Various mechanisms involved in the electronic momentum and energy relaxation processes will be reviewed based on current and noise transport experiments. Graphene is a model 2D system where situation is enriched by ubiquitous coupling to environmental excitations such as phonons, photons, plasmons or substrate polaritons. In strongly out-of-equilibrium situations, high-frequency noise thermometry is used and cooling pathways are identified by monitoring the Joule power dependence of electronic noise temperature Te(P).
Phonon cooling pathways dominate in diffusive graphene which are characterized by power laws 𝑃∝𝑇𝑒𝛼, with 𝛼=4 for acoustic phonons at low temperature and 𝛼=3 at high temperature unveiling efficient electron-phono-impurity supercollisions [1,2]. The activated optical-phonon cooling is reported in suspended graphene .
In h-BN supported graphene, the electron mobility can be so high that the electron fluid becomes decoupled from the host lattice, and prone to otherwise elusive coupling to its environment. Suppressing heat conduction with velocity saturation, we observe a drop-down of the noise temperature (Figure) in the interband Zener-Klein transport regime, which signals the ignition of a new and very efficient cooling pathway: the electroluminescent emission of hyperbolic phonon-polaritons (HPhPs) in the h-BN substrate [4,5].
In quantizing magnetic fields phonon/HPhP relaxation is suppressed. Transport is ballistic and noiseless below a breakdown drift velocity associated with inter Landau level tunneling. Critical velocity is revealed by a steep onset of shot-noise . Breakdown velocity and noise are explained by a collective magneto-excitons instability, which is reminiscent of the roton instability of superfluids.
HPhP emission also controls the photoresponse, limiting photo-carrier lifetimes to a few picoseconds. This efficient recombination pathway can be switched off for photon energies below the HPhP 0.2eV Restrahlen band. The signature is a carrier lifetime peak at charge neutrality  that can be suppressed by switching on HPhP emission above an optical/Zener-Klein pumping power threshold.
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