INVESTIGATION OF KONDO IMPURITY - KONDO-LATTICE TRANSITION IN CEXLA1-XRU2

The Kondo impurity - Kondo lattice (KI-KL) transition in the intermeta llic compounds CexLa1-xRu2 is induced by variation of cerium concentra tion x. It allows to observe the change of magnetic state of the syste m which involves Kondo effect, intermediate valence fluctuations pheno mena and formation of heavy-fermion state. This problem affects also t he coexistence and interplay of superconductivity and magnetism, that will be especially investigated. We have measured specific heat down t o 4.5 K. susceptibility down to 1 K, resistivity down to 0.6 K in magn etic 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 T-C behaviour as a function of x: a t first we use the concurrence between the Kondo and the RKKY interact ions; second, we take into account the variation of the density of sta tes at the Fermi level versus x and the value of the discontinuity in specific heat Delta C = 1.85 gamma(T-C)T-C in the CeRu2 sample. Publis hed by Elsevier Science B.V.