  We report on THz time-domain spectroscopy on multiferroic GeV4S8, which
undergoes orbital ordering at a Jahn-Teller transition at 30.5 K and exhibits
antiferromagnetic order below 14.6 K. The THz experiments are complemented by
dielectric experiments at audio and radio frequencies. We identify a low-lying
excitation close to 15 cm-1, which is only weakly temperature dependent and
probably corresponds to a molecular excitation within the electronic level
scheme of the V4 clusters. In addition, we detect complex temperature-dependent
behavior of a low-lying phononic excitation, closely linked to the onset of
orbitally-driven ferroelectricity. In the high-temperature cubic phase, which
is paramagnetic and orbitally disordered, this excitation is of relaxational
character, becomes an overdamped Lorentzian mode in the orbitally ordered phase
below the Jahn-Teller transition, and finally appears as well-defined phonon
excitation in the antiferromagnetic state. Abrupt changes in real and imaginary
parts of the complex dielectric permittivity show that orbital ordering appears
via a structural phase transition with strong first-order character and that
the onset of antiferromagnetic order is accompanied by significant structural
changes, which are of first-order character, too. Dielectric spectroscopy
documents that, at low frequencies, significant dipolar relaxations are present
in the orbitally ordered, paramagnetic phase only. In contrast to the closely
related GaV4S8, this relaxation dynamics that most likely mirrors coupled
orbital and polar fluctuations does not seem to be related to the dynamic
processes detected in the THz regime.
