METAL-INSULATOR-TRANSITION IN DIRTY KONDO INSULATORS

We consider arbitrary clusters of Kondo holes in a Kondo insulator des cribed by the nondegenerate symmetric Anderson lattice with a nearest- neighbor tight-binding conduction band on a simple cubic lattice. The f-electron self energy is considered within the local approximation. E ach Kondo hole introduces a boundstate in the gap. The quantum interfe rence in the scattering off the impurities gives rise to interactions among the Kondo holes. The spectral weight of the bound states is pred ominantly localized on the sites neighboring the Kondo holes. Clusters of impurities separated by more than one lattice site are disconnecte d for boundstates at the Fermi level. On a simple cubic lattice the me tal-insulator transition in the impurity band then reduces to the site percolation of Kondo holes with first, second and fourth nearest neig hbors. We use the low density mean cluster size expansion and a small cell renormalization to estimate the critical concentration. Hopping i n the conduction band beyond nearest neighbors reduces the percolation threshold. Hence, 9.9% of Kondo holes is an upper bound for the insul ator to become a metal. (C) 1996 American Institute of Physics.