On the Spectral Shape of the Structural Relaxation in Deeply Supercooled Liquids

TL;DR

This paper reviews the spectral shape of structural relaxation in deeply supercooled liquids, highlighting the generic ^{-1/2} high-frequency behavior and discussing factors causing deviations, with implications for understanding glassy dynamics.

Contribution

It synthesizes recent findings on the spectral shape of structural relaxation, emphasizing the universality of the ^{-1/2} behavior and the effects of molecular and orientational dynamics.

Findings

High-frequency ^{-1/2} behavior is common across many supercooled liquids.

Orientational cross-correlations can cause deviations from the generic spectral shape.

Intramolecular dynamics significantly influence spectral shape in complex molecules.

Abstract

Structural relaxation in deeply supercooled liquids is non-exponential. In susceptibility representation, $\chi^{\prime\prime}(\nu)$, the spectral shape of the structural relaxation is observed as an asymmetrically broadened peak with a $\nu^{1}$ low- and $\nu^{-\beta}$ high-frequency behavior. In this perspective article we discuss common notions, recent results and open questions regarding the spectral shape of the structural relaxation. In particular, we focus on the observation that a high-frequency behavior of $\nu^{-1/2}$ appears to be a generic feature in a broad range of different deeply supercooled liquids. Moreover, we review extensive evidence that contributions from orientational cross-correlations can lead to deviations from the generic spectral shape in certain substances, in particular in dielectric loss spectra. Additionally, intramolecular dynamics can contribute significantly to the spectral shape in substances containing more complex and flexible molecules. Finally, we discuss the open questions regarding potential physical origins of the generic $\nu^{-1/2}$ behavior and the evolution of the spectral shape towards higher temperatures.