Flat bands and gaps in twisted double bilayer graphene

TL;DR

This paper investigates the electronic properties of twisted double bilayer graphene, revealing angle-dependent band gaps and the emergence of flat bands at low twist angles, with potential tunability via external electric fields.

Contribution

It provides first-principles calculations showing how twist angle and surface effects influence the band gap and flat band formation in TDBG, a novel multilayer graphene structure.

Findings

TDBG is semiconducting with a twist-angle-dependent band gap.

Flat bands emerge at low twist angles, localizing electrons at AA regions.

External electric fields can tune the band gap.

Abstract

We present electronic structure calculations of twisted double bilayer graphene (TDBG): A tetralayer graphene structure composed of two AB-stacked graphene bilayers with a relative rotation angle between them. Using first-principles calculations, we find that TDBG is semiconducting with a band gap that depends on the twist angle, that can be tuned by an external electric field. The gap is consistent with TDBG symmetry and its magnitude is related to surface effects, driving electron transfer from outer to inner layers. The surface effect competes with an energy upshift of localized states at inner layers, giving rise to the peculiar angle dependence of the band gap, which reduces at low angles. For these low twist angles, the TDBG develops flat bands, in which electrons in the inner layers are localized at the AA regions, as in twisted bilayer graphene.