Multiband Effects in the Superconducting Phase Diagram of Oxide Interfaces

Abstract 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 Schrödinger–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 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.