Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The linear dispersion for electrons in graphene leads to a non-equidistant Landau-level (LL) spectrum. This enables one to investigate single Landau-level transitions by resonant pump-probe experiments. So far, however, only the quasi-continuum of high-index LL states has been studied in near-infrared pump-probe experiments. Here we report on terahertz pump-probe measurements at a fixed photon energy of 14 meV, where the free-electron laser FELBE at Dresden-Rossendorf served as radiation source. The magnetic field was varied between 0 T and 7 T. For these measurements we used a sample with 50 layers of graphene, which were epitaxially grown on the C-face of silicon carbide. While the interface layers are highly doped, the major part of the layers is quasi neutral with a Fermi energy in the range of ~10 meV.
At a magnetic field of 0.165 T the photon energy of 14 meV becomes resonant with the inter-LL transition LL-1(0) -> LL0(1). The amplitude of the pump-probe signal is increased by roughly a factor of 4 compared to zero field. Only the quasi neutral layers contribute to this signal, since the first LL is fully occupied for the interface layers with a higher doping. Surprisingly a decreased relaxation time is observed on resonance. We will discuss how the contribution of the LL-1 -> LL0 and LL0 -> LL1 and Auger-type processes lead to a unique relaxation scenario. At higher magnetic fields higher intraband LL transitions can be tuned into resonance (LLn -> LLn+1, n=1,2,3,…). The observed pump-probe signals then stem from these transitions in the interface layers, where the elevated Fermi energy ensures that the ground state LL is populated. With increasing magnetic field both amplitude and decay time constants vary non-monotonically. In the magnetic field region around 3 T negative pump-probe signals occur. The main features observed in the experiments are explained by a model for the electron temperature after optical excitation, which describes the absorption in the layers via the Kubo formalism and takes into account the doping level of the interface layers. The doping levels agree well with values previously obtained from pump-probe experiments without magnetic field.