Dipolar dynamics for orbital-order driven ferroelectricity

Ferroelectricity, the ordering of electrical dipoles formed by molecules or atoms in specific materials, has recently found renewed interest and plays an important role in many areas of modern technology (e.g., non-volatile data storage). While the polarization in classical ferroelectrics usually arises from structurally driven displacements of charged atoms, recently various new mechanisms have been discovered, including the so-called orbital-order driven ferroelectricity. There ferroelectricity is triggered by the ordering of orbitals, dumbbell-shaped clouds of electrons around an atom. Generally, the investigation of the dipole fluctuations, which occur in all ferroelectrics, is the key for achieving a thorough understanding of the microscopic mechanisms behind this ordering phenomenon. By combining THz and MHz-GHz spectroscopy, for the first time we succeeded in measuring the coupled orbital and dipolar fluctuations expected in this new type of ferroelectric. We find highly non-canonical behavior of the detected dynamics, which helps unravelling the nature of orbital-order driven ferroelectricity and the intriguing entanglement between the polar and orbital dynamics.

orbital glass

Temperature-dependent dielectric loss of an orbital-order driven ferroelectric showing a dramatic change at the simultaneous orbital and ferroelectric phase transition at 44 K. At this temperature, the orbitals of the vanadium atoms (shown in red) order, the V4 tetrahedra in the crystalline structure become elongated, and ferroelectric polarization arises. This is accompanied by an astonishing slowing down of the relaxation time, characterizing the orbital and dipolar fluctuations, by about five orders of magnitude (see inset).


To learn more, see:
Zhe Wang, E. Ruff, M. Schmidt, V. Tsurkan, I. Kézsmárki, P. Lunkenheimer, and A. Loidl, Polar dynamics at the Jahn-Teller transition in ferroelectric GaV4S8, Phys. Rev. Lett., 115, 207601 (2015)