Nonreciprocal linear dichroism observed in electron spin resonance spectra of the magnetoelectric multiferroic Pb(TiO)Cu_{4}(PO_{4})_{4}

It is known that magnetic excitations in magnetoelectric multiferroics can be induced by the oscillation of electric polarization, called electromagnons, which often cause nonreciprocal optical phenomena. Energy diagrams of the excitations in the magnetoelectric multiferroic Pb(TiO)Cu_{4}(PO_{4})_{4} were obtained in wide ranges of magnetic fields up to 50 T and frequencies below 2 THz. Some of the observed resonance modes were reproduced qualitatively by a numerical analysis based on the generalized spin-wave theory. Moreover, we found that electron spin resonance (ESR) signals on some of the modes can be modulated by applying an electric field. This result implies the presence of nonreciprocal linear dichroism on the ESR signals in Pb(TiO)Cu_{4}(PO_{4})_{4}.