Theory of absorption in the frustrated spin gap system SrCu₂(BO₃)₂

In magnetoelectric mulitferroics, there are strong couplings between magnetization and electric polarization, which induces magnetoelectric effects. Moreover, such magnetoelectric couplings induce anomalous behaviors even in dynamical processes. It is well known that an electromagnon, i.e., an electro active magnon, process arises in the cycloidal magnets. We show that electromagnetic couplings can exist even in spin gap systems and induce anomalous spin excitations. As a typical example, we discuss the absorption process in the frustrated two-dimensional spin gap system SrCu₂(BO₃)₂. SrCu₂(BO₃)₂ is a realization of the Shastry-Sutherland model of which the ground state is exactly described as a direct product of dimer singlet states. Spin excitation spectra have been investigated by ESR and IR and spin gap and bound states of two triplets excitations are observed. However, the origin and mechanism of the observation are still mysetry. We show that the spin gap excitation and the bound states of two triplets are active due to the electric components of light in the dimer singlets phase in the Shastry-Sutherland material SrCu₂(BO₃)₂.