Tuning the magnetism of ordered and disordered strongly-correlated electron nanoclusters

Recently, there has been a resurgence of intense experimental and theoretical interest on the Kondo physics of nanoscopic and mesoscopic systems due to the possibility of making experiments in extremely small samples. We have carried out exact diagonalization calculations to study the effect of energy spacing $\Delta$ in the conduction band states, hybridization, number of electrons, and disorder on the ground-state and thermal properties of strongly-correlated electron nanoclusters. For the ordered systems, the calculations reveal for the first time that $\Delta$ tunes the interplay between the {\it local} Kondo and {\it non local} RKKY interactions, giving rise to a "Doniach phase diagram" for the nanocluster with regions of prevailing Kondo or RKKY correlations. The interplay of $\Delta$ and disorder gives rise to a $\Delta$ versus concentration T=0 phase diagram very rich in structure. The parity of the total number of electrons alters the competition between the Kondo and RKKY correlations. 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. This interplay may be relevant to experimental realizations of small rings or quantum dots with tunable magnetic properties.