Nonlinear Dielectric Response of Plastic Crystals

Abstract

This article summarizes ongoing experimental efforts on nonlinear dielectric spectroscopy on plastic crystals. In plastic crystals, the relevant dipolar orientational degrees of freedom are fixed on a crystalline lattice with perfect translational symmetry. However, while they can reorient freely in the high-temperature plastic phase, they often undergo glassy freezing at low temperatures. Hence, plastic crystals are often considered as model systems for structural glass formers. It is well known that plastic crystals reveal striking similarities with phenomena of conventional supercooled liquids. However, in most cases, they can be characterized as rather strong glass formers. Nonlinear dielectric spectroscopy is an ideal tool to study glass-transition phenomena, providing insight into cooperative phenomena or hidden phase transitions, undetectable by purely linear spectroscopy. In the present article, we discuss dielectric experiments using large electric ac fields probing the nonlinear 1ω and the third-order harmonic 3ω susceptibility. In the 1ω experiments, we find striking differences compared with observations on conventional structural glass formers: at low frequencies plastic crystals do not approach the trivial response, but reveal strong additional nonlinearity. These phenomena document the importance of entropic effects in this class of glassy materials. The harmonic third-order susceptibility reveals a hump-like shape, similar to observations in canonical glass formers, indicating the importance of cooperativity dominating the glass transition. In the frequency regime of the secondary relaxations, only minor nonlinear effects can be detected, supporting arguments in favor of the non-cooperative nature of these faster dynamics processes. Based on a model by Bouchaud and Biroli, from the hump observed in the 3ω susceptibility spectra, the temperature dependence of the number of correlated particles can be determined. We document that the results in plastic crystals perfectly well scale with the results derived from measurements on conventional glass formers, providing evidence that in plastic crystals the non-Arrhenius behavior of the relaxation times also arises from a temperature dependence of the energy barriers due to growing cooperative clusters.