  New ideas for low-mass dark matter direct detection suggest that narrow band
gap materials, such as Dirac semiconductors, are sensitive to the absorption of
meV dark matter or the scattering of keV dark matter. Here we propose
spin-orbit semiconductors - materials whose band gap arises due to spin-orbit
coupling - as low-mass dark matter targets owing to their ~10 meV band gaps. We
present three material families that are predicted to be spin-orbit
semiconductors using Density Functional Theory (DFT), assess their electronic
and topological features, and evaluate their use as low-mass dark matter
targets. In particular, we find that that the tin pnictide compounds are
especially suitable having a tunable range of meV-scale band gaps with
anisotropic Fermi velocities allowing directional detection. Finally, we
address the pitfalls in the DFT methods that must be considered in the ab
initio prediction of narrow-gapped materials, including those close to the
topological critical point.
