Hot Electron Cooling by Acoustic Phonons in Graphene

We have investigated the energy loss of hot electrons in metallic graphene by means of GHz noise thermometry at liquid helium temperature. We observe the electronic temperature T proportional to V at low bias in agreement with the heat diffusion to the leads described by the Wiedemann-Franz law. We report on T proportional to root V behavior at high bias, which corresponds to a T-4 dependence of the cooling power. This is the signature of a 2D acoustic phonon cooling mechanism. From a heat equation analysis of the two regimes we extract accurate values of the electron-acoustic phonon coupling constant Sigma in monolayer graphene. Our measurements point to an important effect of lattice disorder in the reduction of Sigma, not yet considered by theory. Moreover, our study provides a strong and firm support to the rising field of graphene bolometric detectors.