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O: Fachverband Oberflächenphysik

O 2: Focus Session: Frontiers of Electronic-Structure Theory – Advances in Time-Dependent and Nonequilibrium Ab Initio Methods I

O 2.1: Talk

Monday, March 18, 2024, 10:30–10:45, HE 101

Electron-phonon coupling in photoexcited graphene — •Nina Girotto and Dino Novko — Institute of Physics, Zagreb, Croatia

Understanding the out-of-equilibrium interactions and how they evolve in time is one of the main goals of ultrafast experiments. Here, we present an extensive study of the phonon spectrum and electron-phonon coupling in pristine and doped graphene during a strong optical excitation, calculated from first principles density-functional perturbation theory with adjusted occupations of the electronic states. By promoting a substantial number of electrons from the valence to the conduction band with a laser pulse, the thermalization process of photoexcited carriers includes three distinct electron distributions; photoexcited state is achieved exactly after the laser pulse and is followed by the photo-inverted state (cca 100 fs), and finally at timescales at the order of the electron thermalization time, the electrons follow a hot Fermi-Dirac distribution. In the three time regimes, we find renormalizations of the real and imaginary parts of the phonon self-energy, resulting from the interplay between ultrafast scatterings between nonequilibrium electrons and strongly coupled optical phonons and changes in the electron-phonon coupling matrix elements. Some of the compelling features are the photo-induced negative phonon linewidth, or 'phonon gain', found for some wave vectors, as well as anomalous phonon frequency shifts of strongly-coupled modes. Our results shed light on the intricate effects of photoexcitation on electron-phonon coupling, and track their changes in time.

Keywords: nonequilibrium; electron-phonon coupling; graphene