the superconductivity in other sulpho-iron seleno-tellurides and anti-PbO-type structures. Obviously, the pressure effect on the crystallographic structure can lead to changes in the electronic structure, which is not excluded.