Anomalies in G and 2D Raman Modes of Twisted Bilayer Graphene Near the Magic Angle

TL;DR

This study investigates how twist angles near the magic angle affect phononic and electronic properties of twisted bilayer graphene, revealing phonon hybridization, mode splitting, and enhanced electron-phonon interactions.

Contribution

It provides systematic Raman spectroscopy analysis of twist angle effects, especially near the magic angle, highlighting phonon hybridization and electron-phonon coupling phenomena.

Findings

G mode splits near the magic angle due to moiré potential

Enhanced broadening of G$^-$ and 2D modes near the magic angle

Significant increase in phonon anharmonicity at the magic angle

Abstract

The role of twist angle ($\theta_t$) in tailoring the physical properties of heterostructures is emerging as a new paradigm in two-dimensional materials. The influence of flat electronic bands near the magic angle ($\sim$1.1$^{\circ}$) on the phononic properties of twisted bilayer graphene (t-BLG) is not well understood. In this work, we systematically investigate the G and 2D Raman modes of t-BLG samples with twist angles ranging from $\sim$0.3$^{\circ}$ to $\sim$3$^{\circ}$ using micro-Raman spectroscopy. A key finding of our work is the splitting of the G mode near the magic angle due to moir\'e potential induced phonon hybridization. The linewidth of the low-frequency component of the G mode (G$^-$), as well as the main component of the 2D mode, exhibits enhanced broadening near the magic angle due to increased electron-phonon coupling, driven by the emergence of flat electronic bands. Additionally, temperature-dependent Raman measurements (6-300 K) of magic-angle twisted bilayer graphene sample ($\theta_t \sim$ 1$^{\circ}$) reveal an almost tenfold increase in phonon anharmonicity-induced temperature variation in both components of the split G mode, as compared to Bernal-stacked bilayer graphene sample, further emphasizing the role of phonon hybridization in this system. These studies could be important for understanding the thermal properties of the twisted bilayer graphene systems.