Unconventional Multiband Superconductivity in Bulk SrTiO3 and LaAlO3/SrTiO3 Interfaces

Although discovered many decades ago, superconductivity in doped SrTiO3 remains a topic of intense research. Recent experiments revealed that, upon increasing the carrier concentration, multiple bands cross the Fermi level, signaling the onset of Lifshitz transitions. Interestingly, Tc was observed to be suppressed across the Lifshitz transition of oxygen-deficient SrTiO3; a similar behavior was also observed in gated LaAlO3/SrTiO3 interfaces. Such a behavior is difficult to explain in the clean theory of two-band superconductivity, as the additional electronic states provided by the second band should enhance Tc. Here, we show that this unexpected behavior can be explained by the strong pair-breaking effect promoted by disorder, which takes place if the interband pairing interaction is subleading and repulsive. A consequence of this scenario is that, upon moving away from the Lifshitz transition, the two-band superconducting state changes from opposite-sign gaps to same-sign gaps.