Enhanced superconductivity in FeSe/SrTiO₃ mediated by hybrid plasmon–phonon modes in a micro cavity

Monolayer FeSe grown on SrTiO₃ exhibits a superconducting transition temperature an order of magnitude higher than in bulk FeSe, making it a prime example of interface-enhanced high-T_c superconductivity, not explained by conventional acoustic phonon coupling. The SrTiO₃ substrate hosts polar optical surface phonons (Fuchs–Kliewer modes) that strongly couple to the FeSe electrons. These phonons hybridize with charge plasmon excitations in FeSe, forming two distinct plasmon–phonon branches. The coupling between these modes can be tuned in a plasmonic micro cavity. We test the hypothesis that these hybrid modes are responsible for high T_c in FeSe+SrTiO₃ system. Using a simplified effective interaction that captures this hybridization, we solve the Eliashberg self-consistency equations numerically to determine the resulting superconducting T_c. We show how the strength of coupling of these modes to the electrons and energy of the hybridized modes affect the superconducting transition temperature.