New high-T _c iron-selenide superconductor with hydroxide spacer layers

Abstract

Iron-based superconductors include pnictides and chalcogenides. So far, the chalcogenides are much fewer in number, thus it is particularly valuable to find new superconductors in iron chalcogenides, for the “ultimate” understanding of iron-based superconductivity. The tetragonal binary iron selenide (β-Fe_1+δSe) is the structurally simplest iron-based superconductor with a super-conducting critical temperature (T _c) of 8.5 K at ambient pressure. Under a hydrostatic pressure of ∼7 GPa, the T _c can be greatly enhanced to 37 K. By intercalation of alkali metal ions into adjacent iron-selenide layers, which induces more electrons into the FeSe layers, the T _c can also be increased to above 30 K. Upon applying high pressures around ∼12 GPa, an unknown secondary superconducting phase re-emerges at 48 K. Unfortunately, there exist phase separations in these intercalated compounds with only the minority phase responsible for superconductivity, which prevents revealing the intrinsic properties. Another remarkable progress is the observation of superconductivity in one-unit-cell FeSe thin film on a Sr-TiO₃ substrate with impressively high T _c values up to 65 K and more recently exceeding 100 K. These findings inspire the exploration of high-T _c superconductivity in bulk FeSe-based materials.