The Urbach-Martienssen Rule Revisited

Abstract

Many theories have been presented so far for about thirty five years after the first discovery of the Urbach-Martienssen rule on the low energy tail of the fundamental absorption edge of a non-metallic solid. They have been subject to selection and three kinds of theories have survived now. These are a series of theories initiated by Toyozawa, the one proposed by Skettrup and the one by Dow and Redfield. Fundamental assumptions underlying these theories are critically examined, especially in the light of a reformulation with a coherent state for the third one, and problems contained in the latter two are pointed out.