General Theory for Bilayer Stacking Ferroelectricity

TL;DR

This paper develops a comprehensive group theory-based framework for understanding bilayer stacking ferroelectricity in 2D materials, explaining previous phenomena and enabling new ferroelectric designs through stacking configurations.

Contribution

It introduces a general theoretical model for bilayer stacking ferroelectricity, encompassing all layer groups and revealing rules for symmetry changes and polarization behavior.

Findings

All possible bilayer stacking ferroelectricities identified

Stacking can induce ferroelectricity in centrosymmetric monolayers

Out-of-plane polarization in CrI3 can be manipulated via in-plane electric fields

Abstract

Two-dimensional (2D) ferroelectrics, which is rare in nature, enable high-density non-volatile memory with low energy consumption. Here, we propose a theory of bilayer stacking ferroelectricity (BSF), in which, two stacked layers of the same 2D material, with different rotation and translation, exhibits ferroelectricity. By performing systematic group theory analysis, we find out all the possible BSF in all the 80 layer groups (LGs) and discover the rules about the creation and annihilation of symmetries in the bilayer. Our general theory can not only explain all the previous findings (including sliding ferroelectricity), but also provide new perspective. Interestingly, the direction of the electric polarization of the bilayer could be totally different from that of the single layer. In particular, the bilayer could become ferroelectric after properly stacking two centrosymmetric non-polar monolayers. By means of first-principles simulations, we demonstrate that the ferroelectricity and thus multiferroicity can be introduced to the prototypical 2D ferromagnetic centrosymmetric material CrI3 by stacking. Furthermore, we find that the out-of-plane electric polarization in bilayer CrI3 is interlocked with the in-plane electric polarization, suggesting that the out-of-plane polarization can be manipulated in a deterministic way through the application of an in-plane electric field. The present BSF theory lays a solid foundation for designing a large number of bilayer ferroelectrics and thus colorful platforms for fundamental studies and applications.