Modulation doping of the FeSe monolayer on SrTiO₃

The FeSe monolayer on STO has a higher superconducting T_c than its bulk counterpart and exhibits interesting characteristics such as missing hole pockets at the BZ center and replica bands in photoemission experiments. It remains ambiguous whether interactions possibly involving the STO substrate behind those experimental observations contribute to the T_c increase. One consensus reached to date is the modulation doping of FeSe by STO is critical for the enhancement. Monitoring the T_c change with a variable carrier density would provide valuable information for the electron pairing itself and the significance of interlayer interactions. However, varying the carrier density via the modulation doping method is challenging, and many groups have reported similar doping levels. We have developed the theory underlying the charge transfer at the FeSe/STO interface and found the key factor limiting the FeSe doping level. This information provides an experimental pathway to accommodate a variable carrier density in the FeSe monolayer via modulation doping.