Effect of pressure on the structural phase transition and superconductivity in (B$_{1-x}K_x)Fe_2As_2 (x = 0 and 0.45) and SrFe_2As_2 single crystals

The effects of pressure up to $\sim 20$ kbar, on the structural phase transition of SrFe${}_2$As${}_2$ and lightly Sn-doped BaFe${}_2$As${}_2$, as well as on the superconducting transition temperature and upper critical field of (Ba${}_{0.55}$K${}_{0.45}$)Fe${}_2$As${}_2$ 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${}_{0.55}$K${}_{0.45}$)Fe${}_2$As${}_2$ 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.