Network rigidity and dynamics of oxides

TL;DR

This paper explores how the rigidity and low-frequency vibrational modes in oxide materials influence their physical properties, including relaxation, crystallization, and superconductivity, by viewing these materials as mechanical networks.

Contribution

It introduces the concept of rigid unit modes (RUMs) in oxides and discusses their impact on various properties across different materials, including glasses and superconductors.

Findings

RUMs govern relaxation and other properties in SiO2 glass.

RUM flexibility influences low-energy excitations in glasses.

RUMs are relevant to superconductivity in cuprates.

Abstract

If a hierarchy of interatomic interactions exists in a solid, low-frequency modes can be found from viewing this solid as a mechanical network. In this case, the low-frequency modes are determined by the network rigidity. We study the low-frequency modes (rigid unit modes, or RUMs) in several important oxide materials and discuss how the RUMs affect their properties. In SiO$_2$ glass, the ability to support RUMs governs its relaxation in the wide range of pressures and temperatures, giving rise to the non-trivial pressure window. It also affects other properties, including crystallization, slow relaxation and compressibility. At ambient pressure and low temperature, RUM flexibility is related to the large-scale localized atomic motions. Whether these motions are interpreted as independent two-level systems or collective density excitations, the RUM flexibility determines whether and to what extent low-energy excitations can exist in a given glass structure. Finally, we discuss the RUM in, perhaps unexpectedly, cuprate superconductors, and its relevance for superconductivity, including the d-wave symmetry of the order parameter and other properties.