Interplay of Jahn-Teller effect and Kondo effect for orbitally degenerate impurity ions

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