Why does SrTiO3 raise the transition temperature of monolayer FeSe?
In plain words
Electrons donated by the substrate explain part of the rise, since FeSe doped with electrons in other ways reaches about 40 to 48 K. Whether vibrations of the substrate supply the rest is disputed.
Precise statement
Monolayer FeSe/SrTiO3(001) has only electron pockets at M (about 0.1 e per Fe), with a gap of $\sim 15-20\,\mathrm{meV}$ that closes near $55-65\,\mathrm{K}$ in photoemission, while electron-doped FeSe without SrTiO3 (K dosing, ionic gating) reaches $\sim 40-48\,\mathrm{K}$. Determine the separate contributions to $T_c$ of electron doping, interfacial electron-phonon coupling to SrTiO3 polar (Fuchs-Kliewer) phonons at $\sim 80-100\,\mathrm{meV}$ through small-momentum forward scattering, and substrate screening.
What would settle it
$T_{c}$ of monolayer FeSe at fixed doping on substrates with systematically varied polar-phonon coupling, compared with a quantitative pairing calculation.
Status in the literature
Unverified note
Inserting a metallic SrVO3 layer that screens interfacial phonons lowered the pairing temperature to about 48 K (Guo et al., Nano Letters, 2024, arXiv 2407.09880), supporting a role for interfacial electron-phonon coupling.