Phase Coherence of Conduction Electrons Below the Kondo Temperature

We have measured the phase decoherence rate ${\ensuremath{\tau}}_{\ensuremath{\phi}}^{\ensuremath{-}1}$ of conduction electrons in disordered Ag wires implanted with 2 and 10 ppm Fe impurities, by means of the weak-localization magnetoresistance. The Kondo temperature of Fe in Ag, ${T}_{K}\ensuremath{\approx}4\text{ }\text{ }\mathrm{K}$, is in the ideal temperature range to study the progressive screening of the Fe spins as the temperature $T$ falls below ${T}_{K}$. The contribution to ${\ensuremath{\tau}}_{\ensuremath{\phi}}^{\ensuremath{-}1}$ from the Fe impurities is clearly visible over the temperature range 40 mK---10 K. Below ${T}_{K}$, ${\ensuremath{\tau}}_{\ensuremath{\phi}}^{\ensuremath{-}1}$ falls rapidly until $T/{T}_{K}\ensuremath{\approx}0.1$, in agreement with recent theoretical calculations. At lower $T$ ${\ensuremath{\tau}}_{\ensuremath{\phi}}^{\ensuremath{-}1}$ deviates from theory with a flatter $T$-dependence. Understanding this anomalous dephasing for $T/{T}_{K}<0.1$ may require theoretical models with larger spin and number of channels.