Effect of pressure on the structural phase transition and superconductivity in (Ba1-xKx)Fe2As2 (x=0 and 0.45) and SrFe2As2 single crystals

The effects of pressure up to similar 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 (Ba0.55K0.45)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 (Ba0.55K0.45)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 AFe(2)As(2) (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.