Ferroionic generation of superconductivity in a non-superconducting cuprate*

We have studied ferroelectric tunnel junctions with La_0.7Sr_0.3MnO₃ (LSMO) bottom electrode, a ferroelectric (FE) BaTiO₃ barrier and La₅SrCu₆O₁₅ (LSCO) top electrode. Here, migration of oxygen atoms is triggered by the large strain modulations building up at the junction interfaces combined with the strong electric field produced at moderate applied voltages. We show that this mechanism can stabilize a superconducting phase at the interface between a ferroelectric and a non-superconducting cuprate. As probed by tunneling conductance, such phase is characterized by a 22 meV superconducting gap and a critical temperature T_C as high as 70 K, as well as by spectral features specific of d-wave superconductors such as YBa₂Cu₃O₇. Furthermore, we show that ferroelectric switching modulates the superconducting gap, which implies the that coupling between ferroelectric polarization and the ionization of oxygen vacancies controls the interfacial phase doping. This ferroionic route appears as promising new strategy to explore phase diagrams at doping levels far beyond the ones reached with field effect or ionic liquid doping.