# Terahertz Dielectric Analysis and Spin-Phonon Coupling in Multiferroic GeV$_4$S$_8$

## TL;DR

This study uses terahertz spectroscopy to explore the dielectric and magnetic properties of GeV$_4$S$_8$, revealing coupled excitations, a magnon, and spectral collapse behavior across phase transitions.

## Contribution

It provides new insights into spin-phonon coupling and dielectric behavior in multiferroic GeV$_4$S$_8$ through detailed terahertz spectral analysis.

## Key findings

- Identification of three terahertz absorptions linked to magnetic transition
- Observation of isosbestic points indicating spectral collapse
- Detection of a magnon appearing below the Néel temperature

## Abstract

We present an investigation of the multiferroic lacunar spinel compound GeV$_4$S$_8$ using time-domain terahertz spectroscopy. We find three absorptions which either appear or shift at the antiferromagnetic transition temperature, T$_N = 17$ K, as S=1 magnetic moments develop on vanadium tetrahedra. Two of these absorptions are coupled to the magnetic state and one only appears below the N\'{e}el temperature, and is interpreted as a magnon. We also observe isosbestic points in the dielectric constant in both the temperature and frequency domains. Further, we perform analysis on the isosbestic features to reveal an interesting collapse into a single curve as a function of both frequency and temperature, behavior which exists throughout the phase transitions. This analysis suggests the importance of spectral changes in the terahertz range which are linear in frequency and temperature.