Competition between disorder and Coulomb interaction in a two-dimensional plaquette Hubbard model

We have studied a disordered Nc×Nc plaquette Hubbard model on a two-dimensional square lattice at half-filling using a coherent potential approximation (CPA) in combination with a single-site dynamical mean field theory (DMFT) approach with a paramagnetic bath. Such a model conveniently interpolates between the ionic Hubbard model at Nc=2 and the Anderson model at Nc= and enables the analysis of the various limiting properties. We confirmed that within the CPA approach a band insulator behavior appears for noninteracting strongly disordered systems with a small plaquette size Nc=4, while the paramagnetic Anderson insulator with nearly gapless density of states is present for large plaquette sizes Nc=48. When the interaction U is turned on in the strongly fluctuating random potential regions, the electrons on the low energy states push each other into high energy states in DMFT in a paramagnetic bath and correlated metallic states with a quasiparticle peak and Hubbard bands emerge, though a larger critical interaction U is needed to obtain this state from the paramagnetic Anderson insulator (Nc=48) than from the band insulator (Nc=4). Finally, we observe a Mott insulator behavior in the strong interaction U regions for both Nc=4 and Nc=48 independent of the disorder strength. We discuss the application of this model to real materials.