  Collective excitations in condensed matter systems, such as phonons and
magnons, have recently been proposed as novel detection channels for light dark
matter. We show that excitation of i) optical phonon polaritons in polar
materials in an ${\mathcal O}$(1 T) magnetic field (via the axion-photon
coupling), and ii) gapped magnons in magnetically ordered materials (via the
axion wind coupling to the electron spin), can cover the difficult-to-reach
${\mathcal O}$(1-100) meV mass window of QCD axion dark matter with less than a
kilogram-year exposure. Finding materials with a large number of optical phonon
or magnon modes that can couple to the axion field is crucial, suggesting a
program to search for a range of materials with different resonant energies and
excitation selection rules; we outline the rules and discuss a few candidate
targets, leaving a more exhaustive search for future work. Ongoing development
of single photon, phonon and magnon detectors will provide the key for
experimentally realizing the ideas presented here.
