Renormalized Magic Angles in Asymmetric Twisted Graphene Multilayers

TL;DR

This paper investigates how asymmetries in Dirac points and Fermi velocities affect the flat band phenomena in twisted multilayer graphene, revealing increased magic angles and tunability through substrate screening and layer twist control.

Contribution

It introduces models showing that flat bands persist despite asymmetries and demonstrates how to tune magic angles via screening and twist adjustments.

Findings

Flat band phenomena remain despite Dirac point asymmetry.

Magic angles can be tuned by controlling screening and twist angles.

Quantitative magic angles are increased in asymmetric models.

Abstract

Stacked graphene multilayers with a small relative twist angle between each of the layers have been found to host flat bands at a series of magic angles. We consider the effect that Dirac point asymmetry between the layers, and in particular different Fermi velocities in each layer, may have on this phenomenon. Such asymmetry may be introduced by unequal Fermi velocity renormalizations through Coulomb interactions with a dielectric substrate. It also arises in an approximate way in tetralayer systems, in which the outer twist angles are large enough that there is a dominant moire periodicity from the stacking of the inner two layers. We find in such models that the flat band phenomenon persists in spite of this asymmetry, and that the magic angles acquire a degree of tunability through either controlling the screening in the bilayer system or the twist angles of the outer layers in the tetralayer system. Notably, we find in our models that the quantitative values of the magic angles are increased.