Understanding the superconducting state in SrTiO3 interfaces: Possible two-band superconductivity

We examine the possibility of multi-band superconductivity in SrTiO$_3$ interfaces by investigating the effects of a two-dimensional two-band model. In undoped SrTiO$_3$, one of the bands is occupied, while the upper band is empty. As the chemical potential shifts, due to doping by negative charge carriers or application of an electric field, the second band becomes occupied, giving rise to a strongenhancement of the transition temperature and a sharp feature in the gap functions, which is manifested in the local density of states spectrum. By comparing our results with tunneling experiments in Nb-doped SrTiO$_3$, we find that intra-band pairing dominates over inter-band pairing, unlikeother known multi-band superconductors. Given the similar transition temperature and band structure of LaAlO$_3$/SrTiO$_3$ heterostructures, we speculate that thesuperconductivity observed in SrTiO$_3$ interfaces may be similar in nature to that of bulk SrTiO$_3$,involving multiple bands with distinct electronic occupations.