Anisotropic superconductivity and Fermi surface reconstruction in the spin-vortex antiferromagnetic superconductor CaK(Fe$_{0.95}$Ni$_{0.05}$)$_4$As$_4$

TL;DR

This study investigates how spin-vortex antiferromagnetic order in Ni-doped CaKFe4As4 causes Fermi surface reconstruction and anisotropic superconducting gaps, revealing the interplay between magnetism and superconductivity.

Contribution

It provides the first detailed analysis of Fermi surface changes and gap anisotropy caused by spin-vortex order in this compound.

Findings

Fermi surface reconstruction observed via quasiparticle interference

Superconducting gap exhibits fourfold anisotropy

Spin-vortex order influences electronic structure and superconductivity

Abstract

High critical temperature superconductivity often occurs in systems where an antiferromagnetic order is brought near $T=0K$ by slightly modifying pressure or doping. CaKFe$_4$As$_4$ is a superconducting, stoichiometric iron pnictide compound showing optimal superconducting critical temperature with $T_c$ as large as $38$ K. Doping with Ni induces a decrease in $T_c$ and the onset of spin-vortex antiferromagnetic order, which consists of spins pointing inwards to or outwards from alternating As sites on the diagonals of the in-plane square Fe lattice. Here we study the band structure of CaK(Fe$_{0.95}$Ni$_{0.05}$)$_4$As$_4$ (T$_c$ = 10 K, T$_N$ = 50 K) using quasiparticle interference with a Scanning Tunneling Microscope (STM) and show that the spin-vortex order induces a Fermi surface reconstruction and a fourfold superconducting gap anisotropy.