Electron dynamics in graphene in the presence of an electrical field
Graphene as atomically thin two-dimensional material exhibits remarkable
optical and electronic properties that suggest its technological application
in novel optoelectronic devices, such as graphene-based lasers and photodetectors.
The linear electronic bandstructure and the vanishing band gap at
the Dirac point open up new relaxation channels, such as Auger scattering.
Here, optically excited carriers can eciently bridge the valence and the
conduction band, which might result in an increase of the number of charge
carriers (electrons and holes), i.e. by absorbing a single photon one can create
multiple electron-hole pairs through internal scattering. This many-particle
process is called carrier multiplication (CM) and has a large technological
potential. In the presence of an electric eld, carriers become accellerated in
the momentum space depleting the region around the Dirac point and providing
optimal conditions for Auger scattering and CM. To investigate ultrafast
phenomena characterizing the carrier dynamics in graphene, we develop a
microscopic approach based on the density matrix formalism and the semiconductor
Bloch equations, which provides microscopic access to the timeand
momentum resolved carrier dynamics in the presence of an electric eld.
The aim of the thesis is to investigate the many-particle processes behind the
ultrafast electron dynamics in graphene. The focus lies on understanding the
dynamics in the presence of an electrical eld and in particular providing a
microscopic foundation for the photoconduction eect, which is crucial for
the application of graphene as an ultrafast photodetector. The highlight
of the thesis is the proposal of a very ecient dark carrier multiplication
in the presence of an electrical eld. While scattering processes are generally
considered to reduce the eld-induced current, we have revealed that
in graphene Auger processes give rise to a signicant current enhancement
via dark CM. Furthermore, we have investigated the interplay of optical excitation,
many-particle scattering and eld-induced acceleration of carriers
resulting in asymmetric scattering processes and generation of photocurrents.
density matrix formalism
PJ-Salen, Origo, Kemigården 1, Chalmers University of Technology
Opponent: Assistant Professor Witlef Wieczorek, Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Sweden