Transitions from conventional metals and insulators to Kondo metals and insulators

This paper reports calculations of the spectral weight for the lattice Anderson model, defined on a square (four-site) cluster, by exact diagonalization. The calculations emphasize the change in the density of states as the interaction strength U varies between weak and strong limits. We consider system parameters and occupancies such that the model resembles either a conventional insulator or a conventional metal for weak interactions. Then increasing interaction strength leads in the first case to a Kondo insulator in which there is a gap at the chemical potential between hole and electron states. In the metallic case, several features of the Kondo effect in bulk systems are observed. We also observe the splitting of the f band, leading to the formation of an upper Hubbard band at high energy with some double-site occupancy, while states with two f holes on a site are also separated to form a satellite.