Nematicity with a twist: Rotational symmetry breaking in a moiré superlattice.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 32821828.
- Also identified by DOI 10.1126/sciadv.aba8834 and PMC identifier 7406379.
- Licence recorded as CC BY.
- The licence permits redistribution, so the abstract is shown in full and the full text is available from the publisher.
Abstract
Motivated by recent reports of nematic order in twisted bilayer graphene (TBG), we investigate the impact of the triangular moiré superlattice degrees of freedom on nematicity. In TBG, the nematic order parameter is not Ising like, as in tetragonal crystals, but has a three-state Potts character related to the threefold rotational symmetry (<i>C</i> <sub>3<i>z</i></sub> ) of the moiré superlattice. We find that, even in the presence of static strain that explicitly breaks the <i>C</i> <sub>3<i>z</i></sub> symmetry, the system can still undergo a nematic-flop phase transition that spontaneously breaks in-plane twofold rotations. Moreover, elastic fluctuations, manifested as acoustic phonons, mediate a nemato-orbital coupling that ties the nematic director orientation to certain soft directions in momentum space, rendering the Potts-nematic transition mean field and first order. In contrast to the case of rigid crystals, the Fermi surface hot spots associated with these soft directions are maximally coupled to low-energy nematic fluctuations in the moiré superlattice case.