Glassy Dynamics: From Millihertz to Terahertz

Abstract

In this article, we review broadband dielectric spectroscopy in supercooled liquids, in many cases covering more than 15 decades in frequency and a wide range of temperatures from the low-viscosity liquid to the rigid sub-Tg glass. The access to this extremely broad frequency window allows a detailed study of the complexity of glassy freezing and glassy dynamics in a large variety of materials. Dielectric spectroscopy not only documents the enormous slowing down of the structural relaxation when approaching the glass transition, but also reveals a variety of further relaxation processes, which are important to understand the physics of the transition from a supercooled liquid into a rigid glass. After a short introduction, mainly focusing on long-term experiments on glasses and on the classification of glass formers into strong and fragile, we shortly discuss some basics of relaxation and conductivity contributions when viewed via dielectric spectroscopy. We provide some prototypical examples of dielectric loss spectra covering a large frequency and temperature regime. The glass formers shown can be categorized into two classes, type A and type B. The latter reveal well-defined Johari–Goldstein secondary relaxations, which lead to peaks in the dielectric loss at least at low temperatures. The former exhibit an excess wing, showing only a change of slope of the high-frequency flank of the structural-relaxation loss peaks. Then, we exemplify the phenomenology of glassy dynamics as revealed by these broadband spectra: The structural relaxation, the Johari–Goldstein relaxation, the appearance of a fast process as proposed by the mode-coupling theory, and the boson peak, a well-defined feature in the dielectric loss at THz frequencies, are discussed in detail. In a further chapter, we focus on the importance of sub-Tg experiments: Aging experiments and a possible experimental evidence of the Gardner transition are discussed. Finally, we summarize the experimental dielectric results documenting the universality of glassy freezing, which can be directly derived from these measurements.