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Electric polarization switching in an atomically thin binary rock salt structure

Abstract

Inducing and controlling electric dipoles is hindered in the ultrathin limit by the finite screening length of surface charges at metal–insulator junctions1,2,3, 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 symmetry5,6,7,8. 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.

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Fig. 1: Ultrathin NaCl layers on Cu2N/Cu(001).
Fig. 2: Electric-field-induced switching and calculated decay rates of NaCl bilayer on Cu2N/Cu(001).
Fig. 3: Force measurements for NaCl bilayer on Cu2N/Cu(001).
Fig. 4: Electric-field-induced shift of dipoles in NaCl bilayer on Cu2N/Cu(001).

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Acknowledgements

The authors thank P. Zubko for stimulating discussions. J.M.-C. and C.F.H. acknowledge financial support from Specs GmbH and EPSRC (EP/H002367/1). D.S. and M.Pi. acknowledge funding from Specs GmbH, MINECO (MAT2013-46593-C6-3-P),and the use of SAI-Universidad de Zaragoza. M.Pe. acknowledges computer time allocated on ARCHER through the Materials Chemistry Consortium funded by an EPSRC grant (EP/L000202), on Polaris through N8 HPC funded by an EPSRC grant(EP/K000225/1) and on Chadwick at the University of Liverpool.

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J.M.-C., D.S. and C.F.H. conceived of the project. J.M.-C., M.Pi., S.S. and D.S. performed the experiments and analysed the results. M.Pe. performed the DFT calculations.All authors discussed the results and contributed to the writing of the paper.

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Correspondence to David Serrate or Cyrus F. Hirjibehedin.

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Supplementary Methods, Supplementary Table 1.

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Martinez-Castro, J., Piantek, M., Schubert, S. et al. Electric polarization switching in an atomically thin binary rock salt structure. Nature Nanotech 13, 19–23 (2018). https://doi.org/10.1038/s41565-017-0001-2

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