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, T-c 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 T-c. 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.