Coupling between different degrees of freedom, that is, charge, spin, orbital and lattice, is responsible for emergent phenomena in complex oxide heterostrutures1,2. One example is the formation of a two-dimensional electron gas (2DEG) at the polar/non-polar LaAlO3/SrTiO3 (LAO/STO)3,4,5,6,7 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 discontinuity8,9,10. 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)11,12,13. While recent first-principles modelling predicts the formation of 2DEGs at the ferroelectric/insulator interfaces9,10,12,13,14, 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.
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The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.
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The work was supported by the Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award DE-SC0014430 and partially by the National Science Foundation (NSF) under grants DMR-1506535 and DMR-1629270. L.X. was supported by the National Basic Research Program of China (grant no. 2015CB654901) and National Natural Science Foundation of China (grant no. 51302132). The research at the University of Nebraska-Lincoln was supported by the NSF through the Nebraska Materials Science and Engineering Center (MRSEC) under grant DMR-1420645. J.K., H.W. and R.Q.W. acknowledge support of DOE-BES (grant no. DE-272 FG02-05ER46237) and computing allocation by NERSC. The work at Penn State is supported by the US Department of Energy under award DE-FG02-07ER46417. The work at Cornell University was supported by the NSF (Nanosystems Engineering Research Center for Translational Applications of Nanoscale Multiferroic Systems) under grant EEC-1160504 (C.H. and D.G.S.). Substrate preparation was performed in part at the Cornell Nanoscale Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the NSF (grant ECCS-1542081). The authors would also like to acknowledge the University of California, Irvine’s Materials Research Institute (IMRI) for the use of TEM facilities. Y.Z. would like to thank J. R. Jokisaari for his initial work and help on the PFM measurements carried out at the University of Michigan. The authors also thank T. Aoki (University of California, Irvine) for his help on EELS measurements.
The authors declare no competing interests.
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Zhang, Y., Lu, H., Xie, L. et al. Anisotropic polarization-induced conductance at a ferroelectric–insulator interface. Nature Nanotech 13, 1132–1136 (2018). https://doi.org/10.1038/s41565-018-0259-z
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