Soft atomic motion modes in glasses: Their role in anomalous properties

Abstract

The present review article discusses the nature and properties of soft atomic motion modes, and their role in anomalous properties, including low-energy dynamical properties, of (nonmetallic) glasses. The basic concept and results of the soft-mode model of anomalous properties of glasses are summarized and discussed. The discussion shows that this model is able, in particular, to give a qualitative and scale description of anomalous dynamic and thermal properties of the glasses at low temperatures and frequencies. In accordance with the general suggestion of solid state theory, the “excess” (non-Debye) low-energy excitations are expected to determine the properties under discussion, and the major problem is the description of “excess” excitations. The present review is aimed at demonstrating that the soft-mode model is able to describe the nature of “excess” excitations at low temperatures T ≪ T D and low frequencies ν ≪ ν D , including both very low ( T , ν ) , T ≲ 1 K , and moderately low ( T , ν ) , 1 K < T < 10 2 K . In fact, an essential contribution to the properties is found to be due to soft-mode harmonic vibrational excitations at moderately low T and ν , e.g., to the boson peak as a universal broad asymmetric peak in inelastic scattering spectra of photons or neutrons, while at very low T and ν to be determined by soft-mode nonvibrational excitations like two-level systems. Thus in the model the “excess” excitations in glasses are soft-mode excitations mainly described by one of the two simplest fundamental systems: strongly anharmonic two-level systems or harmonic oscillators. In this sense an important correlation can be predicted in the model between the properties at very low ( T , ν ) and at moderately low ( T , ν ) in the glasses. This prediction naturally corresponds to a recently found empirical correlation: characteristic dynamic and thermal properties of the glasses at very low temperatures are found to be missing in amorphous materials where no boson peak is found. Additional manifestations of the soft modes in glassy phenomena are also described, which concern the origin of the so called negative-U centres in electron properties of semiconducting glasses and the predicted strong suppression of low-energy properties of glasses at moderately high hydrostatic pressure. Other models of the anomalous properties of glasses can either be considered as the limit case of soft-mode model for very low ( T , ν ) or as relevant for only moderately low ( T , ν ) (while not being in terms of soft modes).