Anisotropic Kondo effect in a valence-fluctuating system: CeNiIn

Measurements of magnetic susceptibility, electrical resistivity, thermoelectric power, and thermal expansion have been made on CeNiIn single crystals with a hexagonal ${\mathrm{Fe}}_2$P-type structure. The results indicate that CeNiIn is a valence-fluctuating system with a significantly anisotropic Kondo effect which comes from anisotropic c-f mixing along the a and c axes. Specific-heat measurements on a CeNiIn polycrystal sample reveal that the magnetic contribution divided by temperature $C_m$/T takes a minimum around 20 K and reaches 60 mJ/${\mathrm{K}}^2$ mole around T=2 K, which is still ten times larger than that of LaNiIn with no 4f electron