Spectroscopy on Landau-quantized charge neutral graphene

Landau level spectroscopy is a powerful tool to determine the Fermi velocity in Dirac-like
materials. In our study, we investigate the optical properties of charge neutral graphene on
SiC in presence of strong magnetic fields applied perpendicularly to the sample. For details
about this neutral large-area monolayer graphene, see [1]. We measured the linear
transmission in dependence of the magnitude of the magnetic fields from 0 to 8 T for photon
energies up to 100 meV. We observe a broad resonance resulting from the LL-1(LL0)→LL0(LL1)
transition. It corresponds to a low Fermi velocity of 0.85x106 m/s, which is the limit of low
electron-electron interaction [2]. The efficient screening is supported by the dielectric
environment of the polymer layer on top of the graphene [3]. We support our understanding
of the low Fermi velocity by density functional theory (DFT) calculations. In addition, we
studied the dynamics of the LL-1→LL0 and LL0→LL1 transitions using circularly polarized light
at 75 meV. We observe a fast decay of about 10 ps. Addressing the transitions with all four
combinations of pump and probe beam polarizations, we obtain only one negative
differential transmission signal. Following [4], we attribute this and the fast decay to very
efficient Auger scattering processes. The observed fluence dependence of the relaxation
with time constants decreasing with increasing fluence is also in accordance with this
interpretation.
[1] H. He, K. H. Kim, A. Danilov et al., Nat. Comm., 9, (2018) 3956.
[2] D. Siegel, C.-H. Park, C. Hwang et al., PNAS, 108, 28 (2011) pp. 11365-11369.
[3] P. Yadav and S. Ghosh, AIP Conference Proceedings, 1665, (2015) 050075.
[4] M. Mittendorff, F. Wendler, E. Malic et al., Nat. Phys., 11 (2015) pp. 75-81.