A COMPARATIVE-STUDY OF SUPPRESSION OF THE ENERGY-GAP WITH LA SUBSTITUTION IN THE KONDO INSULATORS - CENISN AND CERHSB

We report structural, transport and magnetic measurements on the solid solutions of Ce1-xLaxNiSn and Ce1-xLaxRhSb (x = 0 to 1). The results have been used to compare and contrast The effect of La substitution o n the ground state properties of the Kondo insulators CeNiSn and CeRhS b. The resistivity measurements reveal that the energy gap in CeNiSn a nd CeRhSb decreases with increasing La concentration; no gap type beha viour was observed for La concentration greater than or equal to 0.2. At low temperature, Ce1-xLaxNiSn alloys show a cross-over from a Kondo insulator (x = 0) to a single ion Kondo ground state (x greater than or equal to 0.2) with increasing La concentration, while Ce1-xLaxRhSb alloys show a cross-over from a Kondo insulator (x = 0) to a coherent Kondo lattice (x = 0.2) and eventually to a single ion Kondo ground st ate at x = 0.5. Magnetic susceptibility measurements of Ce1-xLaxNiSn a lloys exhibit a dramatic change in behaviour with La concentration; Cu rie-Weiss behaviour for x = 0, valence fluctuation behaviour for x = 0 .2 and again C-W behaviour for x greater than or equal to 0.4. On the other hand, susceptibility of Ce1-xLaxRhSb alloys continue to show a v alence fluctuation behaviour up to x = 0.7, and C-W behaviour for high er La concentrations. CeNiSn and CeRhSb exhibit a large negative magne toresistance at 20 T field and at 1.5 K.