Change of the effective spin degeneracy in CeNi_9-xCu _xGe₄ due to the interplay between Kondo and crystal field effects

Elastic and inelastic neutron scattering experiments were carried out on the heavy-fermion systems CeNi_8.6Cu_0.4Ge₄ and CeNi₈CuGe₄ to study i) the influences of Ni/Cu substitution on the crystal field parameters and to identify ii) the driving forces of quantum criticality in CeNi_9-xCu_xGe₄. The relevance of competing RKKY and Kondo interactions and changes of the crystal field parameters is discussed. The crystallographic site where the Ni replacement by copper atoms takes place is identified by neutron powder diffraction studies. Furthermore, quasi-elastic and inelastic neutron scattering studies provide detailed information regarding the Kondo properties and the changes of the crystal field parameters resulting from the Ni/Cu replacement. Hence, a reduction of the effective spin degeneracy of the crystal field ground state with increasing Cu concentration is identified as one important control parameter of quantum criticality in CeNi_9-xCu_xGe ₄. The results of these experiments are complemented by measurements of the thermopower.