Study of the magnetic order in CeCu2(Si1−xGex)2: evidence for a critical point at x=0.25

Measurements of the specific heat Cp(T), thermal expansion α(T) and the electrical resistivity ρ(T) evidence the presence of a tetracritical point in CeCu2(Si1−xGex)2 at x=0.25. An analysis of the evolution of the Sommerfeld coefficient and of the anomaly in the specific heat at TN suggests that this critical point corresponds to the crossover from a spin density wave of heavy quasiparticles at x<0.25 to an antiferromagnetic order of local moments for x>0.25.