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

摘要

We present an investigation of the multiferroic lacunar spinel compoundGeV$_4$S$_8$ using time-domain terahertz spectroscopy. We find threeabsorptions which either appear or shift at the antiferromagnetic transitiontemperature, T$_N = 17$ K, as S=1 magnetic moments develop on vanadiumtetrahedra. Two of these absorptions are coupled to the magnetic state and oneonly appears below the N\'{e}el temperature, and is interpreted as a magnon. Wealso observe isosbestic points in the dielectric constant in both thetemperature and frequency domains. Further, we perform analysis on theisosbestic features to reveal an interesting collapse into a single curve as afunction of both frequency and temperature, behavior which exists throughoutthe phase transitions. This analysis suggests the importance of spectralchanges in the terahertz range which are linear in frequency and temperature.