Unusual Coulomb effects in graphene

In many semiconductors Coulomb scattering plays an essential role in the thermalization process of a non-equilibrium carrier distribution. Here we discuss three surprising and fascinating manifestations of Coulomb scattering in graphene.
The first observation concerns a double-bended saturation behavior of bleaching induced by near-infrared radiation. The complete bleaching at high fluences is related to Pauli blocking. At much lower fluences, however, the balance between scattering into or out of the optically probed regions in k-space via Coulomb interaction results in a qualitatively similar behavior [1].
The second phenomenon is the optically induced anisotropy in k-space for excitation with linearly polarized radiation and its relaxation to a Fermi-Dirac distribution. Polarization resolved pump-probe experiments at different photon energies provide strikingly direct insights into role of individual processes: Carrier-phonon scattering rapidly transforms the initial anisotropic non-equilibrium distribution into an isotropic one [2]. When carrier-phonon scattering is quenched by applying photon energies below the optical phonon energy, however, Coulomb interaction is the only strong source of scattering [3]. As Coulomb scattering in graphene is predominantly collinear, the anisotropy persists for fairly long times (a few ps).
The third set of experiments tackles the dynamics of graphene in a magnetic field perpendicular to the graphene layer. In this case, the band structure beaks up into a series of non-equidistant Landau levels (LLs). We study in detail the population and polarization dynamics of the levels with index -1, 0 and 1. Applying circularly polarized radiation allows one to selectively excite the energetically degenerate transitions LL-1  LL0 and LL0  LL1, respectively. Applying all four combinations of pumping and probing with left and right circularly polarized radiation reveals a surprising behavior: The possibility to deplete the zeroth Landau level while it is optically pumped at the same time [4]. This is caused by strong Auger scattering, the Coulomb process that thermalizes the carrier distribution in Landau quantized graphene. It also causes a fast dephasing of the microscopic polarization, as evidenced in four-wave-mixing experiments [5]. We discuss the possibility to apply Landau quantized graphene as a gain medium in a tunable laser and as a tunable nonlinear optical material.
We are grateful to a number of people, most importantly, from the experimental side, to M. Mittendorff, J. C. König-Otto, H. Schneider and M. Helm. Furthermore to E. Malic, A. Knorr and A. Belyanin for microscopic theory, and to C. Berger and W. A. de Heer for sample growth.
[1] T. Winzer, M. Mittendorff, S. Winnerl, H. Mittenzwey, R. Jago, M. Helm, E. Malic, and A. Knorr, Nature Commun. 8, 15042 (2017).
[2] M. Mittendorff, T. Winzer, E. Malic, A. Knorr, C. Berger, W. A. de Heer, H. Schneider, Manfred Helm, and S. Winnerl, Nano Lett. 14, 1504 (2014).
[3] J. C. König-Otto, M. Mittendorff, T. Winzer, F. Kadi, E. Malic, A. Knorr, C. Berger, W. A. de Heer, A. Pashkin, H. Schneider, M. Helm, and S. Winnerl, Phys. Rev. Lett. 117, 087401 (2016).
[4] M. Mittendorff, F. Wendler, E. Malic, A. Knorr, M. Orlita, M. Potemski, C. Berger, W. A. de Heer, H. Schneider, M. Helm, and S. Winnerl, Nature Physics 11, 75 (2015).
[5] J. C. König-Otto, Yongrui Wang, Alexey Belyanin, C. Berger, W. A. de Heer, M. Orlita, A. Pashkin, H. Schneider, M. Helm, S. Winnerl, Nano Lett. 17, 2184 (2017).