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SUMMARY:Ferroelectricity and topological polarization in twisted bilayers 
 - Danny Bennett (University of Liège)
DTSTART:20221110T140000Z
DTEND:20221110T150000Z
UID:TALK178175@talks.cam.ac.uk
CONTACT:Jan Behrends
DESCRIPTION:Recently it has been realized that ferroelectricity can occur 
 in layered systems comprised of stacks of two-dimensional (2D) materials s
 uch as hexagonal boron nitride (hBN)\, provided the stack of layers does n
 ot have inversion symmetry. In an aligned stack of hBN\, which has four no
 n-orthogonal mirror planes and is therefore non-centrosymmetric but still 
 non-polar\, sliding one layer over the other breaks the mirror symmetry ab
 out the plane which is parallel to and half-way between the layers\, resul
 ting in an interlayer transfer of electronic charge and an out-of-plane po
 larization. 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 unconv
 entional when compared to the ferroelectricity observed in ABO_3 oxide per
 ovskites\, in particular because the polarization generated is perpendicul
 ar to the atomic motion.\n\nIn 2020 there were several experimental observ
 ations of ferroelectricity in twisted bilayer systems\, although at the ti
 me 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 confi
 gurations\, which may locally break mirror symmetry. Thus\, the stacking d
 omains in twisted bilayers can be identified as out-of-plane 'moiré polar
  domains' (MPDs). The experimentally observed ferroelectricity has been at
 tributed 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 po
 larization (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 co
 nditions are never ideal\, and a small ferroelectric response is observed 
 [1\,2].\n\nIn a recent study\, we propose that the local symmetry breaking
  in a moiré superlattice also gives rise to an in-plane component of pola
 rization\, and the form of the total polarization is determined purely fro
 m symmetry considerations [3]. This implies that the polar properties of b
 ilayer 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 (hal
 f-skyrmions and half-antiskyrmions). We propose that the polar domains in 
 twisted bilayers may serve as a new platform for topological physics in re
 alistic materials\, and discuss how control over topological phases and ph
 ase transitions may be achieved in such systems.\n\n\n[1] D. Bennett and B
 . Remez\, npj 2D Mater. Appl. 6\, 1 (2022).\n[2] D. Bennett\, Phys. Rev. B
  105\, 235445 (2022).\n[3] D. Bennett et. al.\, arXiv:2210.10786
LOCATION:TCM Seminar Room
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