Inducing and controlling electric dipoles is hindered in the ultrathin limit by the finite screening length of surface charges at metal–insulator junctions^1,2,3, although this effect can be circumvented by specially designed interfaces⁴. Heterostructures of insulating materials hold great promise, as confirmed by perovskite oxide superlattices with compositional substitution to artificially break the structural inversion symmetry^5,6,7,8. Bringing this concept to the ultrathin limit would substantially broaden the range of materials and functionalities that could be exploited in novel nanoscale device designs. Here, we report 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. In particular, 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, producing sharp switching of the tunnel current passing through the junction.

尽管可以通过特殊设计的界面⁴来避免这种作用，但是由于金属-绝缘体接合点^1,2,3处表面电荷的有限屏蔽长度，使得超极薄的感应和控制电偶极子受到了限制。钙钛矿氧化物超晶格具有组成替代作用，可以人为地破坏结构反转对称性，证实了绝缘材料的异质结构具有广阔的前景^5,6,7,8。将此概念带到极薄的极限将大大拓宽可在新型纳米级器件设计中利用的材料和功能的范围。在这里，我们报道了由超薄绝缘体制成的双层（其极化不能单独切换）可以以磁滞方式感应和逆转非零极化。特别是，我们探索了诸如NaCl或KBr之类的离子岩盐碱金属卤化物与极性绝缘的Cu ₂ N终止体铜之间的界面。极性Cu ₂之间的强组成不对称性N和真空间隙破坏了碱金属卤化物层中的反对称性，引发了稳定在一个方向上的平面外偶极子（自极化）。偶极子的方向可以被临界电场反转，从而使通过结的隧道电流产生急剧的切换。