Exactly solvable Kondo lattice model
  In this work, we exactly solve a Kondo lattice model in the thermodynamic limit. The system consists of an electronic conduction band described by unconstrained hopping matrix elements between the lattice sites. The conducting electrons interact with a localized impurity spin at each lattice cell. We have found the exact thermodynamics, the ground state energies of the system. At T=0, we explicitly demonstrate that the system exhibits a metal-insulator phase transition at half-filling. In the limit of strong coupling between the impurity spin and the electrons, J=\infty, we have solved the system on a lattice of any size L. The ground states are the resonating-valence-bond type Jastrow product wavefunctions. Various correlation functions may be computed for the impurity spins, and for the singlets formed by the electrons and impurities. 
D. F. Wang and C. Gruber