Perfect and controllable nesting in the small angle twist bilayer graphene

TL;DR

This paper reveals a fully nested Fermi surface phase in small-angle twist bilayer graphene, controllable via interlayer bias, providing new insights into Fermi surface nesting phenomena in 2D materials.

Contribution

It demonstrates the existence of a completely nested Fermi surface in 2D twist bilayer graphene, controllable by interlayer bias, which was not previously observed.

Findings

Identification of a fully nested Fermi surface phase in TBLG

Control of Fermi line geometry through interlayer bias

Agreement with STM imaging of topological states

Abstract

Parallel ("nested") regions of a Fermi surface (FS) drive instabilities of the electron fluid, for example the spin density wave in elemental chromium. In one-dimensional materials, the FS is trivially fully nested (a single nesting vector connects two "Fermi dots"), while in higher dimensions only a fraction of the FS consists of parallel sheets. We demonstrate that the tiny angle regime of twist bilayer graphene (TBLG) possess a phase, accessible by interlayer bias, in which the FS consists entirely of nestable "Fermi lines": the first example of a completely nested FS in a 2d material. This nested phase is found both in the ideal as well as relaxed structure of the twist bilayer. We demonstrate excellent agreement with recent STM images of topological states in this material and elucidate the connection between these and the underlying Fermiology. We show that the geometry of the "Fermi lines" network is controllable by the strength of the applied interlayer bias, and thus that TBLG offers unprecedented access to the physics of FS nesting in 2d materials.