Antiferromagnetic order and its interplay with superconductivity in CaK(Fe$_{1-x}$Mn$_x$)$_4$As$_4$

TL;DR

This study reveals that Mn-doped CaK(Fe$_{1-x}$Mn$_x$)$_4$As$_4$ exhibits a hedgehog spin vortex crystal magnetic order that coexists with superconductivity, with a quantum phase transition near x≈0.01, and the magnetic moment remains stable below T_c.

Contribution

It demonstrates the presence of a hedgehog spin vortex crystal magnetic order coexisting with superconductivity in Mn-doped CaK(Fe$_{1-x}$Mn$_x$)$_4$As$_4$, revealing a quantum phase transition under the superconducting dome.

Findings

Hedgehog spin vortex crystal order observed in Mn-doped CaK(Fe$_{1-x}$Mn$_x$)$_4$As$_4$

Coexistence of magnetic order and superconductivity

Quantum phase transition at x≈0.01

Abstract

The magnetic order for several compositions of CaK(Fe$_{1-x}$Mn$_x$)$_4$As$_4$ has been studied by nuclear magnetic resonance (NMR), M\"ossbauer spectroscopy, and neutron diffraction. Our observations for the Mn-doped 1144 compound are consistent with the hedgehog spin vortex crystal (hSVC) order which has previously been found for Ni-doped $\text{Ca}\text{K}\text{Fe}_4\text{As}_4$. The hSVC state is characterized by the stripe-type propagation vectors $(\pi\,0)$ and $(0\,\pi)$ just as in the doped 122 compounds. The hSVC state preserves tetragonal symmetry at the Fe site, and only this SVC motif with simple AFM stacking along $\textbf{c}$ is consistent with all our observations using NMR, M\"ossbauer spectroscopy, and neutron diffraction. We find that the hSVC state in the Mn-doped 1144 compound coexists with superconductivity (SC), and by combining the neutron scattering and M\"ossbauer spectroscopy data we can infer a quantum phase transition, hidden under the superconducting dome, associated with the suppression of the AFM transition temperature ($T_\text{N}$) to zero for $x\approx0.01$. In addition, unlike several 122 compounds and Ni-doped 1144, the ordered magnetic moment is not observed to decrease at temperatures below the superconducting transition temperature ($T_\text{c}$).