Localization effects in Kondo semimetals CeNiSn and CeRhSb

We have found that the semiconductor-like increase in resistivity previously reported on CeNiSn and CeRhSb below about 8 K is suppressed with decreasing impurity concentration in single-crystalline samples. Nevertheless, the absolute values of the Hall coefficient strongly increase as temperature decreases. Therefore, we interpret the metallic behavior found in the a-axis resistivity of a purified crystal of CeNiSn as a consequence of the significant increase of relaxation time of carriers. Even for the purified crystal, the residual gamma value is 40 mJ/K(2) mol at 0.03 K. These results suggest that the energy gap is closed along the a-axis and that residual carriers are strongly localized in the presence of impurities.