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Phonon cooling pathways of hot electrons in graphene - Bernard Plaçais

Laboratoire Pierre Aigrain, ENS, Paris

LPS – amphi Moyen

Graphene is a versatile platform to study electron-phonon relaxation in 2D conductors. It can be investigated by combining high-frequency noise thermometry and Joule heating to access the power-law dependence P∝Te^α of acoustic-phonon cooling power P with the electronic temperature Te. In the low-temperature Bloch-Gruneïsen regime we observe α=4 [1], whereas a supercollision mechanism (α=3) circumvents the one-phonon bottleneck (α=1) at high temperature. Optical-phonon cooling shows up at high energy, with an activation law observed in bilayer graphene [3,4].

Due to weak acoustic phonon scattering, the Dirac fluid becoming essentially decoupled from the host lattice in high-mobility graphene. It becomes subject to interactions with its environment. At current saturation, where heat conduction is suppressed and interband Zener-Klein transport prevails, we observe a spectacular drop of noise signaling the ignition of a new and very efficient cooling pathway. It is associated with the emission of hyperbolic phonon-polaritons in the h-BN substrate [5] under Zener-Klein population inversion.

Phonon emission is suppressed in the quantum Hall regime as electrons are localized along cyclotron orbits. The noiseless ballistic quantum-Hall regime breaks down at a critical field associated with inter Landau level tunneling. Above that field we observe a surge of shot-noise which eventually approaches the full shot noise limit [6]. We show that the breakdown velocity and the post-breakdown shot-noise obbey a Quantum Hall scaling and are controlled by electron-electron interactions with the spontaneous emission of magneto-excitons in the bulk.

[1] A. Betz et al., Phys. Rev. Lett. 109, 056805 (2012)
[2] A. Betz et al., Nat. Phys. 9, 109-112 (2013)
[3] A. Laitinen et al., Phys. Rev. B 91, 121414(R) (2015)
[4] D. Brunel al., J. Phys. Cond-M. 27, 164208 (2015)
[5] W. Yang et al., Nat. Nanotech. 13, 47-52 (2018)
[6] W. Yang et al., Phys. Rev. Lett. 121, 136804 (2018)


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