Detecting Dark Matter with Polar Materials using ab initio calculations.

Dark matter (DM) comprises ~25% of the mass-energy density of the universe, yet to date has eluded direct detection. Ultra-light DM has emerged as a promising possibility for DM, and is only now becoming experimentally viable for direct searches. In this work we propose the direct detection of DM with polar materials, considering both the scattering of optical and acoustic phonons by light DM, and the absorption of dark photons by optical phonons. Using Density Functional Theory, we calculate the material-specific matrix elements, and show that DM scattering in an anisotropic crystal has a strong directional dependence. We find that phonon-based detectors have comparable or greater sensitivity to sub-MeV dark matter scattering and sub-eV dark matter absorption than other current proposals.