Exact diagonalization results for Kondo nanoclusters alloyed with mixed valence impurities show that tuning the { \it energy spacing}, $\Delta$, drives the system from the Kondo to the RKKY regime. The interplay of $\Delta$ and disorder gives rise to a $\Delta$ versus concentration T=0 phase diagram very rich in structure, where regions with prevailing Kondo or RKKY correlations alternate with domains of ferromagnetic order. The local Kondo temperatures, $T_K$, and RKKY interactions depend strongly on the local environment and are overall { \it enhanced} by disorder, in contrast to the hypothesis of ``Kondo disorder'' single-impurity models.
Categories: cond-mat.str-el