Electron-Phonon Interaction at Magnetic Exciton State in Europium Chalcogenides*

Temperature and magnetic field dependences of the photoluminescence spectra of EuS, EuSe, and EuTe have been studied systematically. Photoluminescence band having a nearly Gaussian line shape has been analyzed by the configuration coordinate model, and the Huang-Rhys factors and phonon energies for the relaxed exciton state have been determined for the first time. From an oscillatory structure in the spectra, it has been found that the magnetic exciton state couples with the LO phonons strongly. Further, it has been found that the half-width of the photoluminescence band changes depending on the phase transition from paramagnetic to antiferromagnetic or ferromagnetic phase, which has been interpreted by the change in the electron-phonon coupling strength due to the change in the spatial extension of the exciton wave function at the magnetic phase transition.