Evolution of magnetism and superconductivity in CeCu2(Si1−xGex)2

Abstract

We have determined the magnetic phase diagram of CeCu${}_{2}$(Si${}_{1\ensuremath{-}x}$Ge${}_{x})$${}_{2}$ using specific heat, susceptibility, and resistivity measurements and additional thermal expansion and muon spin resonance experiments. The system evolves continuously from the heavy-fermion state at $x=0$ to an antiferromagnetically (AF) ordered state at $x=1$, though the magnetic structure undergoes significant modifications. The results strongly suggest that the AF ordering emerges from the $A$ phase of pure CeCu${}_{2}$Si${}_{2}$. The phase diagram can be divided into three regions: Low Ge content $(x<~0.2)$ leads to an enhancement of the $A$ phase of pure CeCu${}_{2}$Si${}_{2}$ and to a moderate decrease of the Kondo temperature. Despite this increase of the magnetic character, we could observe superconductivity up to $x=0.1$. At large Ge content $(x>0.5)$ the behavior resembles that of pure CeCu${}_{2}$Ge${}_{2}$. The intermediate region is characterized by the appearance of a second magnetic transition below ${T}_{N},$ which seems to be of first-order type. The appearance of this transition leads to a well-defined critical point at $x=0.3$ and suggests the possibility of another critical point at $x\ensuremath{\simeq}0.5$.