Kondo lattice model revisited: Vacuum state and elementary excitations

We present an approach to the Kondo lattice model based on the introduction of unitary transformations that lead to an effective Hamiltonian containing a kinetic part, a renormalized Kondo interaction without terms coupling electrons and holes, and an RKKY term. Using this formalism, we discuss the origin of the magnetic structure of the ground state, and the nature and physical role of the various elementary excitations of the system. In particular, we carry out a detailed study of the charged excitations which reveals the existence of "normal", uncorrelated modes, and strongly correlated charged modes in which low-energy electrons or holes combine with spin fluctuations forming collective states. Finally, we show that, for sufficiently strong couplings, these modes condensate in the vacuum state leading to an instability.