Importance of interfacial layers of FeSe/SrTiO₃ superconductors

The electron-phonon coupling at the interfaces between two distinct materials can lead to emergent physical properties and an unexpectedly high superconducting temperature. A notable example is the FeSe monolayer grown on doped SrTiO₃, which was found to exhibit a transition temperature (T_c) over 100 K, much higher than its bulk value of 8 K. This has inspired extensive research efforts to investigate the mechanism behind such extraordinary enhancement. Yet, the role of the oxide substrate remains unclear to date. Here, we discuss how interfacial phonon bands and electron-phonon interactions are influenced by complex reconstructions and segregation at the FeSe/STO interface. A notable discovery is that the formation of an unusual interfacial structure plays an important role in the enhancement of T_c. Our research provides a layer-by-layer breakdown of the electron and phonon states’ contributions to the superconductivity.