Investigation of Kondo impurity - Kondo lattice transition in CexLa1-xRu2

The Kondo impurity - Kondo lattice (KI-KL) transition in the intermetallic compounds CexLa1-xRu2 is induced by variation of cerium concentration x. It allows to observe the change of magnetic state of the system which involves Kondo effect, intermediate valence fluctuations phenomena and formation of heavy-fermion state. This problem affects also the coexistence and interplay of superconductivity and magnetism, that will be especially investigated. We have measured specific heat down to 4.5 K, susceptibility down to 1 K, resistivity down to 0.6 K in magnetic field up to 2 T and X-ray photoabsorption spectra for x=0, 0.25, 0.5, 0.75, 1. It is shown that with increasing x the system goes to an intermediate valence state in which the Kondo effect is suppressed. We propose two interpretations of TC behaviour as a function of x: at first we use the concurrence between the Kondo and the RKKY interactions; second, we take into account the variation of the density of states at the Fermi level versus x and the value of the discontinuity in specific heat ΔC=1.85γ(TC)TC in the CeRu2 sample. (orig.)