Relaxation of optically excited carriers in graphene: Anomalous diffusion and Lévy flights

We present a theoretical analysis of the relaxation cascade of a photoexcited electron in graphene in the presence of RPA screened electron-electron interaction. We calculate the relaxation rate of high energy electrons and the jump-size distribution of the random walk constituting the cascade which exhibits fat tails. We find that the statistics of the entire cascade are described by L\'evy flights with constant drift instead of standard drift-diffusion in energy space. The L\'evy flight manifests nontrivial scaling relations of the fluctuations in the cascade time, which is related to the problem of the first passage time of L\'evy processes. Furthermore we determine the transient differential transmission of graphene after an excitation by a laser pulse taking into account the fractional kinetics of the relaxation dynamics.