Carrier relaxation in Landau-quantized graphene

We report on pump-probe measurements on quasi-neutral sheets of multilayer epitaxial graphene in magnetic fields. Using THz radiation, generated by the free electron laser, we directly probe the relaxation of Dirac fermions among well-resolved Landau levels (LLs). In contrast to conventional semiconductors, the Landau level spacing in graphene is not equidistant. Hence it is possible to investigate a single LL transition selectively. To this end we performed pump-probe measurements at a wavelength of 16.5 µm and applied a magnetic field of up to 7T. We varied the magnetic field for resonant measurements at different LL transitions. For the transitions LL-1(-2) -> LL2(1) and LL-2(-3) -> LL3(2) we could observe a slight increase of the pump-probe signal but no change in the relaxation time. For the transition LL-1(0) -> LL0(1) the pump-probe signal increased by a factor of 2.5 while the relaxation time decreased from 20 ps to 5 ps. These measurements were done with linear polarized radiation. To understand the decrease in the relaxation time additional measurements were carried out with circularly polarized radiation. The circularly polarized radiation enables to distinguish between the transition LL-1->LL0 and LL0->LL1. We performed the experiment in all combinations of left and right polarized radiation for the pump and probe beam. This revealed complex dynamics involving positive and negative signals. We suggest this to result from different relaxation channels including Auger processes.