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From quantum Hall to Hubbard physics in twisted bilayer graphene

May 10, 2024 @ 10:00 am - 11:30 am

Quantum Matter in Mathematics and Physics Seminar

Speaker: Eslam Khalaf (Harvard)

Title: From quantum Hall to Hubbard physics in twisted bilayer graphene

Abstract: Early on it was noticed that twisted bilayer graphene (TBG) has elements in common with two paradigmatic examples of strongly correlated physics: Hubbard physics and quantum Hall physics. On the one hand, TBG hosts flat topological Landau-level-like bands which realize quantum anomalous Hall states and orbital ferromagnetism under the right conditions. On the other hand, these bands are characterized by concentrated charge density and show experimental signs of fluctuating magnetism, and unconventional superconductivty; all characteristics of Hubbard-model-like physics. The emergence of fluctuating moments is particularly surprising, as localized Wannier states do not exist in topological bands. I will discuss a phenomenological model for the flat bands in TBG that centers the concentration of charge density and, relatedly, the concentration of Berry flux. The bands obtained have excellent quantitative agreement with the Bistritzer-Macdonald model for realistic parameters. I will show that, rather remarkably, the model hosts decoupled flavor moments which despite being only power-law delocalized with infinite localization length, have parametrically small overlap with each other. I will show how this unifies many of the observations in TBG and leads to a novel Mott semimetal phase for intermediate temperatures where moments are thermally disordered but charge fluctuations are mostly frozen.


May 10, 2024
10:00 am - 11:30 am
Event Category:


Juven Wang


CMSA Room G10
CMSA, 20 Garden Street
Cambridge, MA 02138 United States
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