Plasmon modes in magnetically doped single-layer and multilayer helical metals

We study the plasmon excitations and the electromagnetic response of the magnetically doped single-layer and multilayer "helical metals," which emerge at the surfaces of topological insulators. For the single-layer case, we find a "spin-plasmon" mode with the rotating spin texture due to the combination of the spin-momentum locking of helical metal and the Hall response from magnetization. For the multilayer case, we investigate the electromagnetic response due to the plasmon excitations, including the Faraday rotation for the light propagating normal to the helical metal layers and an additional optical mode with the frequency within the conventional plasmon gap for the light propagating along the helical metal layers.