Localized states in glasses

Abstract The properties of amorphous materials determined by the presence of localized atomic and electronic states are reviewed. Experimental data on atomic dynamics in glasses are described. The two-level-system (TLS) model is presented, and the limits of its applicability analysed. The concept of soft atomic potentials, allowing generalization of the TLS model to higher energies, is introduced. Atomic dynamics in soft potentials, as well as soft-potential coupling with phonons and thermal expansion of glasses, are described. Localized electronic states in glasses are investigated and different theoretical models of such states in glassy semiconductors are reviewed. The connection between localized electronic states and soft potentials is studied. Particular attention is paid to low-relaxation processes in glasses controlled by two-well soft potentials; specifically, low-frequency noise in disordered conductors is considered. The possibility of investigating the localized states in question with the help of point-contact spectroscopy is discussed.