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# New effects of the electron—phonon interaction in dielectrics

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Institute of Spectroscopy, Russian Academy of Sciences, ul. Fizicheskaya 5, Troitsk, Moscow, 108840, Russian Federation

We have studied the electron—phonon interaction (EPI) in multiferroic ${\text PrFe}_3({\text BO}_3){}_4$ using far-infrared (terahertz) reflection and theoretical simulation. A specific feature of ${\text PrFe}_3({\text BO}_3){}_4$ is that the 4f electronic excitation of the ${\text Pr}^{\,3+}$ ion falls into a range between the longitudinal (LO) and transverse (TO) phonon frequencies. Inversion of the electronic oscillator occurs: its LO frequency becomes smaller than the TO frequency. As temperature diminishes, a coupled electron—phonon mode emerges, and a new effect is observed: the reststrahlen band corresponding to a nondegenerate phonon mode splits. Another effect caused by the EPI is the existence 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.