Polaron-Plasmon Superconductivity in Strontium Titanate

Strontium titanate (STO) is a bulk insulator that becomes a semiconducting superconductor at remarkably low carrier densities - below 10¹⁷ cm^-3 - with a characteristic superconducting dome as a function of doping which peaks at T_c~300mK, all in very close proximity to a ferroelectric quantum critical point. We investigate a scenario of superconductivity mediated by coupling to a strongly anti-adiabatic longitudinal optic phonon, by extending a Fröhlich electron-phonon Hamiltonian to include the effects of electronic Coulomb interactions. For the carrier densities of interest, there is a "cavity enhancement" of superconductivity by the material's own plasmons, which hybridize strongly with LO mode. Working within the cumulant expansion, we calculate the spectral signatures of this unusual regime to compare to photoemission[1] and tunneling[2] experiments, as well as the superconducting phase diagram.

    

[1]Wang et al. Nat Mater 15, 835 (2016)

[2]Swartz et al. PNAS 115, 1475 (2018)