  A review of the low temperature properties of Kondo lattice systems is
presented within the mean-field approximation, focusing on the different
characteristic energy scales. The Kondo temperature, T_K, and the Fermi liquid
coherence energy, T_0, are analyzed as functions of the electronic filling, the
shape of the non-interacting density of states, and the concentration of
magnetic moments. These two scales can vanish, corresponding to a breakdown of
the Kondo effect when an external magnetic field is applied. The Kondo
breakdown can also be reached by adding a superexchange term to the Kondo
lattice model, which mimics the intersite magnetic correlations neglected at
the mean-field level.
