Kondo effect in non-equilibrium - Theory of energy relaxation induced by dynamical defects in diffusive nanowires

In diffusive Cu and Au quantum wires at finite transport voltage $U$ the non-equilibrium distribution function $f(E,U)$ exhibits scaling behavior, $f(E,U)=f(E/eU)$, indicating anomalous energy relaxation processes in these wires. We show that in nonequilibrium the Kondo effect, generated either by magnetic impurities (single-channel Kondo effect) or possibly by non-magnetic, degenerate two-level systems (two-channel Kondo effect), produces such scaling behavior as a consequence of a Korringa-like (pseudo)spin relaxation rate $\propto U$ and of damped powerlaw behavior of the impurity spectral density as a remnant of the Kondo strong coupling regime at low temperature but high bias. The theoretical, scaled distribution functions coincide quantitatively with the experimental results, the impurity concentration being the only adjustable parameter. This provides strong evidence for the presence of Kondo defects, either single- or two-channel, in the experimental systems. The relevance of these results for the problem of dephasing in mesoscopic wires is discussed briefly.