Spatially Coherent Tip-Enhanced Raman Spectroscopy Measurements of Electron-Phonon Interaction in a Graphene Device

Coherence length (L (c)) ofthe Ramanscattering process in graphene as a function of Fermi energy is obtainedwith spatially coherent tip-enhanced Raman spectroscopy. L (c) decreases when the Fermi energy is moved into the neutralitypoint, consistent with the concept of the Kohn anomaly within a ballistictransport regime. Since the Raman scattering involves electrons andphonons, the observed results can be rationalized either as due tounusually large variation of the longitudinal optical phonon groupvelocity v (g), reaching twice the valuefor the longitudinal acoustic phonon, or due to changes in the electronenergy uncertainty, both properties being important for optical andtransport phenomena that might not be observable by any other technique.