Orbital-Order Driven Ferroelectricity and Dipolar Relaxation Dynamics in Multiferroic GaMo₄S₈

GaMo₄S₈, a compound of the lacunar spinel family, was recently shown to exhibit non-canonical, orbitally-driven ferroelectricity [1]. Our dielectric spectroscopy measurements on this multiferroic material reveal complex relaxation dynamics, above as well as below its Jahn-Teller transition at TJT= 47 K [2]. Above the Jahn-Teller transition, two types of coupled dipolar-orbital dynamics were found: On the one hand, relaxations within cluster-like regions with short-range polar order, as known from relaxor ferroelectrics. On the other hand, critical fluctuations of only weakly interacting dipoles, resembling the typical dynamics of order-disorder type ferroelectrics. Below TJT, the system is driven into long range ferrolectric order by the onset of orbital order and dipolar dynamics within the ferroelectric domains is observed: The found marked differences to the skyrmion host GaV₄S₈ seem to be related to the different structural distortions in these systems.
[1] E. Neuber, et al., J. Phys.: Condens. Matter 30, 445402 (2018)
[2] K. Geirhos, et al., arXiv:1810.07145