Unusual magnetotransport in twisted bilayer graphene from strain-induced open Fermi surfaces

TL;DR

This paper demonstrates that uniaxial strain significantly alters the electronic properties of twisted bilayer graphene, explaining unusual magnetotransport phenomena and predicting new features in transport behavior.

Contribution

The study extends the Bistritzer-MacDonald model to include uniaxial heterostrain, revealing its impact on band structure and magnetotransport in twisted bilayer graphene.

Findings

Reproduces non-saturating $B^2$ magnetoresistance near filling $ u= ext{±}2$

Shows Hall coefficient remains nearly unaffected by strain

Predicts rotation of transport principal axes with filling

Abstract

Anisotropic hopping in a toy Hofstadter model was recently invoked to explain a rich and surprising Landau spectrum measured in twisted bilayer graphene away from the magic angle. Suspecting that such anisotropy could arise from unintended uniaxial strain, we extend the Bistritzer-MacDonald model to include uniaxial heterostrain. We find that such strain strongly influences band structure, shifting the three otherwise-degenerate van Hove points to different energies. Coupled to a Boltzmann magnetotransport calculation, this reproduces previously-unexplained non-saturating $B^2$ magnetoresistance over broad ranges of density near filling $\nu=\pm 2$, and predicts subtler features that had not been noticed in the experimental data. In contrast to these distinctive signatures in longitudinal resistivity, the Hall coefficient is barely influenced by strain, to the extent that it still shows a single sign change on each side of the charge neutrality point -- surprisingly, this sign change no longer occurs at a van Hove point. The theory also predicts a marked rotation of the electrical transport principal axes as a function of filling even for fixed strain and for rigid bands. More careful examination of interaction-induced nematic order versus strain effects in twisted bilayer graphene could thus be in order.