Two-band superconductivity in doped SrTiO$_{3}$ films and interfaces

We investigate the possibility of multi-band 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, giving 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 SrTiO$_{3}$, we find that intra-band pairing dominates over inter-band pairing, unlike other known multi-band superconductors. Given the similarities with the value of the transition temperature and with the band structure of LaAlO$_{3}$/SrTiO$_{3}$ heterostructures, we speculate that the superconductivity observed in SrTiO$_{3}$ interfaces may be similar in nature to that of bulk SrTiO$_{3}$, involving multiple bands with distinct electronic occupations.
Comment: revised expanded version