Combined external pressure and Cu-substitution studies on BaFe₂As₂ single crystals

We report a combined study of external pressure and Cu-substitution on BaFe₂As₂ single crystals grown by the in-flux technique. At ambient pressure, the Cu-substitution is known to suppress the spin density wave (SDW) phase in pure BaFe₂As₂(T_SDW ≈ 140 K) and to induce a superconducting (SC) dome with a maximum transition temperature T_c^max ≃ 4.2 K. This T_c^max is much lower than the T_c ∼ 15-28 K achieved in the case of Ru, Ni and Co substitutions. Such a lower T_c is attributed to a Cu²⁺ magnetic pair-breaking effect. The latter is strongly suppressed by applied pressure, as shown herein, T_c can be significantly enhanced by applying high pressures. In this work, we investigated the pressure effects on Cu²⁺ magnetic pair-breaking in the BaFe_2-xCu_xAs₂ series. Around the optimal concentration (x_opd = 0.11), all samples showed a substantial increase of T_c as a function of pressure. Yet for those samples with a slightly higher doping level (over-doped regime), T_c presented a dome-like shape with maximum T_c ≃ 8 K. Remarkably interesting, the under-doped samples, e.g. x = 0.02 display a maximum pressure induced T_c ≃ 30 K which is comparable to the maximum T_c's found for the pure compound under external pressures. Furthermore, the magnetoresistance effect as a function of pressure in the normal state of the x = 0.02 sample also presented an evolution consistent with the screening of the Cu²⁺ local moments. These findings demonstrate that the Cu²⁺ magnetic pair-breaking effect is completely suppressed by applying pressure in the low concentration regime of Cu²⁺ substituted BaFe₂As₂.