Electric field controlled magnetism and hole doping in monolayer FeSe on a polar NaCl substrate

The discovery of high-${T}_{\text{c}}$ superconductivity at FeSe/${\mathrm{SrTiO}}_{3}$ interface under ambient pressure has inspired great interest in exploring interfacial superconductivity in Fe-based systems. While most high-${T}_{\text{c}}$ FeSe superconducting systems are electron-doped, the hole-doped counterparts are much more rare, let alone the effective tuning of the hole doping level. Herein, we have theoretically designed a tunable hole-doped FeSe interfacial system by epitaxially growing FeSe monolayer on a polar substrate made of NaCl bilayer on ${\mathrm{Cu}}_{2}\mathrm{N}$-terminated Cu bulk, in which the polar substrate has already been realized in recent experiment. Through first-principles electronic structure calculations, we find that the external electric field can effectively modulate the magnetic properties of FeSe monolayer, converting it from a stripe antiferromagnetic (AFM) state to a dimer AFM state when the applied field is larger than $\ensuremath{\sim}$ 0.24 V/\AA{}. This conversion indicates that the magnetic fluctuations in FeSe on polar NaCl substrate can be controlled by the gate electric field. Furthermore, due to the work function difference, the polar NaCl substrate could dope holes to the FeSe monolayer while the doping concentration of 0.02 $\ensuremath{\sim}$ 0.04 holes/Fe can be tuned when the electric field ranges from -0.4 V/\AA{} to 0.4 V/\AA{}. Our findings suggest that the FeSe monolayer on the polar substrate made of NaCl bilayer on ${\mathrm{Cu}}_{2}\mathrm{N}$/Cu(001) is an ideal platform to manipulate the magnetism and doping effect of FeSe via the gate electric field, therefore possibly tuning the FeSe-derived interfacial superconductivity.