Pressure-induced superconductivity in Eu_0.5Ca _0.5Fe₂As₂: Wide zero-resistivity region due to suppression of Eu magnetic order and chemical pressure

To clarify competition between FeAs-based superconductivity and antiferromagnetism of Eu²⁺, and the superconducting properties in EuFe₂As₂, we investigated a Ca-substituted sample, Eu _0.5Ca_0.5Fe₂As₂, under high pressure. Under ambient pressure, the sample exhibits a spin-density-wave (SDW) transition at T_SDW = 191K and antiferromagnetic order at T _N = 4K, but no evidence of superconductivity down to 2 K. The Ca-substitution certainly weakens the antiferromagnetism. With increasing pressure, T_SDW shifts to lower temperature and becomes more unclear. Above 1.27 GPa, pressure-induced superconductivity with zero resistivity is observed at around T_c = 20K. At 2.14 GPa, T_c reaches a maximum value of 24 K and the superconducting transition becomes sharpest. The chemical pressure and the suppression of Eu magnetic order induced by Ca substitution enlarge the zero resistivity region compared to pure EuFe ₂As₂.