Experimentally accessible signatures of Auger scattering in graphene

Torben Winzer, Roland Jago, and Ermin Malic

Phys. Rev. B 94, 235430 – Published 23 December 2016

Abstract

The gapless and linear electronic band structure of graphene opens up Auger scattering channels bridging the valence and the conduction band and changing the charge carrier density. Here, we reveal experimentally accessible signatures of Auger scattering in optically excited graphene. To be able to focus on signatures of Auger scattering, we apply a low excitation energy, weak pump fluences, and a cryostatic temperature, so that all relevant processes lie energetically below the optical phonon threshold. In this regime, carrier-phonon scattering is strongly suppressed and Coulomb processes govern the carrier dynamics. Depending on the excitation regime, we find an accumulation or depletion of the carrier occupation close to the Dirac point. This reflects well the behavior predicted from Auger-dominated carrier dynamics. Based on this observation, we propose a multicolor pump-probe experiment to uncover the extreme importance of Auger channels for the nonequilibrium dynamics in graphene.

Received 3 August 2016
Revised 29 November 2016

DOI:https://doi.org/10.1103/PhysRevB.94.235430

Physics Subject Headings (PhySH)

Graphene

Many-body techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Torben Winzer¹, Roland Jago², and Ermin Malic^2,*

¹Institut für Theoretische Physik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
²Chalmers University of Technology, Department of Physics, SE-412 96 Gothenburg, Sweden

^*ermin.malic@chalmers.se

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Issue

Vol. 94, Iss. 23 — 15 December 2016

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Images

Figure 1
Schematic illustration of Coulomb scattering processes. (a) Impact excitation gives rise to an accumulation of carriers close to the Dirac point and a redshift of the optically excited carrier occupation, (b) Coulomb-induced intraband scattering accounts for a spectral broadening around the excitation energy, and (c) Auger recombination reduces the low-energy carrier occupation and leads to a blueshift of the excited carriers.
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Figure 2
Signatures of Auger processes. Carrier occupation $ρ_{k, φ}$ weighted by the density of states (DOS) is shown as a function of carrier energy and angle $φ$ at the maximum of the excitation pulse ( $t = 0$ fs) for (a) a low pump fluence of $0.54 nJ / {cm}^{2}$ and (b) a higher pump fluence of $170 nJ / {cm}^{2}$ . A qualitatively different behavior is observed: While in the low excitation regime, the carriers accumulate at low energies close to the Dirac point (reflecting impact excitation); at higher pump fluences we observe a strong spectral broadening of the carrier distribution including an asymmetric smearing towards higher energies (reflecting Auger recombination). The orange-shaded vertical area corresponds to the spectral width of the excitation pulse.
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Figure 3
Auger scattering rates in different excitation regimes. The rates for impact excitation $γ_{I E}$ and Auger recombination $γ_{A R}$ are shown for (a) $0.54 nJ / {cm}^{2}$ and (b) $170 nJ / {cm}^{2}$ . In the low excitation regime, there is a clear asymmetry in favor of $γ_{I E}$ , while for higher pump fluences $γ_{A R}$ overtakes impact excitation. The Gaussian excitation pulse is depicted in the background.
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Figure 4
Experimentally accessible signatures of Auger processes. Differential transmission spectra (DTS) in the (a) weak and (b) higher excitation regime. The pump energy is fixed to 88 meV, while the probe energy is varied (20 meV red, 88 meV green, and 118 meV blue). We observe in the weak excitation regime a drastic decrease in the DTS amplitude as the probe pulse increases. This reflects the IE-dominated carrier dynamics resulting in a carrier accumulation close to the Dirac point and a vanishing carrier population in the states above the excitation energy [cf. Fig. 2]. The change of the amplitude is much smaller in the AR-dominated higher excitation regime. This reflects the large spectral broadening of the carrier distribution covering energies below and above the excitation energy [cf. Fig. 2]. The solid and dashed lines show the parallel and the perpendicular polarization of the pump and the probe pulse, respectively.
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