Carrier dynamics in graphene near the Dirac point

The relaxation dynamics of charge carriers in graphene is characterized by interesting properties such as impact ionization resulting in carrier multiplication [1,2]. Apart from the fundamental interest, knowledge about the carrier dynamics is important for the development of graphene-based electronic and optoelectronic devices. In our study we apply picosecond mid-infrared and terahertz pulses to explore the carrier dynamics in the vicinity of the Dirac point. In particular the role of carrier-phonon and carrier-carrier scattering is discussed. The carrier dynamics was studied in single-color pump-probe experiments in a wide spectral range (photon energies: 10 meV – 245 meV). A significant increase of the relaxation time was observed as the photon energy was decreased to values below the optical phonon energy of about 200 meV [3]. Microscopic modelling based on the density matrix formalism revealed a suppression of optical-phonon scattering for low photon energies, however, this process is still more efficient than scattering via acoustic phonon [3,4].
The pump-probe study was extended to graphene in magnetic fields (B = 4.2 T, photon energy 75 meV), where the Landau level (LL) transitions LL-1 -> LL0 and LL0 -> LL1 were resonantly excited. Applying circularly polarized radiation allows one to distinguish between these energetically degenerate transitions. Pump-probe signals for all four different combinations of left- and right-circularly polarized radiation are characterized by complex dynamics involving positive and negative signals as well as fast and slow components. An analysis of the results indicates that the carrier occupation of the three LLs is strongly influenced by Auger-type scattering processes. In particular situations the population change by Auger scattering even dominates over the change induced by optical pumping. In summary the role of carrier-phonon scattering in graphene in the energetic vicinity of the Dirac point is clarified by a joint experiment-theory study. Furthermore we report strong evidence for efficient Auger-type scattering in Landau-quantized graphene. M. Mittendorff, H. Schneider, M. Helm, M. Orlita, and M. Potemski planned and performed the experiments, T. Winzer, F. Wendler, E. Malic, and A. Knorr carried out the microscopic modelling. The samples were grown by M. Sprinkle, C. Berger, and W. A. de Heer.