Formation of an anisotropic energy gap in the valence-fluctuating system of CeNiSn

Measurements of susceptibility χ, resistivity ρ, and thermoelectric power S have been performed on single-crystal CeNiSn. Only along the a axis of the orthorhombic structure do χ(T) and ρ(T) exhibit pronounced peaks at 12 K, whereas no anomaly was found in the specific heat. The gap energies estimated from ρ(T) are 2.4, 5.5, and 5.0 K along the a, b, and c axes, respectively. Near 3 K, Sa(T) and Sc(T) exhibit extremely sharp peaks, which indicate the presence of a density of states within the gap. The magnetic contribution to the specific heat divided by temperature, Cm/T versus T reveals a maximum of 0.19 J/K2 mol near 6.7 K. These results suggest that an antiferromagnetic correlation develops near 12 K, which induces the formation of the pseudogap in the narrow band of heavy quasiparticles