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

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≪TDT≪TD and low frequencies ν≪νDν≪νD, including both very low (T,ν),T≲1K, and moderately low (T,ν)(T,ν), 1 K