Electromagnon excitation and longitudinal mode studied on the basis of symmetric spin-dependent electric polarization

Abstract

Since a symmetric spin-dependent electric polarization makes quantum spin systems couple to an electric field, it is one of a possible origin of electromagnon excitations in noncollinear (spiral) magnetically ordered states. In collinear ordered states, however, it is believed that the electromagnon excitation is difficult to realize. In the present study, we focus on the symmetric spin-dependent electric polarization and study the electromagnon excitation in both collinear and noncollinear states. In one-magnon excitation process, it is found that the symmetric spin-dependent polarization couples to a longitudinal fluctuation of the ordered moment and that the electromagnon measurements probe the longitudinal (Higgs) mode selectively. In noncollinear states, we report that both Higgs and Nambu-Goldstone (transverse) modes are observable.