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

Abstract

Measurements of susceptibility \ensuremath{\chi}, resistivity \ensuremath{\rho}, and thermoelectric power S have been performed on single-crystal CeNiSn. Only along the a axis of the orthorhombic structure do \ensuremath{\chi}(T) and \ensuremath{\rho}(T) exhibit pronounced peaks at 12 K, whereas no anomaly was found in the specific heat. The gap energies estimated from \ensuremath{\rho}(T) are 2.4, 5.5, and 5.0 K along the a, b, and c axes, respectively. Near 3 K, ${\mathit{S}}_{\mathit{a}}$(T) and ${\mathit{S}}_{\mathit{c}}$(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, ${\mathit{C}}_{\mathit{m}}$/T versus T reveals a maximum of 0.19 J/${\mathrm{K}}^{2}$ 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.