Slow Noncollinear Coulomb Scattering in the Vicinity of the Dirac Point in Graphene

König-Otto, Jacob C.; Mittendorff, Martin; Winzer, Torben; Kadi, Faris; Malić, Ermin; Knorr, Andreas; Berger, Christian; Heer, Walter A. de; Pashkin, Alexej; Schneider, Harald; Helm, M.; Winnerl, Stephan

doi:10.1103/physrevlett.117.087401

Cited by 47 publications

(65 citation statements)

References 54 publications

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“…In agreement with Ref. [13], we find that the anisotropy is much more pronounced in the low-excitation regime, where it persists on a timescale of 10 ps, cf. Fig.…”

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confidence: 76%

“…The carrier distribution is shown 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 . For both excitation strengths, the excited anisotropy of the carrier distribution due to the linear polarization of the excitation pulse [25,26] is conserved on the timescale of the pulse (FWHM of 4 ps) [13]. However, the energetic distribution strongly depends on the excitation strength and exhibits qualitative differences between the low and the strong excitation regime.…”

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confidence: 99%

“…Our work is motivated by a very recent experimental study performed by J. Koenig-Otto and co-workers [13], which demonstrated that phonon-driven carrier relaxation can be strongly suppressed by applying low excitation energies, weak pump fluences, and cryostatic temperatures. This allows a study of purely Coulomb-induced carrier dynamics.…”

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confidence: 99%

“…This is achieved by applying a cryostatic temperature of 16 K, an excitation energy of 88 meV, and low pump fluences in the range of nJ/cm 2 , so that all relevant processes lie energetically below the optical phonon threshold (∝ 200 meV) [13]. Recently, it has been demonstrated that under these conditions carrier-phonon scattering is strongly suppressed resulting in a long-lived anisotropic carrier distribution after excitation with linearly polarized light [13]. Figure 2 illustrates the carrier distribution as a function of carrier energy ε = ν F |k| and angle ϕ, where ν F is the Fermi velocity.…”

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confidence: 99%

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Experimentally accessible signatures of Auger scattering in graphene

2016

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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.

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“…In agreement with Ref. [13], we find that the anisotropy is much more pronounced in the low-excitation regime, where it persists on a timescale of 10 ps, cf. Fig.…”

supporting

confidence: 76%

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Experimentally accessible signatures of Auger scattering in graphene

2016

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“…The existence of anisotropic photocarrier distributions in graphene was predicted [1,2] and observed in optical pump-probe experiments [3][4][5][6], which showed a pronounced difference in the time-dependent optical response for different probe polarizations. The decay of the anisotropy extracted in this manner was attributed to optical phonon emission [2][3][4][5]7,8].…”

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confidence: 99%

Ultrafast momentum imaging of pseudospin-flip excitations in graphene

et al. 2017

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The pseudospin of Dirac electrons in graphene manifests itself in a peculiar momentum anisotropy for photoexcited electron-hole pairs. These interband excitations are in fact forbidden along the direction of the light polarization and are maximum perpendicular to it. Here, we use time-and angle-resolved photoemission spectroscopy to investigate the resulting unconventional hot carrier dynamics, sampling carrier distributions as a function of energy, and in-plane momentum. We first show that the rapidly-established quasithermal electron distribution initially exhibits an azimuth-dependent temperature, consistent with relaxation through collinear electron-electron scattering. Azimuthal thermalization is found to occur only at longer time delays, at a rate that depends on the substrate and the static doping level. Further, we observe pronounced differences in the electron and hole dynamics in n-doped samples. By simulating the Coulomb-and phonon-mediated carrier dynamics we are able to disentangle the influence of excitation fluence, screening, and doping, and develop a microscopic picture of the carrier dynamics in photoexcited graphene. Our results clarify new aspects of hot carrier dynamics that are unique to Dirac materials, with relevance for photocontrol experiments and optoelectronic device applications. DOI: 10.1103/PhysRevB.96.020301The existence of anisotropic photocarrier distributions in graphene was predicted [1,2] and observed in optical pump-probe experiments [3-6], which showed a pronounced difference in the time-dependent optical response for different probe polarizations. The decay of the anisotropy extracted in this manner was attributed to optical phonon emission [2][3][4][5]7,8]. However, a complete picture for these nonequilibrium phenomena can only be obtained by tracking both carrier energy and momentum in the time domain.Here we use time-and angle-resolved photoemission spectroscopy (tr-ARPES) at extreme ultraviolet (XUV) wavelengths to track the temporal evolution of the photoexcited carrier distribution as a function of energy and momentum. We establish a hierarchy of events that redistribute carriers on the Dirac cone, including the formation of a quasithermal state with an azimuth-dependent anisotropic electron temperature, which indicates that primary thermalization occurs through collinear electron-electron scattering. Azimuthal relaxation through phonon emission and noncollinear electron-electron scattering plays a role only at later time delays and is found to be strongly influenced by the substrate and the type of static doping of the graphene layer. Furthermore, the finite doping in our samples breaks the electron-hole symmetry and results in different dynamics for electrons and holes. Microscopic simulations of the anisotropic carrier dynamics indicate that the observed dynamics are due to a subtle interplay between doping that affects the scattering phase space and substrate screening which reduces the influence of electron-electron scattering.* sven.aeschlimann@mpsd.mpg.de † is...

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