Bending Stiffness Collapse, Buckling, Topological Bands of Freestanding Twisted Bilayer Graphene

TL;DR

This paper investigates the mechanical and electronic instabilities of freestanding twisted bilayer graphene at small twist angles, revealing collapse of bending rigidity, buckling phenomena, and unusual electronic band behavior.

Contribution

It introduces a comprehensive analysis of mechanical collapse, buckling, and electronic properties of freestanding TBG, highlighting new phenomena and their underlying physics.

Findings

Collapse of bending rigidity below critical angle

Persistence of buckling at higher temperatures

Unusual electronic band structure with eight narrow bands

Abstract

The freestanding twisted bilayer graphene (TBG) is unstable, below a critical twist angle {\theta}_c~3.7 degrees, against a moire (2 \times 1) buckling distortion at T=0. Realistic simulations reveal the concurrent unexpected collapse of the bending rigidity, an unrelated macroscopic mechanical parameter. An analytical model connects bending and buckling anomalies at T=0, but as temperature rises the former fades, while buckling persists further. The (2 \times 1) electronic properties are also surprising. The magic twist angle narrow bands, now eight in number, fail to show zone boundary splittings despite the new periodicity. Symmetry shows how this is dictated by an effective single valley physics. These structural, critical, and electronic predictions promise to make the freestanding state of TBG especially interesting.