Hot electron relaxation and phonon dynamics in graphene

Butscher, S.; Milde, Frank; Hirtschulz, Matthias; Malić, Ermin; Knorr, Andreas

doi:10.1063/1.2809413

Cited by 179 publications

(208 citation statements)

References 20 publications

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“…The ultrashort time needed for a hot-electron distribution to thermalize is a consequence of e-e collisions [9][10][11][29][30][31][32] . We now theoretically investigate the possible Coulomb-mediated two-body collisions in SLG (Fig.…”

Section: Resultsmentioning

confidence: 99%

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Ultrafast collinear scattering and carrier multiplication in graphene

et al. 2013

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Graphene is emerging as a viable alternative to conventional optoelectronic, plasmonic and nanophotonic materials. The interaction of light with charge carriers creates an out-ofequilibrium distribution, which relaxes on an ultrafast timescale to a hot Fermi-Dirac distribution, that subsequently cools emitting phonons. Although the slower relaxation mechanisms have been extensively investigated, the initial stages still pose a challenge. Experimentally, they defy the resolution of most pump-probe setups, due to the extremely fast sub-100 fs carrier dynamics. Theoretically, massless Dirac fermions represent a novel many-body problem, fundamentally different from Schrödinger fermions. Here we combine pump-probe spectroscopy with a microscopic theory to investigate electron-electron interactions during the early stages of relaxation. We identify the mechanisms controlling the ultrafast dynamics, in particular the role of collinear scattering. This gives rise to Auger processes, including charge multiplication, which is key in photovoltage generation and photodetectors.

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Section: Resultsmentioning

confidence: 99%

“…The effect of quantum coherences can be taken into account by employing the semiconductor Bloch equations 29 or, to a greater degree of accuracy, quantum kinetic equations 37 . Including quantum coherences is expected to significantly affect the results only on the very short timescale below 10 fs (see, in particular, Fig.…”

Section: Discussionmentioning

confidence: 99%

Ultrafast collinear scattering and carrier multiplication in graphene

et al. 2013

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“…While graphene is by no means a unique example of a system with these properties, it is believed to fit the bill better than other materials. This has motivated an intense investigation of photoexcitation processes in graphene-based systems [1][2][3][4][5][6][7][8][9][10][11][12].…”

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confidence: 99%

Photoexcited carrier dynamics and impact-excitation cascade in graphene

et al. 2013

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Photo-excitation in solids can trigger a cascade in which multiple particle-hole excitations are generated. We analyze the carrier multiplication cascade of impact excitation processes in graphene and show that the number of pair excitations has a strong dependence on doping, which makes carrier multiplication gate-tunable. We also predict that the number of excited pairs as well as the characteristic time of the cascade scale linearly with photo-excitation energy. These dependences, as well as sharply peaked angular distribution of pair excitations, provide clear experimental signatures of carrier multiplication.Converting light to electrical currents or voltages is a complex, multi-step process which involves photoexcited particle-hole pairs undergoing scattering by ambient charge carriers, by other photoexcited carriers and by lattice vibrations. One of the key questions in the field of optoelectronics is identifying materials in which carrier multiplication can occur, i.e. a single absorbed photon yielding a large number of particle-hole pairs as a result of the primary photoexcited pair producing secondary pairs. Efficient carrier multiplication relies on a combination of characteristics such as a wide band of states with a large phase space density for pair excitations, strong electron-electron scattering, and not too strong electronphonon interaction. While graphene is by no means a unique example of a system with these properties, it is believed to fit the bill better than other materials. This has motivated an intense investigation of photoexcitation processes in graphene-based systems [1][2][3][4][5][6][7][8][9][10][11][12].One aspect of graphene that distinguishes it from other materials is its truly two-dimensional structure which renders electronic states fully exposed. Photoexcitation in such a system generates photoexcited carriers that can in principle be extracted by a vertical transfer process, e.g. in a sandwich-type tunneling structure. Vertical carrier extraction eliminates carrier loss in a lateral transport betwen photoexcitation region and contacts, often an important limiting factor for optoelectronic response in semiconductor systems.Despite intense interest, the photo-excitation cascade in graphene remains poorly understood. Theory predicts that the linear dispersion of charge carriers acquires a negative curvature due to electron-electron interactions, d 2 (k)/dk 2 < 0 [13], which inhibits decay via electronelectron scattering in undoped graphene [11]. However, while the prediction of negative curvature appears to be in agreement with transport measurements [14], ARPES experiments support the notion of interaction-mediated decay [15,16]; interaction-induced quasiparticle decay remains the subject of ongoing debate [11,12,17].Besides its immediate utility for optolectronics, photoexcitation cascade in graphene is of interest because of its analogy with jets in particle physics, which are nar- row cones of hardrons and other particles produced in high-energy particle detectors. Massless...

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“…Such a distribution has been predicted in a recent microscopic calculation taking into account e-p scattering. 24 Because of a hot phonon distribution and a phase-space bottleneck near the K points, a Fermi-Dirac distribution with a high electronic temperature is achieved at much later time ͓0.5 ps ͑Refs. 8 and 24͔͒.…”

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confidence: 99%

Ultrafast electron-phonon decoupling in graphite

et al. 2008

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We report the ultrafast dynamics of the 47.4 THz coherent phonons of graphite interacting with a photoinduced nonequilibrium electron-hole plasma. Unlike conventional materials, upon photoexcitation the phonon frequency of graphite upshifts, and within a few picoseconds relaxes to the stationary value. Our first-principles density functional calculations demonstrate that the phonon stiffening stems from the light-induced decoupling of the nonadiabatic electron-phonon interaction by creating a nonequilibrium electron-hole plasma. Timeresolved vibrational spectroscopy provides a window on the ultrafast nonquilibrium electron dynamics. Graphite possesses highly anisotropic crystal structure, with strong covalent bonding of atoms within and weak van der Waals bonding between the hexagonal symmetry graphene sheets. The layered lattice structure translates to a quasi-two-dimensional ͑2D͒ electronic structure, in which the electronic bands disperse linearly near the Fermi level ͑E F ͒ and form pointlike Fermi surfaces. The discovery of massless relativistic behavior of quasiparticles at E F of graphene and graphite has aroused great interest in the nature of carrier transport in these materials.1-3 Because of the linear dispersion of the electronic bands in graphene, the quasiparticle mass associated with the charge carrier interaction with the periodic crystalline lattice nearly vanishes, leading to extremely high electron mobilities and unusual halfinteger quantum Hall effect.1,2 Since graphite has a quasi-2D band structure very similar to that of graphene, these electronic properties may be expressed also in graphite.The electron-phonon ͑e-p͒ interaction contributes to the carrier mass near E F and limits the high-field transport through the carrier scattering. The strong e-p interaction in graphite is a distinctive characteristic of ineffective screening of the Coulomb interaction in semimetals. 4,5 It is expressed in the phonon frequency shift by carrier doping 6 and the strong electronic renormalization of the phonon bands ͑Kohn anomalies͒.7 Time-resolved measurements on the optically generated nonthermal electron-hole ͑e-h͒ plasma in graphite provide evidence for the carrier thermalization within 0.5 ps both through electron-electron ͑e-e͒ scattering and optical phonon emission. 8 The nonthermal carriers decay nonuniformly in phase space because of the anisotropic band structure of graphite. 5,9 Quasiparticle correlations in nonthermal plasmas can also be probed from the perspective of the coherent optical phonons. In the present work, through the time-dependent complex self-energy ͑frequency and lifetime͒ of the 47 THz E 2g2 phonon of graphite, we study the transient changes in the e-p coupling induced by the optical perturbation of the nonadiabatic Kohn anomaly.To probe the ultrafast response of the coherent phonons, we perform transient anisotropic reflectivity measurements on a natural single crystal and highly oriented pyrolytic graphite ͑HOPG͒ samples. Because the phonon properties were identical, we repor...

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Hot electron relaxation and phonon dynamics in graphene

Cited by 179 publications

References 20 publications

Ultrafast collinear scattering and carrier multiplication in graphene

Ultrafast collinear scattering and carrier multiplication in graphene

Photoexcited carrier dynamics and impact-excitation cascade in graphene

Ultrafast electron-phonon decoupling in graphite

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