Tunable superconductivity and its origin at KTaO₃ interfaces

What causes Cooper pairs to form in unconventional superconductors is often elusive because experimental signatures that connect to a specific pairing mechanism are rare. Here, we observe distinct dependences of the superconducting transition temperature T_c on carrier density n_2D for electron gases formed at KTaO₃ (111), (001) and (110) interfaces. For the (111) interface, a remarkable linear dependence of T_c on n_2D is observed over a range of nearly one order of magnitude. Further, our study of the dependence of superconductivity on gate electric fields reveals the role of the interface in mediating superconductivity. We find that the extreme sensitivity of superconductivity to crystallographic orientation can be explained by pairing via inter-orbital interactions induced by an inversion-breaking transverse optical phonon and quantum confinement. This mechanism is also consistent with the dependence of T_c on n_2D. Our study may shed light on the pairing mechanism in other superconducting quantum paraelectrics.