Doping effects of Se vacancies in monolayer FeSe

Following the discovery of the potentially very high-temperature superconductivity in monolayer FeSe, we investigate the doping effect of Se vacancies in these materials. We find that Se vacancies pull a vacancy-centered orbital below the Fermi energy that absorbs most of the doped electrons. Furthermore, we find that the disorder-induced broadening causes an effective hole doping. The surprising net result is that, in terms of the Fe-d bands, Se vacancies behave like hole dopants rather than electron dopants. Our results exclude Se vacancies as the origin of the large electron pockets measured by angle-resolved photoemission spectroscopy. Furthermore, the unexpected doping effects not only lead to numerous consequences for the debated role of anion vacancies in the iron-based superconductors, but also demonstrate the surprisingly rich physics of vacancies in materials in general.