Resonant Pair Exchange and Percolation in the Disordered Hubbard Model

We show that the effect of disorder on a Mott-Hubbard insulator of interacting electrons produces a quantum phase transition from an antiferromagnetic to a paramagnetic phase governed by a percolation of the singly occupied weakly disordered sites. Near the transition, we propose that a new type of defect, formed by the resonant exchange between a spin singlet and a doubly occupied site with an attractive disorder potential, plays an important role. These resonant pair exchange defects reduce the staggered magnetization but enhance the coupling of the two spins and produce characteristic signatures in the temperature dependent specific heat and the non-Curie spin susceptibility, at temperatures on the order the hopping t, higher than the typical exchange scale J.