Ferroelectricity and topological polarization in twisted bilayers

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• Danny Bennett (University of Liège)
• Thursday 10 November 2022, 14:00-15:00
• TCM Seminar Room.

If you have a question about this talk, please contact Jan Behrends.

Recently it has been realized that ferroelectricity can occur in layered systems comprised of stacks of two-dimensional (2D) materials such as hexagonal boron nitride (hBN), provided the stack of layers does not have inversion symmetry. In an aligned stack of hBN, which has four non-orthogonal mirror planes and is therefore non-centrosymmetric but still non-polar, sliding one layer over the other breaks the mirror symmetry about the plane which is parallel to and half-way between the layers, resulting in an interlayer transfer of electronic charge and an out-of-plane polarization. Applying an electric field, the polarization can be inverted via a relative sliding between the layers (van der Waals sliding) in order to align the polarization with the field. This mechanism is highly unconventional when compared to the ferroelectricity observed in ABO _3 oxide perovskites, in particular because the polarization generated is perpendicular to the atomic motion.

In 2020 there were several experimental observations of ferroelectricity in twisted bilayer systems, although at the time of discovery the physical origin for the ferroelectric response was not known. The moiré superlattice formed by a small angle twist between non-Bravais lattice monolayers has local regions with different stacking configurations, which may locally break mirror symmetry. Thus, the stacking domains in twisted bilayers can be identified as out-of-plane ‘moiré polar domains’ (MPDs). The experimentally observed ferroelectricity has been attributed to the motion of the domain walls separating the MPDs in response to an applied out-of-plane electric field. As a result, the MPDs with polarization (anti-)aligned to the field (shrink) grow in size. Under ideal conditions, the average polarisation at zero electric field is zero, and this mechanism would not result in ferroelectricity, but experimental conditions are never ideal, and a small ferroelectric response is observed [1,2].

In a recent study, we propose that the local symmetry breaking in a moiré superlattice also gives rise to an in-plane component of polarization, and the form of the total polarization is determined purely from symmetry considerations [3]. This implies that the polar properties of bilayer systems are much richer than previously thought. Furthermore, the in-plane component reveals that the polar domains in twisted bilayers are topologically nontrivial, forming a network of merons and antimerons (half-skyrmions and half-antiskyrmions). We propose that the polar domains in twisted bilayers may serve as a new platform for topological physics in realistic materials, and discuss how control over topological phases and phase transitions may be achieved in such systems.

[1] D. Bennett and B. Remez, npj 2D Mater. Appl. 6, 1 (2022). [2] D. Bennett, Phys. Rev. B 105 , 235445 (2022). [3] D. Bennett et. al., arXiv:2210.10786

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