New effects of the electron–phonon interaction in dielectrics (50th anniversary of the Institute of Spectroscopy, Russian Academy of Sciences)

We have studied the electron–phonon interaction (EPI) in multiferroic $\rm{PrFe_3(BO_3)_4}$ applying reflection methods in far-infrared (terahertz) spectral range and theoretical simulations. A specific feature of $\rm{PrF_3(BO_3)_4}$ is that the 4f-electron excitation of the $\rm{Pr^{3+}}$ ion falls into a range between the longitudinal (LO) and transverse (TO) phonon mode frequencies. Simultaneously, inversion of the electronic oscillator occurs: its LO frequency becomes smaller than the TO frequency. With lowering temperature, a coupled electron–phonon mode emerges, and a new effect is observed: the reststrahlen band corresponding to a nondegenerate phonon mode splits. Yet another effect caused by the EPI is the appearance of a gap in the spectrum of electronic excitations of a stoichiometric rare-earth easy-axis antiferromagnet placed into an arbitrarily weak external magnetic field directed along the easy magnetization axis.