Transparent mirror effect in twist-angle-disordered bilayer graphene

TL;DR

This paper explores how spatially disordered twist angles in bilayer graphene create a mirror-like effect for electrons, leading to near-perfect reflection at certain angles due to interference and Klein tunneling effects.

Contribution

It introduces a theoretical framework for understanding electron localization and transmission in twisted bilayer graphene with twist angle disorder, revealing mechanisms for electron collimation and filtering.

Findings

Localization length diverges at normal incidence due to Klein tunneling

Disorder causes angle-dependent shifts in perfect transmission

Potential for disorder-induced electron collimation and valley filtering

Abstract

When light is incident on a medium with spatially disordered index of refraction, interference effects lead to near-perfect reflection when the number of dielectric interfaces is large, so that the medium becomes a "transparent mirror." We investigate the analog of this effect for electrons in twisted bilayer graphene (TBG), for which local fluctuations of the twist angle give rise to a spatially random Fermi velocity. In a description that includes only spatial variation of Fermi velocity, we derive the incident-angle-dependent localization length for the case of quasi-one-dimensional disorder by mapping this problem onto one dimensional Anderson localization. The localization length diverges at normal incidence as a consequence of Klein tunneling, leading to a power-law decay of the transmission when averaged over incidence angle. In a minimal model of TBG, the modulation of twist angle also shifts the location of the Dirac cones in momentum space in a way that can be described by a random gauge field, and thus Klein tunneling is inexact. However, when the Dirac electron's incident momentum is large compared to these shifts, the primary effect of twist disorder is only to shift the incident angle associated with perfect transmission away from zero. These results suggest a mechanism for disorder-induced collimation, valley filtration, and energy filtration of Dirac electron beams, so that TBG offers a promising new platform for Dirac fermion optics.