  In van der Waals bonded or rotationally disordered multilayer stacks of
two-dimensional (2D) materials, the electronic states remain tightly confined
within individual 2D layers. As a result, electron-phonon interactions occur
primarily within layers and interlayer electrical conductivities are low. In
addition, strong covalent in-plane intralayer bonding combined with weak van
der Waals interlayer bonding results in weak phonon-mediated thermal coupling
between the layers. We demonstrate here, however, that Coulomb interactions
between electrons in different layers of multilayer epitaxial graphene provide
an important mechanism for interlayer thermal transport even though all
electronic states are strongly confined within individual 2D layers. This
effect is manifested in the relaxation dynamics of hot carriers in ultrafast
time-resolved terahertz spectroscopy. We develop a theory of interlayer Coulomb
coupling containing no free parameters that accounts for the experimentally
observed trends in hot-carrier dynamics as temperature and the number of layers
is varied.
