Lattice dynamics and mixing of polar phonons in the rare-earth orthoferrite TbFeO$_{3}$

TL;DR

This study investigates the lattice dynamics and mode mixing of polar phonons in TbFeO₃, revealing complex LO-TO interactions and ion displacement contributions, which are crucial for understanding nonlinear phononics in rare-earth orthoferrites.

Contribution

It provides a comprehensive experimental and theoretical analysis of phonon modes in TbFeO₃, clarifying the complex LO-TO phonon relations and ion contributions, advancing the understanding of nonlinear phononics.

Findings

Mode mixing between LO and TO phonons is complex and deviates from the LO-TO rule.

Magnetic Fe ions are not involved in Raman-active phonons.

The study establishes a basis for further research in nonlinear phononics and magnetophononics.

Abstract

Rare-earth orthoferrites are a promising platform for antiferromagnetic spintronics with a rich variety of terahertz spin and lattice dynamics phenomena. For instance, it has been experimentally demonstrated that the light-driven optical phonons can coherently manipulate macroscopic magnetic states via nonlinear magnetophononic effects. Here using TbFeO$_{3}$ as an example, we reveal the origin of the mode mixing between the LO and TO phonons, which is important for understanding of nonlinear phononics. We performed a comprehensive study of the lattice dynamics of the TbFeO$_{3}$ single crystal by polarized infrared and Raman scattering spectroscopic techniques, and experimentally obtained and carefully analyzed the spectra of anisotropic complex dielectric functions in the far-infrared spectral range. This allowed us to reliably identify the symmetries and parameters of most infrared- and Raman-active phonons. Next, the experimental studies were supplemented by the lattice dynamics calculations which allowed us to propose the normal mode assignments. We reveal that the relation between LO and TO polar phonons is complex and does not strictly follow the ``LO-TO rule'' due to the strong mode mixing. We further analyze how displacements of different ions contribute to phonon modes and reveal that magnetic Fe ions are not involved in Raman-active phonons, thus shedding light on a lack of spin phonon coupling for such phonons. The obtained results establish a solid basis for further in-depth experimental research in the field of nonlinear phononics and magnetophononics in rare-earth orthoferrites.