Two-band superconductivity in doped SrTiO₃ films and interfaces

We investigate the possibility of multiband superconductivity in SrTiO ₃ films and interfaces using a two-dimensional two-band model. In the undoped compound, one of the bands is occupied whereas the other 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 and gives rise to a strong enhancement 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 SrTiO₃, we find that intraband pairing dominates over interband pairing, unlike other known multiband superconductors. Given the similarities with the value of the transition temperature and with the band structure of LaAlO₃/SrTiO₃ heterostructures, we speculate that the superconductivity observed in SrTiO₃ interfaces may be similar in nature to that of bulk SrTiO₃, involving multiple bands with distinct electronic occupations.