Enhancement of superconducting state properties of Fe_0.994Ni_0.007Te_0.66Se_0.34 single crystal with increasing pressure: a correlation with pressure-induced crystallinity degradation

Detailed investigations of Ni substituted Fe_0.994Ni_0.007Te_0.66Se_0.34 and unsubstituted Fe_0.99Te_0.66Se_0.34 crystals were performed at ambient pressure and under hydrostatic pressure in order to understand the complex relationship between structure, magnetism, and superconductivity in iron selenides. The aim of the investigations was to determine the structural and superconducting state properties and study their evolution under pressure, up to 11 kbar. A comparison of Fe_0.99Te_0.66Se_0.34, a material with much less developed phase separation, with FeTe_0.5Se_0.5, a material exhibiting pronounced nanoscale phase separation, was also performed. Under ambient pressure, a weakening of superconducting state properties was observed in the Fe_0.994Ni_0.007Te_0.66Se_0.34 crystal, with disorder introduced by Ni substitution, as compared with those in Fe_0.99Te_0.66Se_0.34. For Fe_0.994Ni_0.007Te_0.66Se_0.34, the x-ray diffraction studies have revealed a degradation of crystal quality under applied elevated pressure. Superconducting state properties of the single phase Fe_0.99Te_0.66Se_0.34 crystal, such as upper and lower critical fields, were found to be poorer, at both ambient and hydrostatic pressure, than those observed for FeTe_0.5Se_0.5 crystals exhibiting pronounced nanoscale phase separation. Comprehensive studies of the impact of pressure on crystal structure and on superconducting state properties confirm that enhancement of superconductivity correlates with the appearance of mosaicity under pressure. The above observation leads to the expectation of a pressure-induced increase in inhomogeneity, leading to the enhancement of superconducting properties in other sulpho-iron seleno-tellurides of anti-PbO-type structure. (paper)