Pressure-temperature phase diagrams of CaK(Fe1−xNix)4As4 superconductors

The pressure dependence of the magnetic and superconducting transitions and that of the superconducting upper critical field are reported for $\mathrm{CaK}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_x)}₄{\mathrm{As}}₄$, the first example of an Fe-based superconductor with spin-vortex-crystal-type magnetic ordering. Resistance measurements were performed on single crystals with two substitution levels $(x=0.033,0.050)$ under hydrostatic pressures up to 5.12 GPa and in magnetic fields up to 9 T. Our results show that, for both compositions, magnetic transition temperatures ${T}_{\mathrm{N}}$ are suppressed upon applying pressure; the superconducting transition temperatures ${T}_{\mathrm{c}}$ are suppressed by pressure as well, except for $x=0.050$ in the pressure region where ${T}_{\mathrm{N}}$ and ${T}_{\mathrm{c}}$ cross. Furthermore, the pressure associated with the crossing of the ${T}_{\mathrm{N}}$ and ${T}_{\mathrm{c}}$ lines also coincides with a minimum in the normalized slope of the superconducting upper critical field, consistent with a likely Fermi-surface reconstruction associated with the loss of magnetic ordering. Finally, at $p\ensuremath{\sim}4$ GPa, both Ni-substituted $\mathrm{CaK}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_x)}₄{\mathrm{As}}₄$ samples likely go through a half-collapsed-tetragonal phase transition, similar to the parent compound ${\mathrm{CaKFe}}₄{\mathrm{As}}₄$.