A suggestion concerning the origin of the broad distribution of tunneling system relaxation rates in glasses

Abstract

While it has been known for some time that the coupling between low-temperature tunneling degrees of freedom and phonons in glasses isstrong, this fact is rarely appreciated in theoretical descriptions based on the tunneling model. It is surprising, in view of this situation, how well these theories work. In this article, it is shown that the presence of strong phonon coupling may lead to a broad distribution of relaxation rates of the tunneling systems which is rather insensitive to the detailed shape ofP(Δ, λ), the distribution of asymmetry and overlap parameters Δ and λ. The (energy-)rate distribution $$\hat P$$ (E,r) turns out to be very similar to the one obtained in weak-coupling theory. However, whereas in the latter theoretical framework this is due to the flatness ofP(Δ, λ), it is proposed here that the experimentally visible part of $$\hat P$$ (E,r) isgenerated by the strong phonon coupling, i.e., it is to be considered adynamic effect. Appropriately normalishing the relaxation rate distribution, one can recover, within strong-coupling theory, many results of weak-coupling theory, whose undeniable success is thus rendered understandable.