Dynamical Phonons Following Electron Relaxation Stages in Photo-excited Graphene

TL;DR

This paper investigates the ultrafast electron-phonon relaxation in graphene using first-principles calculations, revealing complex phenomena like phonon hardening, nonadiabatic effects, and coherent phonon production during non-equilibrium states.

Contribution

It provides detailed first-principles insights into the dynamical phonon behavior and electron-phonon interactions in graphene under photo-excitation, highlighting new mechanisms of phonon dressing and relaxation.

Findings

Photo-induced phonon hardening observed

Increase in relaxation rate and nonadiabaticity during excitation

Coherent phonon modes produced in population inversion state

Abstract

Ultrafast electron-phonon relaxation dynamics in graphene hides many distinct phenomena, such as hot phonon generation, dynamical Kohn anomalies, and phonon decoupling, yet still remains largely unexplored. Here, we unravel intricate mechanisms governing the vibrational relaxation and phonon dressing in graphene at a highly non-equilibrium state by means of first-principles techniques. We calculate dynamical phonon spectral functions and momentum-resolved linewidths for various stages of electron relaxation and find photo-induced phonon hardening, overall increase of relaxation rate and nonadiabaticity as well as phonon gain. Namely, the initial stage of photo-excitation is found to be governed by strong phonon anomalies of finite-momentum optical modes along with incoherent phonon production. Population inversion state, on the other hand, allows production of coherent and strongly-coupled phonon modes. Our research provides vital insights into the electron-phonon coupling phenomena in graphene, and serves as a foundation for exploring non-equilibrium phonon dressing in materials where ordered states and phase transitions can be induced by photo-excitation.