Electron dynamics in graphene in the presence of an electrical field

Sammanfattning: Graphene as atomically thin two-dimensional material exhibits remarkableoptical and electronic properties that suggest its technological applicationin novel optoelectronic devices, such as graphene-based lasers and photodetectors.The linear electronic bandstructure and the vanishing band gap atthe Dirac point open up new relaxation channels, such as Auger scattering.Here, optically excited carriers can eciently bridge the valence and theconduction band, which might result in an increase of the number of chargecarriers (electrons and holes), i.e. by absorbing a single photon one can createmultiple electron-hole pairs through internal scattering. This many-particleprocess is called carrier multiplication (CM) and has a large technologicalpotential. In the presence of an electric eld, carriers become accellerated inthe momentum space depleting the region around the Dirac point and providingoptimal conditions for Auger scattering and CM. To investigate ultrafastphenomena characterizing the carrier dynamics in graphene, we develop amicroscopic approach based on the density matrix formalism and the semiconductorBloch equations, which provides microscopic access to the timeandmomentum resolved carrier dynamics in the presence of an electric eld.The aim of the thesis is to investigate the many-particle processes behind theultrafast electron dynamics in graphene. The focus lies on understanding thedynamics in the presence of an electrical eld and in particular providing amicroscopic foundation for the photoconduction eect, which is crucial forthe application of graphene as an ultrafast photodetector. The highlightof the thesis is the proposal of a very ecient dark carrier multiplicationin the presence of an electrical eld. While scattering processes are generallyconsidered to reduce the eld-induced current, we have revealed thatin graphene Auger processes give rise to a signicant current enhancementvia dark CM. Furthermore, we have investigated the interplay of optical excitation,many-particle scattering and eld-induced acceleration of carriersresulting in asymmetric scattering processes and generation of photocurrents.