Coexistence of Replica Bands and Superconductivity in FeSe Monolayer Films

To elucidate the mechanisms behind the enhanced T-c in monolayer (1 ML) FeSe on SrTiO3 (STO), we grew highly strained 1 ML FeSe on the rectangular (100) face of rutile TiO2, and observed the coexistence of replica bands and superconductivity with a T-c of 63 K. From the similar T-c between this system and 1ML FeSe on STO (001), we conclude that strain and dielectric constant are likely unimportant to the enhanced T-c in these systems. A systematic comparison of 1 ML FeSe on TiO2 with other systems in the FeSe family shows that while charge transfer alone can enhance T-c, it is only with the addition of interfacial electron-phonon coupling that T-c can be increased to the level seen in 1 ML FeSe on STO.