Unusual Coulomb scattering processes in graphene


Unusual Coulomb scattering processes in graphene

Winnerl, S.

Graphene, the two-dimensional allotrope of carbon, is characterized by a gapless linear band structure. This unique band structure is directly related to some unusual phenomena regarding the ultrafast Coulomb dynamics in the material. The understanding of this dynamics is of vital interest, both from a fundamental as well as from an application oriented point of view. The elastic Coulomb scattering is the main mechanism for thermalization of a non-equilibrium carrier distribution.
We investigate the carrier dynamics in a regime, where scattering via optical phonons is strongly suppressed. To this end, time resolved spectroscopy was performed with radiation of photon energies around 75 meV, which is smaller than the optical phonon energy (~200 meV). In polarization resolved experiments using co- and cross-polarized linearly polarized pump and probe beams, respectively, the Coulomb scattering dynamics is investigated. We find a two-fold nature of this process, namely very fast (fs timescale) collinear Coulomb scattering but surprisingly slow (ps-timescale) non-collinear scattering [1].
In the presence of a magnetic field, the linear band structure of graphene splits up into a series of non-equidistant Landau levels. Studying the transitions between the lowest Landau levels in pump-probe and four-wave mixing experiments, we find evidence for strong Auger scattering [2, 3].

[1] 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).

[2] 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 Phys. 11, 75 (2015).

[3] J. C. König-Otto, Y. Wang, A. Belyanin, C. Berger, W. A. de Heer, M. Orlita, A. Pashkin, H. Schneider, M. Helm, and S. Winnerl (submitted).

Keywords: graphene; carrier dynamics; Coulomb scattering; Auger scattering; ultrafast spectroscopy

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