Limits on graphene's charge carrier mobility and what we can do to overcome them --- interactions and disorder in epitaxial graphene on silicon carbide

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Epitaxial graphene on SiC(0001) is the favorite realization of graphene for wafer-sized growth with precise thickness control. Due to the small interaction with the substrate, the conical band structure is well-preserved. While the material system is in principle suitable for the application in electronic devices --- back gating and doping have been successfully implemented --- the reduced carrier mobility with respect to suspended graphene sheets seems to be an insuperable obstacle. Transport in high-speed electronic graphene devices is intrinsically limited by the scattering of the charge carriers from hot optical phonons. In addition, the charge carrier mobility is impaired by extrinsic effects, such as the presence of a substrate, defects or impurities. Despite previous efforts, a better understanding of both intrinsic and extrinsic effects is highly desirable. We will use time- and angle- resolved photoemission spectroscopy (ARPES) to access the decay channels of hot carriers in graphene and combine it with high-resolution static ARPES as well as simple model calculations to investigate the effects of different substrates, defects and impurities on the electronic structure of graphene. The graphene/SiC(0001) system offers an ideal playground for such investigations as it allows for the intercalation of various atoms (e.g. Au, Ge and H) in between the graphene-SiC interface as well as chemical doping by atom or molecule adsorption. Furthermore, controlled defect densities can be obtained by deliberately destroying the graphene lattice with Argon ion bombardment. In a second step, we will build on the knowledge gained through these studies to engineer a graphene/SiC(0001) system with an optimized graphene-SiC interface and doping level. For this purpose, we will exploit the interplay of different quasiparticle interactions with substrate screening, chemical doping, as well as deliberately created defect concentrations, and find a combination that results in the best possible conditions for graphene's charge carriers. Finally, using selective coherent phonon excitation with femtosecond optical pulses at mid-infrared wavelengths, we will attempt to tip the equilibrium balance between different quasiparticle interactions in graphene and induce novel electronic properties absent in equilibrium. Resonant excitation of the IR-active A_2u out-of-plane phonon will result in an average non-zero coupling between the otherwise uncoupled pi- and sigma-bands. This coupling might trigger phase transitions into the predicted quantum spin Hall phase or even into a transient superconducting phase. The combination of static and time-resolved ARPES is essential for a more complete understanding of interactions and disorder as only both a good time and energy resolution will ultimately reveal the intricacies of the peculiar electronic structure in graphene.