We present the results of broadband dielectric spectroscopy of GaMo4S8, a lacunar spinel system that recently was shown to exhibit non-canonical, orbitally-driven ferroelectricity. Our study reveals complex relaxation dynamics of this multiferroic material, both above and below its Jahn-Teller transition at TJT=47 K. Above TJT, two types of coupled dipolar-orbital dynamics seem to compete: relaxations within cluster-like regions with short-range polar order like in relaxor ferroelectrics and critical fluctuations of only weakly interacting dipoles, the latter resembling the typical dynamics of order-disorder type ferroelectrics. Below the Jahn-Teller transition, the onset of orbital order drives the system into long-range ferroelectric order and dipolar dynamics within the ferroelectric domains is observed. The coupled dipolar and orbital relaxation behavior of GaMo4S8 above the Jahn-Teller transition markedly differs from that of the skyrmion host GaV4S8, which seems to be linked to differences in the structural distortions of the two systems on the unit-cell level.
@article{arxiv.1810.07145,
title = {Orbital-Order Driven Ferroelectricity and Dipolar Relaxation Dynamics in Multiferroic GaMo$_4$S$_8$},
author = {K. Geirhos and S. Krohns and H. Nakamura and T. Waki and Y. Tabata and I. Kézsmárki and P. Lunkenheimer},
journal= {arXiv preprint arXiv:1810.07145},
year = {2018}
}