  After decades of effort in a number of dark matter direct detection
experiments, we still do not have any conclusive signal yet. Therefore it is
high time we broaden our horizon by building experiments with increased
sensitivity for light dark matter, specifically in the sub-MeV mass regime. In
this paper, we propose a new detector material, a bilayer stack of graphene for
this purpose. Its voltage-tunable low energy sub-eV electronic band gap makes
it an excellent choice for the detector material of a light dark matter search
experiment. We compute its dielectric function using the random phase
approximation and estimate the projected sensitivity for sub-MeV dark
matter-electron scattering and sub-eV dark matter absorption. We show that a
bilayer graphene dark matter detector can have competitive sensitivity as other
candidate target materials, like a superconductor, in this mass regime. The
dark matter scattering rate in bilayer graphene is also characterized by a
daily modulation from the rotation of the Earth which may help us mitigate the
backgrounds in a future experiment.
