Imaging Se diffusion across the FeSe/ SrTiO3 interface

Monolayer FeSe on ${\mathrm{SrTiO}}_3$ superconducts with reported $T_c$ as high as 100 K, but the dramatic interfacial $T_c$ enhancement remains poorly understood. Oxygen vacancies in ${\mathrm{SrTiO}}_3$ 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 and electron energy loss spectroscopy to FeSe/${\mathrm{SrTiO}}_3$ to image the diffusion of selenium into ${\mathrm{SrTiO}}_3$ to an unexpected depth of several unit cells, consistent with the simultaneously observed depth profile of oxygen vacancies. Our density functional theory 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/${\mathrm{SrTiO}}_3$ interface that is typically removed during postgrowth annealing, the diffused Se remains in the top few unit cells of the ${\mathrm{SrTiO}}_3$ bulk after the extended postgrowth annealing that is necessary to achieve superconductivity. Thus, the unexpected Se in ${\mathrm{SrTiO}}_3$ may contribute to the interfacial electron doping and electron-phonon coupling that enhance $T_c$, suggesting another important role for oxygen vacancies as facilitators of Se diffusion.