Ground state and elementary excitations in the one-dimensional f² Kondo lattice model

The ground state properties of the one-dimensional Kondo lattice with an f² configuration at each site are studied by the density matrix renormalization group method. At half-filling, competition between the Kondo exchange J and the antiferromagnetic intra f-shell exchange I leads to the reduction of the characteristic energy scale. The remarkable change of the quasi-particle gap is driven by the change of the spin- 1/2 excitation character from the itinerant one to the localized one. The attractive force among conduction electrons is induced by the competition and a bound electron pair is formed. Off half-filling, the spin gap remains but the charge gap is closed. This constitutes a new mechanism of superconductivity in multi-orbital systems. For small J/I an effective model is derived by perturbation theory and is studied by bosonization and scaling theory.