Engineering superconductivity in single-layer FeSe on SrTiO₃ (001) with TiO₂- and SrO- terminations

The discovery of high-temperature superconductivity in single-layer FeSe grown on a SrTiO₃(001) (STO) substrate raises a fundamental question about the role of the interface in superconductivity. Extensive research so far suggests that the origin of the enhanced T_C is two-fold: electron doping from the substrate and interfacial coupling. Interestingly, most studies have been conducted on films grown on the TiO₂-terminated STO substrate. In this work, we synthesized single-layer FeSe on both TiO₂- and SrO-terminated STO substrates by molecular beam epitaxy and compared their superconducting properties by scanning tunneling microscopy/spectroscopy measurements. Nb-doped STO was first annealed under an oxygen partial pressure of 1 × 10^-5 Torr at 900 ° C for 30 min, resulting in a surface with mixed TiO₂ and SrO terminations. After further annealing under Se flux at 350 °C for 60 min, dI/dz spectroscopy revealed a higher work function for the SrO termination than the TiO₂ termination. For single-layer FeSe films grown on such STO substrates, a superconducting gap of ~15 meV was observed in the FeSe/TiO₂ regions by dI/dV tunneling spectroscopy, which agrees well with previous reports. In contrast, a smaller gap ~10 meV is observed on the FeSe/SrO. By comparing these experimental observations with DMFT calculations, our findings indicate the interfacial spacing and Se-Fe-Se angle of the FeSe tetrahedra are optimal at the FeSe-TiO₂ interface for enhancing superconducting transition temperature in single-layer FeSe/STO.