Polarization multistates in antiferroelectric van der Waals materials

TL;DR

This paper reveals multiple polarization states in ultra-thin van der Waals antiferroelectric materials, enabling new device functionalities beyond traditional ferroelectric bistability through first-principles calculations and electric field manipulation.

Contribution

It uncovers polarization multistates in layered antiferroelectric van der Waals materials and demonstrates their controllability via external electric fields, expanding the potential for advanced ferroelectric devices.

Findings

Bilayers and trilayers of CuInP$_2$S$_6$ have five and seven polarization states.

States can be transformed into each other under external electric fields.

A layer-selective flipping mechanism governs polarization transformations.

Abstract

The bistability of charge polarization in ferroelectric materials has long been the basis of ferroelectric devices. However, the ferroelectricity tends to be vanishing as the thickness of materials is reduced to a few nanometers or thinner due to the depolarization field. Instead, they show a paraelectric or an antiferroelectric ordering in the ultra-thin limit, which is unfavorable for their applications in devices. Here we uncover polarization multistates in thin films of van der Waals materials, in which the individual monolayers have an antiferroelectric ordering with out-of-plane polarizations. This property results from a unique combination of the polarization and layer degrees of freedom. Using first-principles calculations, we demonstrate that bilayers and trilayers of the CuInP$_2$S$_6$ family possess quintuple and septuple polarization states., respectively. Our climbing image nudged elastic band calculations for the bilayers and trilayers of CuInP$_2$S$_6$ and CuCrP$_2$S$_6$ further show that the states can be transformed into each other under appropriate external electric fields, for which a unique layer-selective half-layer-by-half-layer flipping mechanism governs the transformings. Our study opens up a door to design unusual polarization states using intrinsic degrees of freedom of layered antiferroelectrics for the next-generation ferroelectric devices that go beyond the bistability paradigm.