Investigation of Enhanced Superconductivity in the FeSe/SrTiO₃ System

The complex superconducting structure of the single-layer FeSe with T_c above 50 K by Q. Y. Wang et al. in 2012 is consistent with the complex interaction between electron-phonon couplings and doping the charge carriers. It is important to discover the nature of the superconducting state observed in the single-layer FeSe in order to design and manufacture higher T_c superconductors. Atomically flat thin films of monolayer and multilayer FeSe have been grown on SiC and SrTiO₃ substrates by molecular beam epitaxy (MBE). By a combined study of reflection high energy electron diffraction and scanning tunneling microscopy, we identified the temperature criterion for layer-by-layer growth of epitaxial FeSe films. A systematic study of the interfacial properties has clarified the essential roles of the substrate in realizing the high-T_c superconductivity, probably via interface-induced electron-phonon coupling enhancement and charge transfer. Motivated by the high T_c of monolayer FeSe on SrTiO₃, we carried out systematic annealing on a 3D superlattice FeSe-TiO₂ at temperatures between 450 and 700 º C. An unknown transition has been observed above 30 K, which is comparable with the reported T_c for the monolayer FeSe; however, additional measurements are still needed to confirm its origin.