IMPURE KONDO INSULATORS

A Kondo hole is the charge neutral substitution of a rare earth ion by a nonmagnetic analog. We consider an arbitrary cluster of Kondo holes in a Kondo insulator described by the nondegenerate symmetric Anderso n lattice with a nearest-neighbor tight-binding conduction band on a s imple cubic lattice. Each Kondo hole introduces a bound state in the g ap. Quantum interference in the scattering off the impurities gives ri se to interactions among the Kondo holes. The interaction is strong if the impurities are nearest neighbors, but weak if they are further ap art. The wave function of the bound states is predominantly localized on the sites neighboring the Kondo holes. Clusters of impurities separ ated by more than two hoppings are disconnected for bound states at th e Fermi level, i.e. the wave functions do not overlap. The possibility of a metal-insulator transition can then be reduced to a site percola tion of Kondo holes.