Kondo effect in "bad metals"

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We study the low-temperature properties of a Kondo lattice using the large-N formalism. For a singular density of conduction states (DOS), we generalize the single-impurity result of Withoff and Fradkin: the strong-coupling fixed point becomes irrelevant if the DOS vanishes at the Fermi level EF. However, for EF close enough to the singularity, and close to half-filling, the Kondo temperature, Tₖ, can become much smaller than the characteristic Fermi liquid scale. At T=0, a meta-magnetic transition occurs at the critical magnetic field Hc (kB/muB) Tₖ. Our results provide a qualitative explanation for the behavior of the YbInCu₄ compound below the valence-change transition.