<i>Low</i>-energy-plasmon enhanced <i>High</i>-transition-temperature superconductivity in monolayered FeSe on SrTiO₃

It was observed nearly a decade ago that the superconducting transition temperature (T_c) of a monolayered FeSe grown on SrTiO₃ can be drastically enhanced from that of bulk FeSe, yet the likely microscopic mechanisms involved are still under active debate. Here we reveal the vital role of the interfacial optical plasmon mode in mediating electron pairing, which so far has been overlooked for this unique system. The low-energy nature of the plasmon known to be comparable to the energy of the interfacial phonon mode is a direct consequence of the low carrier density, demanding the electron-phonon and electron-plasmon couplings to be treated on equal footing. Our detailed analysis within a generalized Eliashberg formalism shows that, whereas the electron-phonon or electron-plasmon coupling alone could only yield a low T_c of a few K, the constructive interplay of the two is able to enhance the T_c by one order of magnitude, to the experimentally observed range. Our theory also predicts a characteristic dependence of T_c on the plasmon frequency, and is expected to find broader applicability in related superconducting systems.