Abstract

One of the major challenges in the study of magic angle twisted bilayer graphene (MATBG) has been the understanding the insulating states that arise as a result of inter-electron interaction. A combination of recent theoretical [1,2,3,4] and experimental [5,6] work has now established the presence of a new type of order which we call “Incommensurate Kekule Spiral” (IKS) order. This order spontaneously breaks both the valley-charge conservation and moiré translation symmetries, but preserves a modified translation symmetry which simultaneously shifts the spatial coordinates and rotates the U(1) angle which characterizes the spontaneous inter-valley coherence. This type of order appears ubiquitous in the presence of miniscule amounts of strain at temperatures above the superconducting transitions. In this talk I will give an introduction to the physics of MATBG emphasizing the strong coupling approximations. I will then turn to the Hartree-Fock calculations we used to predict the IKS order and I will explain the physics that drives the system towards this ground state. I will also briefly discuss the experiments that have observed this phase, the competing phases that are observed in ultra-small strain samples, as well as the closely related physics of magic angle trilayer graphene.

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