Anomalous magnetic properties and non-Fermi-liquid behaviour in single crystals of the Kondo lattice CeNiGe_2-xSi_x

We report on the magnetic susceptibility, specific heat and electrical resistivity of the heavy fermion compounds CeNiGe_2-xSi_x (0 ≤ x ≤ 1). Compounds with x < 1 show antiferromagnetic order, which with increasing x shifts toward lower temperature owing to increased exchange coupling between the localized 4f magnetic moments and conduction electrons. Eventually, the magnetic order almost becomes absent, for x = 1. An anomaly observed in the specific heat is well interpreted by the Kondo model for a degenerate impurity spin J = 1/2 in the Coqblin-Schrieffer limit. A coherence peak indicative of the formation of a Kondo lattice is found in the electrical resistivity, whose features are consistent with the results for the specific heat. Interestingly, there is a significant deviation from Fermi-liquid behaviour at the critical concentration x = 1. This deviation is attributed to a quantum phase transition in a model with two-dimensional antiferromagnetic fluctuations.