A Spiral Twist to the “Normal" State of Moiré Graphene

by Prof. Sid Parameswaran (University of Oxford)

Friday 1 Sept 2023, 11:00 → 12:00 Europe/Zurich

WBGB/019

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 discuss recent scanning tunnelling experiments in twisted bilayer (Nature 620, 525 (2023)) and trilayer (arXiv:2304.10586) graphene that directly visualised IKS order.

References

This talk is based on Phys. Rev. X 11, 041063 (2021), Phys. Rev. Lett. 128,156401 (2022), arXiv:2303.13602, and ongoing work.