Microscopic study of the efficiency of Coulomb- and phonon-induced relaxation channels in graphene

We present a microscopic approach on ultrafast relaxation dynamics of optically excited carriers in graphene. In agreement with experimental differential transmission spectra, we find the dynamics to be characterized by two timescales: the ultrafast femtosecond thermalization and the carrier cooling on a picosecond timescale. The framework of the density matrix theory allows to separately analyze the strength of different relaxation channels, such as carrier-carrier and carrier-phonon scattering, resolved in time, momentum, and angle. Our calculations reveal that the initial thermalization is governed by the Coulomb interaction, however the carrier-phonon scattering also considerably contributes to the process of forming a hot Fermi distribution. The energy dissipation and the cooling of carriers is determined by phonon-induced intra- and interband scattering. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim