Nina Girotto and Dino Novko published a paper in The Journal of Physical Chemistry Letters, where they theoretically examined intriguing consequences of a photo-excited electron distribution on phonons in graphene.

Dynamical Phonons Following Electron Relaxation Stages in Photoexcited Graphene
Nina Girotto, Dino Novko

Journal of Physical Chemistry Letters  14, 8709–8716 (2023)
DOI: 10.1021/acs.jpclett.3c01905

Ultrafast light excitations are a powerful tool to control and create new states of matter, making the effects of optically-induced excitations on the electron-lattice system essential to understand. As opposed to the well studied ultrafast electron dynamics, the dynamics of phonons (vibrations of the lattice) in non-equilibrium states is less explored and often overlooked, but crucial for controlling material properties like structural phase transitions and superconductivity.
Our study addresses this gap, focusing on phonon dynamics in optically excited graphene. We use constrained Density Functional Perturbation Theory (cDFPT) and phonon self-energy calculations to obtain a phonon spectral description of graphene during different electron relaxation stages. We reveal the origin of phonon hardening observed in the ultrafast experiments and identify phonon amplification channels, which aids in understanding of phonon generation processes in coherent phonon spectroscopy. These findings and theoretical tools could be applicable to a broader range of materials where electron-phonon coupling is important.

Fig. 1: Dynamical phonon spectral functions at different stages of electron relaxation in comparison with the adiabatic equilibrium DFPT result (grey dashed line), i.e., for: (a) Equilibrium regime, (b) far non-equilibrium following the laser excitation, (c) population inversion, (d) hot equilibrium distribution. Note the strong renormalizations occurring near the two strongly-coupled optical modes (E_2g and A’₁). Also, the negative linewidth contribution to spectral function is shown in teal. The histograms in the insets of (b)-(d) show the nonadiabatic corrections to the E_2g and A’₁ modes. (e) Phonon linewidth on Γ to K path of LO/TO optical modes, due to EPC. (f-g) Intra- and inter-valley (i.e., K→K and K→K′) electron transitions which contribute to the phonon self-energy. The color-coded arrows reveal positive (brown) and negative (teal) contributions to the linewidth. (h) Same as in (e) but for the photo-inverted (yellow) and hot electron distributions (dark red).