Kondo Effect: Does it Exist?

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Abstract
The experimental situation with regard to the well‐known anomalies associated with magnetic impurities in simple metals is reviewed in terms of recent work on the Kondo effect and spin fluctuations. Detailed evidence is presented to demonstrate the existence of nonperturbative spin correlations in the conduction electron sea at temperatures less than the Kondo temperature. In particular magnetic susceptibility measurements together with NMR and Mössbauer studies in the system CuFe imply an excess nonlocal contribution to the susceptibility which arises from formation of a quasispin in the electron gas. The experiments imply that the excess polarization resides in a region less than 9 Å from the impurity. Similar results are found for Rh(Fe). Measurements of the 63Cu nuclear spin‐lattice relaxation rate in Cu(Cr) provide further confirmation of such spin correlations. For T≪Tk an excess relaxation rate is observed which is more than thirty times that predicted by perturbation theory. It is suggested that the many‐electron quasispin can be viewed as arising from an indirect electron‐electron interaction (via the impurity spin). Experimental studies of the resistivity and electron spin resonance linewidth in Cu(Mn) doped with trace amounts of Pt support this point of view.

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