Abstract
We analyze the field-dependent intensities of the coupled electron-phonon modes observed in the low-temperature far-infrared (terahertz) reflection spectra of PrFe3(BO3)4 and develop a theory based on the Green’s function approach. An excellent agreement between the experimental and theoretical data is achieved. The developed theory of the intensity transfer from phonons to quasi-electronic excitations can be applied to the electron-phonon modes in other compounds, in particular, in magnetodielectric materials, where it can be used to analyze the magnetodielectric response.
Highlights
A strong interaction between electronic, magnetic, and lattice degrees of freedom in multiferroics results in a vast variety of phases and phenomena [1–4], including electronic-structural phase transitions [5], coupled magnon-phonon [6,7] and electron-magnon modes [8], and formation of electromagnons [9,10]
The peculiarity of PrFe3 (BO3 )4 is that the 4f electronic excitation corresponding to the transition from the ground state to the first excited crystal-field (CF) level of the Pr3+ ion falls into the region between the this single quasi-electronic (TO) and LO phonon frequencies
Investigation of the behavior of the coupled electron-phonon modes of PrFe3 (BO3 )4 in an external magnetic field made it possible to discover another new effect caused by the electron-phonon interaction, namely, the existence of a gap in the spectrum of quasi-electronic excitations of an easy-axis antiferromagnet in an arbitrarily small external magnetic field directed along the easy axis of magnetization [12]
Summary
A strong interaction between electronic, magnetic, and lattice degrees of freedom in multiferroics results in a vast variety of phases and phenomena [1–4], including electronic-structural phase transitions [5], coupled magnon-phonon [6,7] and electron-magnon modes [8], and formation of electromagnons [9,10]. The peculiarity of PrFe3 (BO3 ) is that the 4f electronic excitation corresponding to the transition from the ground state to the first excited crystal-field (CF) level of the Pr3+ ion falls into the region between the TO and LO phonon frequencies. In this case, the electronic mode is inverted: the LO frequency becomes lower than the TO frequency. The electron-phonon interaction results in the intensity borrowing from a phonon mode and frequency renormalization, i.e., in a formation of the coupled electron-phonon mode in PrFe3 (BO3 ) , which is observed in the π-polarized reflection spectra [11,12]
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