Effect of pressure on the structural phase transition and superconductivity in (Ba_1−xK_x)Fe₂As₂ (x=0 and 0.45) and SrFe₂As₂ single crystals

The effects of pressure up to ∼20 kbar on the structural phase transition of SrFe₂As₂ and lightly Sn-doped BaFe₂As₂ as well as on the superconducting transition temperature and upper critical field of (Ba_0.55K_0.45)Fe₂As₂ single crystals have been studied. All the transition temperatures decrease with pressure in an almost linear fashion. Under pressure, the upper critical-field curve H_c2(T) for (Ba_0.55K_0.45)Fe₂As₂ shifts down in temperature to follow the zero-field T_c with very little change in slope. Composite P-T phase diagrams for three parent compounds AFe₂As₂ (A=Ba,Sr,Ca) are constructed and appear to be remarkably similar: (i) having a structural (antiferromagnetic) phase-transition line with a negative slope and (ii) showing signs of the emerging superconducting state at intermediate pressures.