(Nanowerk News) The shifting and twisting of the van der Waals layers can produce amazing physical phenomena. In the most recent publication on Nature Communications (“Polar meron-antimeron network in a strained and twisted double layer”), Daniel Bennett, Eric Bousquet and Philippe Ghosez, from the Theoretical Materials Physics (Q-MAT, CESAM Research Unit) group at the University of Liège (BE), with collaborators from the University of Cambridge (UK) demonstrated that moiré polar domains in the hBN bilayer gives rise to topologically non-trivial polarization plane windings, forming a network of merons and antimerons.
Out-of-plane polarization has recently been discovered in layered systems with broken inversion symmetry such as hexagonal boron nitride and transition metal dichalcogenides such as MoS.2. Polarization depends on the relative arrangement between the layers, and when the layers are aligned, the out-of-plane polarization can be shifted through shear between the layers, creating a ferroelectricity.
When there is a relative twist angle or lattice mismatch between layers, forming a supercell known as a moiré superlattice, there is local polarization for each distinct stack, resulting in a network of moiré polar domains (MPD).
These MPDs have been experimentally shown to generate ferroelectricity, making them a promising choice for nanoscale electronics applications such as information processing and memory storage.
It is shown here that this symmetry breaking also gives rise to a previously neglected in-plane polarization component, and the shape of the total polarization is determined purely from symmetry considerations. The in-plane polarization component makes the MPD in a double layer strained and twisted topologically non-trivial. In each individual domain, the polarization completes exactly half the windings forming a topological object known as a meron (half skyrmion).
Therefore, the MPD in a strained or twisted double layer forms a regular network of topological polar merons and antimerons. For the strained bilayer, the polarization flows in and out of the center of the domain (Néel type), whereas for the bent bilayer, the polarization is circular around the center of the domain (Bloch type). This means that the topological properties of these materials can be controlled by aligning the layers in different ways.
MPD in a strained or bent double layer can serve as a new platform for engineering and exploring topological physics in two-dimensional layered materials.