Exciton-induced electric dipole moment in organic ferromagnets

Abstract

Based on an Anderson-like model including electron–lattice interaction and electron–electron (e–e) interactions, charge and spin properties of excitons in quasi-one-dimensional organic ferromagnets with spin radicals are investigated. The results demonstrate the appearance of an unusually large electric dipole moment in the magnetic molecule upon the formation of the exciton. The sign of the dipole moment depends on the spin of the excited electron relative to the magnetization of the spin radicals. The underlying mechanism is analysed based on the different charge distribution and lattice distortion in the two excitation modes with opposite spin. The origin is attributed to the preferred occupation on different domain walls of the exciton distortion for different spin-resolved excitonic levels. The experimental realization of the large dipole moment is discussed. Although the realization of a large dipole moment is impeded by the superposition state formed due to the degeneracy of two excitation modes, we propose an achievable route to break the symmetry and create controllable electric polarization by optical illumination. The dipole moment is robust even if the long-range e–e interaction is included. The effects of the system parameters, including the electron hopping between the main chain and the radicals and the e–e interaction, on the magnitude of the dipole moment are also discussed. This work indicates a novel way to realize organic multiferroic materials with controllable polarization, which can be induced by photon excitation.