Theory of electromagnons in CuO

Recently, an electromagnon excitation was found in the multiferroic phase of CuO for electric field parallel to the [101] direction, with excitation energy $ \sim 3$ meV. We performed symmetry analysis to show that the experimentally measured electromagnon cannot be explained by the exchange-striction mechanism. We then studied the CuO electromagnons using ab-initio calculations and effective model simulations. The experimentally measured electromagnon was found to originate from the Dzyaloshinskii-Moriya interaction, with the magnon part corresponding to a phason mode. We further predict that a new high-energy electromagnon with selection rule E $\parallel$ [010] should also exist in the multiferroic phase of CuO, due to the exchange-striction mechanism.