Coexistence of Replica Bands and Superconductivity in FeSe Monolayer Films

To elucidate the mechanisms behind the enhanced Tc 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 Tc of 63 K. From the similar Tc between this system and 1ML FeSe on STO (001), we conclude that strain and dielectric constant are likely unimportant to the enhanced Tc 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 Tc, it is only with the addition of interfacial electron-phonon coupling that Tc can be increased to the level seen in 1 ML FeSe on STO.