Competing energy scales in the compounds Ce(Ni1−xCux)2(Si2)

A transition from a heavy fermion Kondo lattice CeCu2Si2 to the fluctuating valence CeNi2Si2 compound has been studied for the series Ce(Ni1−xCux)2Si2 employing the magnetic susceptibility, electrical resistivity, thermal conductivity, thermopower, and specific heat measurements methods. Apart from the confirmation of the fluctuating valence and heavy fermion state for the Ni-rich and Cu-rich samples, respectively, a crossing of the energy scales for x of about 0.5-0.6 has been observed and ascribed to the proximity of the Kondo temperature and the crystal electric field (CEF) excitations for these compositions. All the experimental methods used provide compatible values of the characteristic energy scales, like those related to the Kondo temperature and the CEF excitations. Especially, agreement between the low and high temperatures regimes is also possible if the changes of the degeneracy of the Ce 4f states are correctly considered.