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Phys Rev Lett. 2018 Jun 1;120(22):227601. doi: 10.1103/PhysRevLett.120.227601.

Intrinsic Two-Dimensional Ferroelectricity with Dipole Locking.

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NSF Nanoscale Science and Engineering Center (NSEC), University of California, Berkeley, California 94720, USA.
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150080, People's Republic of China.
Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA.
Department of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA.
Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA.
Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA.


Out-of-plane ferroelectricity with a high transition temperature in ultrathin films is important for the exploration of new domain physics and scaling down of memory devices. However, depolarizing electrostatic fields and interfacial chemical bonds can destroy this long-range polar order at two-dimensional (2D) limit. Here we report the experimental discovery of the locking between out-of-plane dipoles and in-plane lattice asymmetry in atomically thin In_{2}Se_{3} crystals, a new stabilization mechanism leading to our observation of intrinsic 2D out-of-plane ferroelectricity. Through second harmonic generation spectroscopy and piezoresponse force microscopy, we found switching of out-of-plane electric polarization requires a flip of nonlinear optical polarization that corresponds to the inversion of in-plane lattice orientation. The polar order shows a very high transition temperature (∼700  K) without the assistance of extrinsic screening. This finding of intrinsic 2D ferroelectricity resulting from dipole locking opens up possibilities to explore 2D multiferroic physics and develop ultrahigh density memory devices.

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