Condensed Matter Theory Seminar: Yves Kwan, University of Oxford, “Kekulé spirals in twisted bilayer graphene”

Abstract: Intense experimental efforts over the past few years have uncovered a rich phenomenology in magic-angle twisted bilayer graphene (TBG). The search for a unifying theoretical framework is complicated by the variability of observations between different samples, which is often attributed to perturbations beyond the pristine limit. Among these is strain, which has been demonstrated via scanning tunnelling experiments to be ubiquitous in TBG devices. In this talk, I will discuss recent work where we address the role of strain alongside other realistic perturbations through comprehensive mean-field calculations. We find that with a small amount of strain, the phase diagram is dominated by a novel translation symmetry-breaking order dubbed the incommensurate Kekulé spiral (IKS), which manifests as a microscopic Kekulé distortion whose phase rotates on the moiré scale. I will describe the properties and origin of the IKS, including its relation to the ‘strong coupling’ phases of TBG, and argue that our findings are broadly consistent with the set of experimental observations accumulated in this material so far. This talk is based on arXiv:2105.05857 and arXiv:2109.09749.
Host: Meng Cheng
Please contact Taylor Dunnigan (taylor.dunnigan@yale.edu) for zoom connection information.