Kondo resonance narrowing in d- and f-electron systems.

We develop a simple scaling theory for the effect of Hund's interactions on the Kondo effect, showing how an exponential narrowing of the Kondo resonance develops in magnetic ions with large Hund's interaction. Our theory accounts for the exponential reduction of the Kondo temperature with spin S of the Hund's coupled moment, first observed in d-electron alloys in the 1960s, and more recently encountered in numerical calculations on multiband Hubbard models. We discuss the consequences of Kondo resonance narrowing for the Mott transition in d-band materials, particularly iron pnictides, and the narrow ESR linewidth recently observed in ferromagnetically correlated f-electron materials.