Samantha O'Sullivan

Monolayer FeSe on SrTiO3 superconducts with reported Tc as high as 100 K, but the dramatic interfacial Tc enhancement remains poorly understood. Oxygen vacancies in SrTiO3 are known to enhance the interfacial electron doping, electron-phonon coupling, and superconducting gap, but the detailed mechanism is unclear. Here we apply scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to FeSe/SrTiO3 to image the diffusion of selenium into SrTiO3 to an unexpected depth of several unit cells, consistent with the simultaneously observed depth profile of oxygen vacancies. Our density functional theory (DFT) calculations support the crucial role of oxygen vacancies in facilitating the thermally driven Se diffusion. In contrast to excess Se in the FeSe monolayer or FeSe/SrTiO3 interface that is typically removed during post-growth annealing, the diffused Se remains in the top few unit cells of the SrTiO3 bulk after the extended post-growth annealing that is necessary to achieve superconductivity. Thus, the unexpected Se in SrTiO3 may contribute to the interfacial electron doping and electron-phonon coupling that enhance Tc, suggesting another important role for oxygen vacancies as facilitators of Se diffusion.