Electronic states in eucu₂(Ge_1−xSi_x)₂ based on the doniach phase diagram

We succeeded in growing single crystals of EuCu₂(Ge₁−_xSi_x)₂ (x = 0–1) by the Bridgman method using a Mo crucible and measured the electrical resistivity, specific heat, magnetic susceptibility, and magnetization, together with the electrical resistivity under pressure. EuCu₂(Ge₁−_xSi_x)₂ compounds follow the Doniach phase diagram as a function of the x content. Namely, EuCu₂(Ge₁−_xSi_x)₂ compounds exhibit antiferromagnetic ordering at a Néel temperature of T_N = 15 K in EuCu₂Ge₂ (x = 0), a maximum of T_N ’ 20 K at x’ 0.5, T_N → 0 at x’ 0.7, and a moderate heavy-fermion state at x = 0.8 and 1 (EuCu₂Si₂). The present result is reasonably consistent with the previous studies carried out using arc-melted polycrystal samples. A similar change in the electronic states was also found to be realized under pressure P for x = 0, 0.45, and 0.6. The Néel temperature of T_N = 15 K for x = 0 (EuCu₂Ge₂) increases slightly as a function of pressure, at a rate of dT_N=dP = 3.0 K=GPa, reaches a maximum of T_N = 27 K at around 5–6 GPa, decreases steeply, and becomes zero at a critical pressure of P_c ’ 7 GPa. A moderate heavy-fermion state is realized at 8 GPa. Similar results were also obtained for x = 0.45 and 0.6, with smaller critical pressures of P_c ’ 4.5 and 2 GPa, respectively.