Observations of Kondo scattering without magnetic impurities: A point contact study of two-level tunneling systems in metals

We study singularities in the differential resistance of metal point contacts containing structural disorder. We find zero-bias features which are well described by theories in which two-level tunneling systems couple strongly to the conduction electrons to produce a Kondo resonance whose origin is not magnetic impurities. Away from zero bias, we observe abrupt resistance steps which indicate transitions in the multichannel Kondo condensate. We suggest that two-level systems are the explanation for many of the zero-bias anomalies seen in tunnel junctions and point contacts.