IMPORTANT ROLE OF COHERENCE FOR THE HEAVY-FERMION STATE IN CECU2SI2

The systeM Ce(CU1-xMx)2.2Si2 was doped with M=Pt, Ir, Os, and Ni, alwa ys maintaining the original CeCu2Si2 structure. The doped samples as w ell as the end points (i.e., x = 1) are nonmagnetic. Measurements on t he specific heat C of lower-doped samples (x < 0.2) showed a monotonic decrease of gamma with increasing doping. This decrease depends only on the amount of doping and not on M, thus not on the variation of the lattice constant or the electronic structure of the dopants. We concl ude that the decrease in gamma is primarily determined by the disturbe d coherence in the heavy-fermion lattice, and that variation of the Ce -M ligand hybridization merely plays a secondary role. This result und erlines the fact that coherence is a decisive prerequisite for the for mation of a heavy-fermion state, and that this coherence is equally de pendent on disturbance of the ligand sublattice as of the f-atom sites .