Observation of Anomalous Phonon Softening in Bilayer Graphene

TL;DR

This paper reports the discovery of an anomalous phonon softening in bilayer graphene, linked to charge-tunable electron-phonon interactions, confirmed through low-temperature Raman spectroscopy, revealing new insights into lattice dynamics and disorder effects.

Contribution

It provides the first experimental evidence of a phonon anomaly caused by resonant deformation-potential electron-phonon coupling in bilayer graphene.

Findings

Observation of phonon softening linked to charge density modulations.

Logarithmic divergence in phonon energy predicted and partially observed.

Sensitivity of phonon behavior to charge density inhomogeneity.

Abstract

The interaction of electron-hole pairs with lattice vibrations exhibits a wealth of intriguing physical phenomena. The Kohn anomaly is a renowned example where electron-phonon coupling leads to non-analytic phonon dispersion at specific momentum nesting the Fermi surface. Here we report evidence of another type of phonon anomaly discovered by low temperature Raman spectroscopy in bilayer graphene where the charge density is modulated by the electric field effect. This anomaly, arising from charge-tunable modulations of particle-hole pairs that are resonantly coupled to lattice vibrations, is predicted to exhibit a logarithmic divergence in the long-wavelength optical-phonon energy. In a non-uniform bilayer of graphene, the logarithmic divergence is abated by charge density inhomogeneity leaving as a vestige an anomalous phonon softening. The observed softening marks the first confirmation of the phonon anomaly as a key signature of the resonant deformation-potential electron-phonon coupling. The high sensitivity of the phonon softening to charge density non-uniformity creates significant venues to explore the interplay between fundamental interactions and disorder in the atomic layers.