High-temperature superconductivity in monolayer FeSe on SrTiO3 and related systems mediated by low energy plasmons

The dominant mechanism of high-transition-temperature $({T}_{\text{c}})$ superconductivity in cuprates remains an agonizing puzzle in condensed matter physics. The more recent discoveries of FeSe-based superconductors provide ideal new platforms to explore the high-${T}_{\text{c}}$ superconducting mechanism. Here we develop a generic mechanism of superconductivity centered at the commonality shared by many high-${T}_{\text{c}}$ materials, namely, their effective carrier densities are universally low, and are therefore necessarily accompanied by low energy plasmons. We first show that the excitations of such plasmons can largely suppress the Coulomb repulsion of the electrons. Furthermore, the electron-phonon and electron-plasmon couplings can inherently join force in mediating electron pairing, and when applied to the monolayered FeSe on ${\mathrm{SrTiO}}_{3}$, the plasmon-enhanced ${T}_{\text{c}}$ is one order of magnitude higher than that due to phonon alone, to the experimentally observed range. The present ``phonon $+$ plasmon'' mechanism also embodies characteristic dependences of ${T}_{\text{c}}$ on the carrier density and isotope substitution, and may find broad applicability in many superconducting systems with low carrier densities, including, most notably, the cuprates.