  Graphene as a zero-bandgap semiconductor is an ideal model structure to study
the carrier relaxation channels, which are inefficient in conventional
semiconductors. In particular, it is of fundamental interest to address the
question whether Auger-type processes significantly influence the carrier
dynamics in graphene. These scattering channels bridge the valence and
conduction band allowing carrier multiplication - a process that generates
multiple charge carriers from the absorption of a single photon. This has been
suggested in literature for improving the efficiency of solar cells. Here we
show, based on microscopic calculations within the density matrix formalism,
that Auger processes do play an unusually strong role for the relaxation
dynamics of photo-excited charge carriers in graphene. We predict that a
considerable carrier multiplication takes place, suggesting graphene as a new
material for high-efficiency solar cells and for high-sensitivity
photodetectors.
