Effect of the substrate interface conditions on the superconducting properties of Fe(Te,Se) thin films

Fe(Te,Se), a member of the iron-based superconductor, is of great interest as a potential candidate for topological superconductors as well as its application in high magnetic fields. Superconducting epitaxial Fe(Te,Se) thin films have been successfully grown on different substrates with a broad range of growth methods, and it has been found that superconductivity can be enhanced by various interface mechanisms such as electron doping and epitaxial strain effect. For example, the superconducting transition temperature (Tc) of a single unit cell of FeSe grown on SrTiO₃ can reach 65-109 K, and thin FeSe films on CaF₂ substrates can reach ~15 K, both higher than that (~8 K) of bulk FeSe crystal. Here, we report the growth of Fe(Te,Se) thin films on CaF₂ substrate with different interface conditions. We show that by applying different surface pretreatment methods, the Tc of Fe(Te,Se) thin films can vary substantially despite having similar crystalline qualities. Our results illustrate that the substrate interface condition is a critical parameter determining the superconducting properties of Fe(Te,Se) thin films, shedding light on the origin of superconductivity in Fe(Te,Se) as well as ways to tune their superconductivity for practical applications.