Coupling between electrons and optical phonons in suspended bilayer graphene

Laitinen, Antti; Kumar, Manohar; Oksanen, Mika; Plaçais, Bernard; Virtanen, Pauli; Hakonen, Pertti

doi:10.1103/physrevb.91.121414

Cited by 30 publications

(32 citation statements)

References 42 publications

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“…Also using electrical transport measurements, signatures of cooling via optical phonons were identified (for bilayer graphene). 54 This cooling mechanism, however, was often thought to only mediate cooling for carriers with high enough energy to couple directly to optical phonons 55 and was typically used for experiments with relatively high fluence and therefore relatively high electron temperatures. For the rest of the carriers in the hot-carrier distribution, and for cooling of systems with relatively low electron temperature, alternative cooling channels were considered.…”

Section: Results and Discussionmentioning

confidence: 99%

Hot-Carrier Cooling in High-Quality Graphene Is Intrinsically Limited by Optical Phonons

et al. 2021

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Many promising optoelectronic devices, such as broadband photodetectors, nonlinear frequency converters, and building blocks for data communication systems, exploit photoexcited charge carriers in graphene. For these systems, it is essential to understand the relaxation dynamics after photoexcitation. These dynamics contain a sub-100 fs thermalization phase, which occurs through carrier–carrier scattering and leads to a carrier distribution with an elevated temperature. This is followed by a picosecond cooling phase, where different phonon systems play a role: graphene acoustic and optical phonons, and substrate phonons. Here, we address the cooling pathway of two technologically relevant systems, both consisting of high-quality graphene with a mobility >10 000 cm 2 V –1 s –1 and environments that do not efficiently take up electronic heat from graphene: WSe 2 -encapsulated graphene and suspended graphene. We study the cooling dynamics using ultrafast pump–probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to substrate phonons is relatively inefficient in these systems, suggesting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a time scale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the high-energy tail of the hot-carrier distribution emit optical phonons. This creates a permanent heat sink, as carriers efficiently rethermalize. We develop a macroscopic model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights will guide the development of graphene-based optoelectronic devices.

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Section: Results and Discussionmentioning

confidence: 99%

Hot-Carrier Cooling in High-Quality Graphene Is Intrinsically Limited by Optical Phonons

et al. 2021

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“…1e shows that the crossing point varies rapidly upon varying T, so that for larger T larger carrier densities are needed for electron motion to become ballistic. This trend is indicative of a microscopic scattering mechanism that causes the electronic mean free path to depend strongly on n and T. Since in the experiments n is varied between a few 10 9 cm -2 and 2×10 11 cm -2 -well above the range associated to carrier density inhomogeneity-and T between 12 and 100 K -below the temperature where electron-phonons scattering sets in 11,12 neither density inhomogeneity nor phonon scattering can account for large changes in mean free path. Furthermore, because the density of states is constant in bilayers, a pronounced dependence of the screening length on n and T can also not be invoked.…”

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confidence: 86%

Electron–hole collision limited transport in charge-neutral bilayer graphene

Nam

Soler-Delgado

et al. 2017

Nature Phys

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Ballistic transport occurs whenever electrons propagate without collisions deflecting their trajectory. It is normally observed in conductors with a negligible concentration of impurities, at low temperature, to avoid electron-phonon scattering. Here, we use suspended bilayer graphene devices to reveal a new regime, in which ballistic transport is not limited by scattering with phonons or impurities, but by electron-hole collisions. The phenomenon manifests itself in a negative four-terminal resistance that becomes visible when the density of holes (electrons) is suppressed by gate-shifting the Fermi level in the conduction (valence) band, above the thermal energy. For smaller densities transport is diffusive, and the measured conductivity is reproduced quantitatively, with no fitting parameters, by including electron-hole scattering as the only process causing velocity relaxation. Experiments on a trilayer device show that the phenomenon is robust and that transport at charge neutrality is governed by the same physics. Our results provide a textbook illustration of a transport regime that had not been observed previously and clarify the nature of conduction through charge-neutral graphene under conditions in which carrier density inhomogeneity is immaterial. They also demonstrate that transport can be limited by a fully electronic mechanism, originating from the same microscopic processes that govern the physics of Dirac-like plasmas.Ever since Sharvin's pioneering work 1 , the occurrence of ballistic transport in metallic conductors has been exploited to investigate the electronic properties of solids. In the quasi-classical regime, for instance, magnetic focusing experiments have allowed probing electron-dynamics and the shape of the Fermi surface in ultra-pure crystals of different metals 2 , in semiconducting heterostructures 3,4 , and -more recently-in graphene-based systems 5,6 . In the quantum regime, when the electron wavelength and the conductor size are comparable, ballistic motion normally leads to conductance quantization and allows probing transport through individual quantum channels 7,8 . In all cases, the observation of ballistic transport requires the elastic mean free path determined by collisions of electrons with impurities to be longer than the system size, and the rate of inelastic processes such as phonon scattering to be sufficiently small. Electron-electron collisions are less detrimental, as they only slowly de-collimate a focused beam of ballistic electrons influencing their trajectories gradually, with effects that usually become relevant at rather high temperature 6,9 .

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“…That is how we can identify thehΩ I 0.1 ± 0.01 eV energy of h-BN HPhPs as the main source of scattering and rule out the scattering by intrinsic graphene optical phonons or the Ω I I -HPhP band, ashΩ OP ∼hΩ I I ∼0.2 eV. Note that the role of intrinsic OPs or Ω I I -HPhPs in energy relaxation can be identified using noise thermometry respectively in suspended [26] and h-BN supported graphene [11,16]. This illustrates the difference between momentum relaxation (the resistance) and energy relaxation (the temperature or electronic distribution) mechanisms (Another example of such a difference, is acoustic phonon scattering which gives rise to a linear temperature dependence for resistivity but a cubic one for relaxation, reflecting single phonon and inelastic supercollision scattering respectively [27]).…”

Section: Substrate Phonon Scattering Modelmentioning

confidence: 92%

High-Frequency Limits of Graphene Field-Effect Transistors with Velocity Saturation

et al. 2020

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The current understanding of physical principles governing electronic transport in graphene field effect transistors (GFETs) has reached a level where we can model quite accurately device operation and predict intrinsic frequency limits of performance. In this work, we use this knowledge to analyze DC and RF transport properties of bottom-gated graphene on boron nitride field effect transistors exhibiting pronounced velocity saturation by substrate hyperbolic phonon polariton scattering, including Dirac pinch-off effect. We predict and demonstrate a maximum oscillation frequency exceeding 20 GHz. We discuss the intrinsic 0.1 THz limit of GFETs and envision plasma resonance transistors as an alternative for sub-THz narrow-band detection.

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Coupling between electrons and optical phonons in suspended bilayer graphene

Abstract: International audienc

Cited by 30 publications

References 42 publications

Hot-Carrier Cooling in High-Quality Graphene Is Intrinsically Limited by Optical Phonons

Hot-Carrier Cooling in High-Quality Graphene Is Intrinsically Limited by Optical Phonons

Electron–hole collision limited transport in charge-neutral bilayer graphene

High-Frequency Limits of Graphene Field-Effect Transistors with Velocity Saturation

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