Twisted Bilayer Graphene Lifetimes At Integer Fillings: An Analytic Result

TL;DR

This paper develops an analytical framework to understand the dispersion and linewidth of correlated excitations in twisted bilayer graphene near integer fillings, using a topological heavy-fermion model.

Contribution

It introduces an analytical approach to compute self-energy, dispersion, and scattering rates of excitations in twisted bilayer graphene, incorporating strong correlations and band topology.

Findings

Derived explicit expressions for dispersion renormalization and scattering rates.

Identified different mechanisms for scattering of Dirac electrons Gamma3 and Gamma1,2.

Compared analytical results with DMFT simulations, confirming validity.

Abstract

Twisted bilayer graphene near integer fillings hosts correlated single-particle excitations whose dispersion and linewidth are increasingly accessible experimentally. We study these excitations using the topological heavy-fermion model, which captures both strong correlations and band topology of twisted bilayer graphene. In the decoupled limit, where both the single-particle fc hybridization and the Hund coupling between f and c electrons are absent, the model admits exact solutions in which free Dirac fermions coexist with interacting f electrons that form zero-width Hubbard bands. By treating the fc hybridization and Hund coupling perturbatively around this solvable limit, we obtain analytical results for the single-particle self-energy. From the resulting self-energy, we derive explicit expressions for both dispersion renormalization and scattering rates of both Hubbard-band excitations and low-energy Dirac modes, thereby establishing an analytical framework for understanding correlated excitations in twisted bilayer graphene. We analyze the scattering of the two kinds, Gamma3 and Gamma1,2, of Dirac electrons and find that they arise from different mechanisms. We also briefly investigate the effect of strain. Finally, we compare these analytical expressions with DMFT results for the same model.