Title: Detection of sub-MeV Dark Matter with Three-Dimensional Dirac Materials,
Abstract: We propose the use of three-dimensional Dirac materials as targets for direct
detection of sub-MeV dark matter. Dirac materials are characterized by a linear
dispersion for low-energy electronic excitations, with a small band gap of
O(meV) if lattice symmetries are broken. Dark matter at the keV scale carrying
kinetic energy as small as a few meV can scatter and excite an electron across
the gap. Alternatively, bosonic dark matter as light as a few meV can be
absorbed by the electrons in the target. We develop the formalism for dark
matter scattering and absorption in Dirac materials and calculate the
experimental reach of these target materials. We find that Dirac materials can
play a crucial role in detecting dark matter in the keV to MeV mass range that
scatters with electrons via a kinetically mixed dark photon, as the dark photon
does not develop an in-medium effective mass. The same target materials provide
excellent sensitivity to absorption of light bosonic dark matter in the meV to
hundreds of meV mass range, superior to all other existing proposals when the
dark matter is a kinetically mixed dark photon.