Phonons and their Coupling to Magnons in n=2 Ruddlesden-Popper Compounds

TL;DR

This paper analyzes phonon and magnon modes in Ruddlesden-Popper compounds Ca$_3$X$_2$O$_7$ (X=Mn, Ti), exploring symmetry changes, coupling effects, and proposing experiments to better understand their phase diagrams.

Contribution

It provides a detailed symmetry analysis of phonons and magnons across different phases and investigates magnon-phonon coupling in these compounds.

Findings

Determined the absorption and Raman modes for each Wyckoff orbit.

Analyzed the effect of symmetry lowering on optical modes.

Discussed magnon-phonon coupling and electromagnon phenomena.

Abstract

The number of absorption and Raman optical modes for each Wyckoff orbit in the high-temperature tetragonal (I4/mmm) parent lattice of the Ruddlesden-Popper compounds Ca$_3$X$_2$O$_7$, with X=Mn or X=Ti is given. We analyze the effect of sequential perturbations which lower the symmetry to Cmcm and Cmc2$_1$ and finally include magnetic ordering. We determine the power law behavior (within mean field theory) for the cross section for photon absorption and Raman scattering of modes which appear as the symmetry is successively lowered. In the Cmc2$_1$ phase we give a symmetry analysis to discuss the magnon-phonon coupling which in other systems gives rise to "electromagnons." From our results we suggest several experiments to clarify the phase diagram and other properties of these systems.