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New effects of the electron—phonon interaction in dielectricsInstitute 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.
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