Network rigidity and dynamics of oxides
  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. 
Kostya Trachenko and Martin T Dove