Inducing and controlling electric dipoles is hindered in the ultrathin limit by the finite screening length of surface charges at metal-insulator junctions, though this effect can be circumvented by specially designed interfaces. We demonstrate that non-zero electric polarization can be induced and reversed in a hysteretic manner in bilayers made of ultrathin insulators whose electric polarization cannot be switched individually . Using scanning tunneling microscopy and atomic force microscopy, we explore the interface between ionic rock salt alkali halides such as NaCl or KBr and polar insulating Cu2N terminating bulk copper. The strong compositional asymmetry between the polar Cu2N and the vacuum gap breaks inversion symmetry in the alkali halide layer, inducing out of plane dipoles that are stabilized in one orientation (self-poling). The dipole orientation can be reversed by a critical electric field applied from the tip of a scanning tunneling microscope, producing sharp switching of the tunnel current passing through the junction. Furthermore, the switching is stabilized in the presence of defects that can be manipulated with atomic-scale precision. These results provide a new way to induce, probe, and manipulate electric dipoles at the atomic scale.
 J. Martinez-Castro et al., Nature Nanotechnology; DOI: 10.1038/s41565-017-0001-2