Fingerprints of interface superconductivity in ferroelectric tunnel junctions*

Ferroelectric tunnel junctions open the possibility of controlling the tunneling current with the ferroelectric state of the barrier. We have studied ferroelectric tunnel junctions with La_0.7Sr_0.3MnO₃ (LSMO) bottom electrode, a ferroelectric (FE) BaTiO₃ barrier and La_0.84Sr_0.16CuO_3-δ (LSCO) top electrode. LSCO is a cuprate capable of hosting large densities of oxygen vacancies (OV), yet superconductivity has never been observed. Coupled switching of oxygen vacancies and the ferroelectric polarization may produce drastic changes of the doping of the interface yielding unexpected electronic phases. We have found that the removal of OV from the cuprate interface has a strong hole doping effect of the material. We have observed the emergence of a new gap in the tunneling differential conductance curves which scales with temperature as a superconducting gap, strongly suggesting the nucleation of a superconducting phase at the interface. Switching the ferroelectric polarization of the barrier with an external electric field enables turning the superconducting gap on and off suggesting the possibility of the field effect control of the interface superconductivity.