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

S. Reschke, Zhe Wang, F. Mayr, E. Ruff, P. Lunkenheimer, V. Tsurkan, and A. Loidl
Phys. Rev. B 96, 144418 – Published 16 October 2017

Abstract

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 0.5 THz, 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 the 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.

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  • Received 19 May 2017
  • Revised 14 September 2017

DOI:https://doi.org/10.1103/PhysRevB.96.144418

©2017 American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

S. Reschke1, Zhe Wang1,*, F. Mayr1, E. Ruff1, P. Lunkenheimer1, V. Tsurkan1,2, and A. Loidl1,†

  • 1Experimental Physics V, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, 86135 Augsburg, Germany
  • 2Institute of Applied Physics, Academy of Sciences of Moldova, MD-2028 Chisinau, Republic of Moldova

  • *Present address: Helmholtz-Zentrum Dresden Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany.
  • alois.loidl@physik.uni-augsburg.de

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Issue

Vol. 96, Iss. 14 — 1 October 2017

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