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We report on THz time-domain spectroscopy on multiferroic GeV4S8, whichundergoes orbital ordering at a Jahn-Teller transition at 30.5 K and exhibitsantiferromagnetic order below 14.6 K. The THz experiments are complemented bydielectric experiments at audio and radio frequencies. We identify a low-lyingexcitation close to 15 cm-1, which is only weakly temperature dependent andprobably corresponds to a molecular excitation within the electronic levelscheme of the V4 clusters. In addition, we detect complex temperature-dependentbehavior of a low-lying phononic excitation, closely linked to the onset oforbitally-driven ferroelectricity. In the high-temperature cubic phase, whichis paramagnetic and orbitally disordered, this excitation is of relaxationalcharacter, becomes an overdamped Lorentzian mode in the orbitally ordered phasebelow the Jahn-Teller transition, and finally appears as well-defined phononexcitation in the antiferromagnetic state. Abrupt changes in real and imaginaryparts of the complex dielectric permittivity show that orbital ordering appearsvia a structural phase transition with strong first-order character and thatthe onset of antiferromagnetic order is accompanied by significant structuralchanges, which are of first-order character, too. Dielectric spectroscopydocuments that, at low frequencies, significant dipolar relaxations are presentin the orbitally ordered, paramagnetic phase only. In contrast to the closelyrelated GaV4S8, this relaxation dynamics that most likely mirrors coupledorbital and polar fluctuations does not seem to be related to the dynamicprocesses detected in the THz regime.

Excitations and relaxation dynamics in multiferroic GeV4S8 studied by THz and dielectric spectroscopy