Multiband Effects in the Superconducting Phase Diagram of Oxide Interfaces

A dome-shaped phase diagram of superconducting critical temperature upon doping is often considered as a hallmark of unconventional superconductors. This behavior, observed in SrTiO3-based interfaces, whose electronic density is controlled by field-effect, has not been explained unambiguously yet. Here, a generic scenario for the superconducting phase diagram of these oxide interfaces is elaborated based on transport experiments on a double-gate LaAlO3/SrTiO3 field-effect device and Schrodinger-Poisson numerical simulations of the quantum well. The optimal doping point of maximum T-c is ascribed to the transition between a single-gap and a fragile two-gap s(+/-)-wave superconducting state involving bands of different orbital character. Close to this point, a bifurcation in the dependence of T-c on the carrier density, which can be controlled by the details of the doping execution, is observed experimentally and reproduced by numerical simulations. Where doping with a back-gate triggers the filling of a new dxy${d_{{\rm{xy}}}}$ subband and initiates the overdoped regime, doping with a top-gate delays the filling of the subband and maintains the 2D electron gaz in the single-gap state of higher T-c. Such a bifurcation, whose branches can be followed reversibly, provides a generic explanation for the dome-shaped superconducting phase diagram that could be extended to other multiband superconducting materials.