Phonons and Magnons in Organic Molecular Crystals as Dark Matter Targets

The enormous design space of conjugated organic molecules provides the potential to revolutionize materials design for dark matter (DM) targets. Current DM proposals in the sub-GeV mass range focus on DM interactions with electrons, phonons and other low-energy excitations in inorganic semiconductors. However, inorganic materials have a narrow energy range of available excitations for DM to couple to, limiting their sensitivity to DM particles within a small mass range. Organic molecular crystals have the potential to circumvent that issue since there is a much larger range of phonons available for DM interactions, potentially within reach of existing read-out schemes. Additionally, magnetic organic crystals can be synthesized allowing for the simultaneous testing of magnon-DM and phonon-DM interactions in one target. To investigate the potential of organic molecular crystals as DM targets, we used DFT calculations to find the phonon and magnon structure factors for a selection of organic molecular crystals. From these structure factors, we present estimates of their reach as DM targets for well-motivated DM models. Finally, we identify key materials requirements for maximizing the experimental reach and discuss prospects for readout schemes based on our sensitivity estimates.