Thermal Expansion of CeCu₂(Si_1−xGe_x)₂

We report low‐temperature thermal expansion measurements on CeCu₂(Si_1−xGe_x)₂ single crystals with 0.01 ⩽ x ⩽ 0.45. The results evidence a new magnetic phase diagram more complex than previously obtained. In addition to the antiferromagnetic transition T_N (x), a first order transition T₁(x) within the magnetic state is observed in the whole doping range studied. For x = 0.25, a tetracritical point is found at which T_N(x) and T₁(x) merge together. Furthermore, the analysis of the Grüneisen parameter suggests that at this concentration a transition from rather localized f electrons for x > 0.25 to composite heavy fermions for x < 0.25 occurs. This strongly supports the itinerant spin‐density‐wave scenario for the quantum‐critical point observed in CeCu₂Si₂.