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Despite being explored for two decades, graphene continues to unveil surprises. Its newest incarnation, capturing the imagination of the quantum condensed matter community, is twisted bilayer graphene (TBG). TBG forms when two layers of graphene are placed on top of each other and mutually rotated by an arbitrary angle. Most attention has been directed to specific small rotation angles known as magic angles. At these angles, the electronic bands in TBG become quasiflat, implying that the kinetic energy of electrons is suppressed, allowing electron-electron interaction to dominate. Under certain conditions, these bands can also become topological. This combination of strong interaction and topology propels the system into a myriad of exotic phases---some of these have already been observed experimentally, while many more have been predicted. However, away from the magic angles, TBG is expected to essentially behave like a single layer of graphene.
In the first half of this talk, I will provide a brief overview of the magic-angle physics in TBG that has come to light so far. In the second half, I will present some of our work challenging the viewpoint that TBG away from magic angles is featureless, demonstrating two instances that contradict this view. First, I will show that at certain large rotation angles, far away from magic angles, the bands can also become quasiflat due to commensuration effects and underlying topology. Second, at small rotation angles away from a magic angle, the wavefunction of TBG differs from that of SLG, resulting in observable consequences. This arises from nonlocal terms in the interlayer coupling, typically ignored in conventional descriptions. |