Thermal Expansion of CeCu2(Si1-xGex)2

We report low-temperature thermal expansion measurements on CeCu2(Si1-xGex)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 TN (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 TN(x) and T1(x) merge together. Furthermore, the analysis of the Gruneisen 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.