Imaging Se diffusion across the FeSe/SrTiO3 interface

Monolayer FeSe on SrTiO$_3$ superconducts with reported $T_\\mathrm{c}$ as\nhigh as 100 K, but the dramatic interfacial $T_\\mathrm{c}$ enhancement remains\npoorly understood. Oxygen vacancies in SrTiO$_3$ are known to enhance the\ninterfacial electron doping, electron-phonon coupling, and superconducting gap,\nbut the detailed mechanism is unclear. Here we apply scanning transmission\nelectron microscopy (STEM) and electron energy loss spectroscopy (EELS) to\nFeSe/SrTiO$_3$ to image the diffusion of selenium into SrTiO$_3$ to an\nunexpected depth of several unit cells, consistent with the simultaneously\nobserved depth profile of oxygen vacancies. Our density functional theory (DFT)\ncalculations support the crucial role of oxygen vacancies in facilitating the\nthermally driven Se diffusion. In contrast to excess Se in the FeSe monolayer\nor FeSe/SrTiO$_3$ interface that is typically removed during post-growth\nannealing, the diffused Se remains in the top few unit cells of the SrTiO$_3$\nbulk after the extended post-growth annealing that is necessary to achieve\nsuperconductivity. Thus, the unexpected Se in SrTiO$_3$ may contribute to the\ninterfacial electron doping and electron-phonon coupling that enhance\n$T_\\mathrm{c}$, suggesting another important role for oxygen vacancies as\nfacilitators of Se diffusion.\n