Superconductivity and Upper Critical Field Behavior in FeSe Thin Films

The Fe-chalcogenide superconductors, FeSe and Fe(Se,Te), have attracted significant interest in recent years because of enhanced superconducting critical temperatures in ultrathin films of FeSe on SrTiO₃ and the potential for topological superconductivity in Fe(Te,Se). A thorough study of the upper critical field, H_c2, in these materials is important for obtaining a comprehensive understanding of superconductivity in this context. Here, we present an experimental study of the temperature dependence of H_c2, obtained in static magnetic fields up to 14 T and pulsed magnetic fields up to 65 T, in few-layer FeSe and Fe(Te,Se) films grown by molecular beam epitaxy on SrTiO₃ substrates. We primarily focus on the measurement and analysis of the temperature dependence of H_c2 in the context of Werthamer-Helfand-Hohenberg (WHH) theory for several FeSe samples with varying degrees of disorder and critical temperatures. We observe a distinct departure from WHH theory at low temperatures and comment on the potential implications of this observation. Finally, we will report on measurements of H_c2 in Fe(Te,Se) as the Te concentration approaches the proposed ideal stoichiometry FeTe_0.55Se_0.45 for topological superconductivity.