Kondo effect versus Jahn-Teller effect: A new class of quantum impurity problems

Impurity ions with orbitally degenerate ground electronic configuration embedded in metals are subjects to both the Kondo effect and the Jahn-Teller instability. While increasing the Jahn-Teller coupling, one passes, from the region of parameters where the perturbation theory is applicable, through a complicated intermediate region, to a region where phonons, localized electrons and conduction electrons form a coherent state.