A model is discussed which for the first time provides a unified description of many electrical and optical properties of chalcogenide glasses. It is proposed that localized gap states are at dangling bonds, and that lattice-distortion effects are sufficiently strong that these states exhibit an effective negative electron-electron correlation energy.

• Received 24 June 1975

DOI:https://doi.org/10.1103/PhysRevLett.35.1293