Anisotropic polarization-induced conductance at a ferroelectric-insulator interface

Coupling between different degrees of freedom, that is, charge, spin, orbital and lattice, is responsible for emergent phenomena in complex oxide heterostrutures^1,2. One example is the formation of a two-dimensional electron gas (2DEG) at the polar/non-polar LaAlO₃/SrTiO₃ (LAO/STO)^3-7 interface. This is caused by the polar discontinuity and counteracts the electrostatic potential build-up across the LAO film³. The ferroelectric polarization at a ferroelectric/insulator interface can also give rise to a polar discontinuity^8-10. Depending on the polarization orientation, either electrons or holes are transferred to the interface, to form either a 2DEG or two-dimensional hole gas (2DHG)^11-13. While recent first-principles modelling predicts the formation of 2DEGs at the ferroelectric/insulator interfaces^9,10,12-14, experimental evidence of a ferroelectrically induced interfacial 2DEG remains elusive. Here, we report the emergence of strongly anisotropic polarization-induced conductivity at a ferroelectric/insulator interface, which shows a strong dependence on the polarization orientation. By probing the local conductance and ferroelectric polarization over a cross-section of a BiFeO₃-TbScO₃ (BFO/TSO) (001) heterostructure, we demonstrate that this interface is conducting along the 109° domain stripes in BFO, whereas it is insulating in the direction perpendicular to these domain stripes. Electron energy-loss spectroscopy and theoretical modelling suggest that the anisotropy of the interfacial conduction is caused by an alternating polarization associated with the ferroelectric domains, producing either electron or hole doping of the BFO/TSO interface.