Supercollision cooling in undoped graphene

We have investigated the electron-lattice cooling rate in graphene by means of GHz Johnson noise thermometry. For phonon temperatures ($T_{ph}$) larger than Bloch-Gr\"{u}neisen temperature ($T_{\mathrm{BG}}$), we find the energy relaxation rate $J$ obeys a cubic law as a function of electron temperature $T_e$. In this regime, the small Fermi surface of graphene drastically restricts the allowed phonon energy in ordinary electron-phonon scattering, and disorder-assisted supercollisions dominate over the conventional electron-phonon collisions. In the low-temperature regime, for $T_{ph} < T_{\mathrm{BG}}$, we regain $J \propto T_e^{4}$ dependence, which is the signature of standard electron-phonon interaction in the 2D graphene. Beside its implication for electron-phonon physics, our observations are of direct relevance for the performance of graphene bolometers and photo-detectors.