Substrate and band bending effects on monolayer FeSe on SrTiO₃(001)

Motivated by the high superconducting transition temperature (T_C) shown by monolayer FeSe on cubic perovskite SrTiO₃(001) and SrTiO₃(001)-2×1 reconstructed surfaces, in this study, we explore the atomic and electronic structures of monolayer FeSe on various SrTiO₃(001)-2×1 surface reconstructions using the CALYPSO method and first-principles calculations. Our search reveals two new Ti₂O₂ and Ti₂O reconstructed surface structures, besides the Ti₂O₃ and double TiO₂ layer reconstructed surfaces, and the two new Ti₂O₂ and Ti₂O reconstructed surface structures are more stable under Ti-rich conditions than under Ti-poor conditions. The Fermi-surface topology of an FeSe monolayer on Ti₂O₃- and Ti₂O₂-type reconstructed STO surfaces is different from that of an FeSe monolayer on a Ti₂O-type STO reconstructed surface. The established structure of monolayer FeSe on a Ti₂O-type STO(001) reconstructed surface can naturally explain the experimental observation of the electronic band structure on the monolayer FeSe superconductor and obtained electrons counting per Fe atom. Surface states in the mid-gap induced by various STO surface reconstructions will result in band bending. The surface-state-induced band bending is also responsible for the electron transfer from the STO substrate to the FeSe films.