  Energy relaxation of hot Dirac fermions in bilayer epitaxial graphene is
experimentally investigated by magnetotransport measurements on Shubnikov-de
Haas oscillations and weak localization. The hot-electron energy loss rate is
found to follow the predicted Bloch-Gr\"uneisen power-law behaviour of $T^4$ at
carrier temperatures from 1.4 K up to $\sim$100 K, due to electron-acoustic
phonon interactions with a deformation potential coupling constant of 22 eV. A
carrier density dependence $n_e^{-1.5}$ in the scaling of the $T^4$ power law
is observed in bilayer graphene, in contrast to the $n_e^{-0.5}$ dependence in
monolayer graphene, leading to a crossover in the energy loss rate as a
function of carrier density between these two systems. The electron-phonon
relaxation time in bilayer graphene is also shown to be strongly carrier
density dependent, while it remains constant for a wide range of carrier
densities in monolayer graphene. Our results and comparisons between the
bilayer and monolayer exhibit a more comprehensive picture of hot carrier
dynamics in graphene systems.
