Anomalously Strong 2D Band Intensity in Twisted Bilayer Graphene: Raman Evidence for Doubly Degenerate Dirac Band

TL;DR

This study reveals an unusually strong 2D Raman band in twisted bilayer graphene caused by quantum interference effects related to nearly degenerate Dirac bands, providing new insights into its electronic structure.

Contribution

It demonstrates the first observation of enhanced 2D Raman band intensity in tBLG due to quantum interference from degenerate Dirac bands.

Findings

2D band intensity can be four times stronger than in single-layer graphene.

Rotation-dependent G-line resonances and folded phonons observed under UV excitation.

Enhanced 2D band linked to constructive quantum interference between Raman pathways.

Abstract

We report the observation of anomalously strong 2D band in twisted bilayer graphene (tBLG) with large rotation angles under 638-nm and 532-nm visible laser excitation. The 2D band of tBLG can reach four times as opposed to two times as strong as that of single layer graphene. The same tBLG samples also exhibit rotation dependent G-line resonances and folded phonons under 364-nm UV laser excitation. We attribute this 2D band Raman enhancement to the constructive quantum interference between two double-resonance Raman pathways which are enabled by nearly degenerate Dirac band in tBLG Moir\'e superlattices.