Ferromagnetic transition of the Kondo lattice with Coulomb repulsion: Exact results

The Coulomb interaction among conduction electrons is introduced in the Kondo-lattice Hamiltonian. According to the Perron-Frobenius theorem, we exactly show that the ground state of the Kondo lattice in one dimension with the open boundary condition is ferromagnetic and has the total spin S=(N-${\mathit{N}}_{\mathit{c}}$)/2 in the limit U→∞, where N is the number of sites and ${\mathit{N}}_{\mathit{c}}$ is the number of conduction electrons. Exact-diagonalization calculations clearly indicate that the ferromagnetic state appears for a wide range of Coulomb strength. Our theory predicts that antiferromagnetic correlations of localized spins change into ferromagnetic ones with the increase of Coulomb interactions.