Efficient orientational carrier relaxation in optically excited graphene

Based on a microscopic theory, we investigate the orientational equilibration of the temporally resolved Coulomb- and phonon-induced carrier relaxation dynamics in the first tens of femtoseconds after an optical excitation. The initially highly anisotropic non-equilibrium carrier distribution becomes completely isotropic already within the first 50 fs. Our calculations clearly demonstrate that the carrier-phonon interaction provides dominant scattering channels for processes across the Dirac cone inducing a rapid orientational relaxation. In contrast, Coulomb interaction prefers scattering along the Dirac cone contributing to a fast thermalization of the excited carrier system.