Phase diagram of pressure-induced superconductivity in EuFe ₂As₂ probed by high-pressure resistivity up to 3.2 GPa

We have constructed a pressure-temperature (P-T) phase diagram of P-induced superconductivity in EuFe₂As₂ single crystals via resistivity (ρ) measurements up to 3.2 GPa. As hydrostatic pressure is applied, the temperature T₀ where an antiferromagnetic (AF) transition of the Fe moments and a structural phase transition occur shifts to lower temperatures, and the corresponding resistive anomaly becomes undetectable for P≥2.5 GPa. This suggests that the critical pressure P_c where T₀ becomes zero is about 2.5 GPa. We have found that the AF order of the Eu2⁺ moments survives up to 3.2 GPa, the highest pressure in the experiments, without significant changes in the AF ordering temperature T _N. The superconducting (SC) ground state with a sharp transition to zero resistivity at T_c~30 K, indicative of bulk superconductivity, emerges in a pressure range from P_c~2.5 to ~3.0 GPa. At pressures close to but outside the SC phase, the ρ(T) curve shows a partial SC transition (i.e., zero resistivity is not attained) followed by a reentrant-like hump at approximately T_N with decreasing temperature. When nonhydrostatic pressure with a uniaxial-like strain component is applied using a solid pressure medium, the partial superconductivity is continuously observed in a wide pressure range from 1.1 to 3.2 GPa.