Abstract

Intense experimental efforts have uncovered a wide range of interaction-driven phenomena in magic-angle twisted bilayer graphene (TBG), including the observation of quantised topological responses, correlated insulating states, and most famously, gate-tunable superconductivity. These phenomena challenge a weak-coupling treatment yet the strong-coupling limit, despite admitting an elegant hidden symmetry structure (which I will review), is also in tension with experiments: for instance, it fails to fully explain the sequence of insulating and semimetallic states at commensurate fillings and their associated Landau fan diagrams. In this talk, I will argue that even very small strain (ubiquitous in experimental samples) drives the system to an intermediate coupling regime, where the normal-state phase diagram is dominated by a novel translation symmetry-breaking order dubbed the incommensurate Kekulé spiral (IKS). This order manifests as a Kekulé distortion on the microscopic graphene scale whose phase rotates on the moiré scale: its existence hence relies essentially on the multiscale nature of moiré materials. I will describe the properties and origin of the IKS state and its link to the ‘strong coupling’ phases of TBG . I will demonstrate that these findings are broadly consistent with extant experimental observations in TBG and comment on its implications for the origin of superconductivity in graphene moiré bilayers.

This talk is based on Phys. Rev. X 11 , 041063 (2021) and Phys. Rev. Lett. 128,156401 (2022).