Theory of superconductivity in doped quantum paraelectrics*

Recent experiments on Nb-doped SrTiO₃ have shown that the superconducting energy gap to the transition temperature ratio maintains the Bardeen-Cooper-Schrieffer (BCS) value throughout its superconducting dome. Motivated by these and related studies, we show that the Cooper pairing mediated by a single soft transverse-optical phonon is the most natural mechanism for such a superconducting dome given experimental constraints, and present the microscopic theory for this pairing mechanism. Furthermore, we show that this mechanism is consistent with the T² resistivity in the normal state. Lastly, we discuss what physical insights SrTiO₃ provides for superconductivity in other quantum paraelectrics such as KTaO₃.