Abstract:

Both steady-state and non-equilibrium spectroscopy probes of lattice dynamics (like Raman or coherent phonon spectroscopy) put forward the electron-phonon coupling (EPC) as the predominant mechanism shaping the observed experimental features in graphene. Extensive research has been conducted on the broadening mechanisms and temperature dependence of the graphene E2g mode using Raman spectroscopy, with efforts to distinguish between anharmonic and EPC contributions, but also falling short of describing the experiment accurately and with microscopic insights. We demonstrate the importance of going beyond the commonly used first-order perturbation theory when calculating EPC. These higher-order electron-phonon scattering effects introduce electron-mediated anharmonicity and lead to further temperature dependence in electron-coupled phonon modes and their linewidths. Contrary to the common belief, we reach the conclusion that EPC plays an important role in explaining temperature-dependent Raman features of graphene, especially for higher dopings. Further insight into the significance of higher-order EPC is gained through the investigation of nonequilibrium phenomena, as in ultrafast Raman experiments. In such scenarios, the timescale leaves EPC as the primary temperature-dependent mechanism affecting graphene spectral features, making the mentioned higher-order contributions even more pronounced and able to reproduce the experimentally observed temperature trends. However, one of the main goals of ultrafast experiments is understanding the out-of-equilibrium interactions and how they evolve in time during carrier relaxation. On these very short time scales, graphene undergoes several distinct electron distributions. We present an extensive study of the phonon spectrum and EPC in pristine and doped graphene during a strong optical excitation and its relaxation. Some of the compelling features we observe are the photo-induced phonon gain, found for some wave vectors, enhanchment of the EPC, as well as anomalous phonon frequency shifts of strongly-coupled modes, resulting from the interplay between ultrafast scatterings between nonequilibrium electrons and strongly coupled optical phonons. These results provide insights into the complex effects of photoexcitation on EPC and track their temporal evolution.