THEORY OF 2-LEVEL SYSTEMS AND THEIR ROLE IN POINT CONTACTS

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