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 approximate to 140 K) and to induce a superconducting (SC) dome with a maximum transition temperature T-c(max) similar or equal to 4.2 K. This T-c(max) is much lower than the T-c similar to 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 BaFe2-xCuxAs2 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 similar or equal to 8K. Remarkably interesting, the under-doped samples, e.g. x = 0.02 display a maximum pressure induced T-c similar or equal to 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