Theory of two-level systems and their role in point contacts

Abstract

Recently found non-Fermi-liquid zero-bias anomalies in metallic point contacts have been interpreted in terms of tunneling centers, which are very good candidates for a realization of the two-channel Kondo model. For experimental realizations the crucial parameter is the Kondo temperature, being the relevant energy scale of the problem. Keeping only the two lowest lying states of the tunneling center (two-level system model) the calculated Kondo temperature is hardly in the observable range. Including the excited states, however, we find the Kondo temperature in the right order of magnitude. Furthermore, it is argued by examining the fixed point structure of the scaling equations that the Hamiltonian at low temperature is equivalent to the spin one-half two-channel Kondo Hamiltonian. The role of disorder is shortly discussed as well.