ENERGY-GAP OF THE GROUND-STATE OF CENISN CAUSED BY LOCAL AND LONG-RANGE MAGNETIC-MOMENT INTERACTIONS

We have studied the thermal expansion of the gapped intermetallic heavy-fermion compounds CeNiSn, CeNi0.9Cu0.1Sn, and Ce1-xLaxNiSn (x = 0.03,0.1) between 0.5 K and 25 K; and the transport (3 < T < 60 K) and magnetic (0.1 < T < 8 K) properties of CeNiSn. The magnetic heavy-fermion compound CeNi0.9Cu0.1Sn displays an antiferromagnetic (AF) transition at T(AF) is similar to 1.5 K; CeNiSn exhibits a gapped (E(g) is similar to 2-8 K) and possibly ''weakly'' magnetic ground state T < 0.5 K; Ce0.97La0.03NiSn shows T(AF) is similar to 6 K, and finally Ce0.9La0.1NiSn does not display heavy-fermion or gapped behavior. We have investigated the transition between the above regimes. The comparison between the thermal-expansion data of CeNiSn, Ce0.97La0.03NiSn, and single-crystalline CeNiSn shows that gap formation may be unstable against the weak magnetic transitions with T(AF) is similar to 6 K and T(AF) is similar to 2 K. The anomalous negative thermal expansion of CeNiSn that is observed below 1 K allows us to propose a simple picture for the electron spectrum near the Fermi level.