Multi-Orbital Superconductivity in SrTiO3/LaAlO3 Interface and SrTiO3 Surface

We investigate the superconductivity in two-dimensional electron systems formed in SrTiO3 nanostructures. Our theoretical analysis is based on the three-orbital model, which takes into account t(2g) orbitals of Ti ions. Because of the interfacial breaking of mirror symmetry, a Rashba-type antisymmetric spin-orbit coupling arises from the cooperation of intersite and interorbital hybridyzation and atomic LS coupling. This model shows a characteristic spin texture and carrier density dependence of Rashba spin-orbit coupling through the orbital degree of freedom. Superconductivity is mainly caused by heavy quasiparticles consisting of d(yz) and d(zx) orbitals at high carrier densities. We find that the Rashba spin-orbit coupling stabilizes a quasi-one-dimensional superconducting phase caused by one of the d(yz) or d(zx) orbitals at high magnetic fields along interfaces. This quasi-one-dimensional superconducting phase is protected against paramagnetic depairing effects by the Rashba spin-orbit coupling and realizes a large upper critical field H-c2 beyond the Pauli-Clogston-Chandrasekhar limit. This finding is consistent with an extraordinarily large upper critical field observed in SrTiO3/LaAlO3 interfaces and its carrier density dependence. The possible coexistence of superconductivity and ferromagnetism in SrTiO3/LaAlO3 interfaces may also be attributed to this quasi-one-dimensional superconducting phase.