Anisotropic pseudogap in CeNiSn and CeRhSb studied by a thermal-conductivity measurement

We have measured the thermal conductivity κ for CeNiSn, ${\mathrm{CeNi}}_{0.95}$ ${\mathrm{M}}_{0.05}$Sn (M=Co,Cu), and CeRhSb single crystals. The pronounced enhancement of κ along the b axis is observed around 5 and 10 K in CeNiSn and CeRhSb, respectively. In CeNiSn, this enhancement is almost completely suppressed either by application of a magnetic field of 15 T along the a axis, which is the easy axis of magnetization, or by the substitution of 5% Co and Cu for Ni. These results indicate that the enhancement of κ along the b axis originates from the opening of the pseudogap in these compounds. On the other hand, the enhancement of κ along the a axis in CeNiSn and those along the a and c axis in CeRhSb are much smaller than that along the b axis. These anisotropic behaviors in κ reflect the anisotropy of the gap in these compounds.