Collective excitations in magnetic topological insulators and axion dark matter search

We investigate collective excitations in magnetic topological insulators (TIs) and their impact on axion detection. In the three-dimensional TI model with the Hubbard term, the effective action of magnons and amplitude modes is formulated by dynamical susceptibility under the antiferromagnetic and ferromagnetic states. One of the amplitude modes is identified as "axionic" quasi-particle and its effective coupling to the electromagnetic fields turns out to be enhanced by about four orders of magnitude larger than the previous estimate, which may drastically change the sensitivity of the axion search using "axion" in magnetic TIs.