In ordinary metals, antiferromagnetic exchange between conduction electrons
and a magnetic impurity leads to screening of the impurity spin below the Kondo
temperature, TK. In systems such as semimetals, small-gap semiconductors and
unconventional superconductors, a reduction in available conduction states near
the chemical potential can greatly depress TK. The behavior of an Anderson
impurity in a model with a power-law density of states, N(ϵ)∼∣ϵ∣r, r>0, for ∣ϵ∣<Δ, where Δ≪D, is
studied using the non-crossing approximation. The transition from the Kondo
singlet to the magnetic ground state can be seen in the behavior of the
impurity magnetic susceptibility χ. The product Tχ saturates at a
finite value at low temperature for coupling smaller than the critical one. For
sufficiently large coupling Tχ→0, as T→0, indicating complete
screening of the impurity spin.Comment: 4 pages + 2 postscript figures, uses RevTeX, psfig.sty, submitted to
the European Conference "Physics of Magnetism", Poznan, June 199