Urbach Rule in Solid State Physics

The paper is dedicated to the 60th anniversary of the Urbach rule. The Urbach edge behaviour, measured by optical absorption spectroscopy, is analysed for different solids. The general regularities of the Urbach rule revealed in crystalline and amorphous solids as well as the temperature behaviour of its main parameters are discussed. The main attention is paid to the studies of the possibilities of the Urbach rule parameters to be used for studying disordering processes in solids, short- and medium-range order in amorphous materials, specific features of the Urbach “bundle” in nanosystems and its variation in the vicinity of phase transitions. It is shown that the parameters obtained from the Urbach rule give an important information about dynamic properties of elementary excitations in condensed matter as well as about the interaction of electronic excitations with phonons.