Kondo scattering from atomic two-level tunneling systems in metals: Enhanced conductance, critical-bias transitions, and the non-Fermi-liquid electronic state

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Abstract
We describe a state of enhanced dc conductance, terminated by an abrupt transition as a function of bias current, that exists at low temperature in metal point contacts that also display a signal near zero bias attributable to Kondo scattering from a class of fast-tunneling atomic two-level systems. We argue that the regime of enhanced conductance is a consequence of the presence of a highly correlated non-Fermi-liquid electronic screening state about these two-level tunneling systems. The critical-current transition marks a sudden change in the electronic state during which this non-Fermi-liquid state is disrupted, and replaced by a lower-conductance normal state at high bias. We describe in detail the temperature and magnetic-field dependence of the critical-current transitions, as well as their hysteresis as a function of bias.