Detection of Light Dark Matter With Optical Phonons in Polar Materials
  We show that polar materials are excellent targets for direct detection of
sub-GeV dark matter due to the presence of gapped optical phonons as well as
acoustic phonons with high sound speed. We take the example of Gallium Arsenide
(GaAs), which has the properties needed for experimental realization, and where
many results can be estimated analytically. We find GaAs has excellent reach to
dark photon absorption, can completely cover the freeze-in benchmark for
scattering via an ultralight dark photon, and is competitive with other
proposals to detect sub-MeV dark matter scattering off nuclei.
