Theory of Dephasing of Impurities in Glasses

Abstract

Twelve years ago ZELLER and POHL [1] showed that below 1 [K] the thermal conductivity and the heat capacity of amorphous solids behave in a strikingly different way from those of crystalline solids. Since then a number of studies found that other low-temperature properties such as acoustic, dielectric and spin-relaxation properties are also unusual [2], These results indicate that there is a continuum of low-energy excitations in the amorphous systems which is absent in the crystalline solids. It is generally recognized that the low-energy modes arise from tunneling of an atom or a group of atoms between two energy minima of a double-well through a potential barrier. A random distribution of the energy asymmetry of the well depths and the barrier heights gives a broad distribution of energies and a nearly constant density of states for the so-called two-level systems (TLS) [3,4]. This model is essentially phenomenological and its justification lies ultimately in the fact that it explains most of the data successfully. In this paper we discuss another unusual effect, namely, the anomalous dephasinq of optically active impurities which seems to arise from interactions with TLS at low temperatures.