Pressure-temperature phase diagrams of CaK(Fe$_{1-x}$Ni$_{x}$)$_4$As$_4$ superconductors

TL;DR

This study maps the pressure-temperature phase diagrams of CaK(Fe$_{1-x}$Ni$_{x}$)$_4$As$_4$ superconductors, revealing how magnetic and superconducting transitions evolve under pressure and identifying a likely phase transition at high pressure.

Contribution

First detailed pressure-dependent phase diagrams for CaK(Fe$_{1-x}$Ni$_{x}$)$_4$As$_4$, highlighting the interplay between magnetic order, superconductivity, and structural transitions.

Findings

Magnetic transition temperatures decrease with pressure.

Superconducting transition temperatures are suppressed by pressure.

A likely half-collapsed-tetragonal phase transition occurs around 4 GPa.

Abstract

The pressure dependence of the magnetic and superconducting transitions, as well as that of the superconducting upper critical field is reported for CaK(Fe$_{1-x}$Ni$_{x}$)$_4$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, $T_\textrm{N}$, are suppressed upon applying pressure, the superconducting transition temperatures $T_\textrm{c}$ are suppressed by pressure as well, except for $x=0.050$ in the pressure region where $T_\textrm{N}$ and $T_\textrm{c}$ cross. Furthermore, the pressure associated with the crossing of the $T_\textrm{N}$ and $T_\textrm{c}$ 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 $p \sim$ 4 GPa, both Ni-substituted CaK(Fe$_{1-x}$Ni$_{x}$)$_4$As$_4$ samples likely go through a half-collapsed-tetragonal (hcT) phase transition, similar to the parent compound CaKFe$_4$As$_4$.