EFFECTS OF DOPING IN KONDO INSULATORS

Kondo insulators like Ce3Bi4Pt3 and CeNiSn are compounds with small-ga p semiconductor properties. Nonmagnetic impurities, so-called Kondo ho les, break the translational invariance and hence the coherence of the ground state. Impurity states can be introduced by (i) substituting t he rare earth (actinide) ion or (ii) by replacing (or adding, removing ) one of the ligand atoms. Isolated impurities usually give rise to bo und states in the pp. Depending on the nature of the impurity (charge neutral or a dopand) the Fermi level is pinned by the impurity level o r lies in the gap. In the former case the Kondo hole has magnetic prop erties (Curie susceptibility and Schottky anomaly in the specific heat ), while in the latter situation the properties are nonmagnetic. For a finite concentration of Kondo holes the situations (i) and (ii) are q ualitatively different. In (i) it gives rise to an impurity band insid e the gap of the semiconductor. The height and width of the impurity b and in the f-electron density of states are proportional to C1/2 for s mall concentrations. If the impurities are charge neutral the Fermi le vel lies in the impurity band giving rise to a specific heat proportio nal to T and a Pauli-like susceptibility. If the impurities dope the b ands the properties remain semiconducting with a strongly reduced gap. For ligand impurities [case (ii)] tails of impurity states develop cl ose to the gap edges, suppressing in this way the gap. The system rema ins a semiconductor if the impurities are charge neutral, but C is-pro portional-to T and chi is finite if they are dopands.