Criteria for achieving superconducting monolayer FeSe films

Monolayer FeSe on SrTiO₃ substrate has an enhanced superconducting transition temperature T_c than bulk FeSe. The criteria for achieving the enhanced superconductivity, specifically, the film stoichiometry and the interfacial lattice and electronic structures, will be discussed. The superconducting FeSe films can only be grown by molecular beam epitaxy with hours’ post-growth annealing. We adjust the film stoichiometry by depositing additional amounts of Fe atoms. The monolayers become superconducting after the Fe deposition without annealing and show similar superconducting transition temperatures in transport measurements. The annealing process in essence removes Fe vacancies and the additional Fe deposition serves as a more efficient way. By a complementary analysis of scanning transmission electron microscopy and in situ surface X-ray diffraction, we resolve the atomic-scale structure of the FeSe/SrTiO₃ interface with a double TiO₂ layer presented. ab initio calculations reveal a stronger tendency for electrons to transfer from an oxygen-deficient SrTiO₃ surface to FeSe when the double TiO₂ layer is present. The interfacial charge transfer together with the stoichiometry of FeSe film has been well-accepted as necessary criteria for achieving the enhanced superconductivity.