Probing Layer Localization in Twisted Graphene Bilayers via Cyclotron Resonance

TL;DR

This paper uses cyclotron resonance spectra in magnetic fields to identify whether electron wavefunctions in twisted bilayer graphene are localized in a single layer or delocalized across layers, revealing their nature and critical points.

Contribution

It introduces a method to distinguish wavefunction localization in twisted bilayer graphene using optical absorption spectra under different configurations and biases.

Findings

Optical absorption spectra reveal wavefunction localization characteristics.

Different configurations show distinct selection rules for transitions.

Intra-Landau level absorption sharply increases at saddle point energies.

Abstract

Electron wavefunctions in twisted bilayer graphene may have a strong single layer character or be intrinsically delocalized between layers, with their nature often determined by how energetically close they are to the Dirac point. In this paper, we demonstrate that in magnetic fields, optical absorption (cyclotron resonance) spectra contain signatures which may be used to distinguish the nature of these wavefunctions at low energies, as well as to locate low energy critical points in the zero-field energy spectrum. Optical absorption for two different configurations -- electric field parallel and perpendicular to the bilayer -- are calculated, which are shown to have different selection rules with respect to which states are connected by the perturbation. Interlayer bias further distinguishes transitions involving states of a single layer nature from those with support in both layers. For doped systems, a sharp increase in intra-Landau level absorption occurs with increasing field as the level passes through the zero-field saddle point energy, where the states change character from single layer to bilayer.