Coherence scale of the Kondo lattice

It is shown that the large-N approach yields two energy scales for the Kondo lattice model. The single-impurity Kondo temperature, T-K, signals the onset of local singlet formation, while Fermi-liquid coherence sets in only below a lower scale, T*. At low conduction electron density n(c) (exhaustion limit), the ratio T*/T-K is much smaller than unity, and is shown to depend only on n(c) and not on the Kondo coupling. The physical meaning of these two scales is demonstrated by computing several quantities as a function of n(c) and temperature.