Superconductivity in Strontium Titanate

Strontium titanate 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. Experimentally, a series of replica bands in photoemission[1] and tunneling[2] experiments reveal very prominent coupling of the electrons to a longitudinal optic phonon. A straightforward theory coupling this phonon to an electron gas produces a superconducting phase diagram with a dome but is not well-controlled due to adiabaticity, and relies on coupled plasmon-phonon modes that are not experimentally observed in the material. In this work we examine how the inclusion of interactions beyond leading order can address these discrepancies.



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

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