The pairing state in KFe2As2 studied by measurements of the magnetic vortex lattice

TL;DR

This study uses small-angle-neutron-scattering to investigate the vortex lattice in KFe2As2, providing insights into its pairing symmetry and ruling out certain gap structures based on vortex symmetry and anisotropy.

Contribution

It presents the first detailed observation of the vortex lattice in KFe2As2, revealing nearly isotropic hexagonal packing and constraining the possible pairing symmetries.

Findings

Vortex lattice is nearly isotropic and hexagonal.

No vortex lattice symmetry transitions up to high fields.

Results exclude d-wave and anisotropic s-wave pairing.

Abstract

Understanding the mechanism and symmetry of electron pairing in iron-based superconductors represents an important challenge in condensed matter physics [1-3]. The observation of magnetic flux lines - "vortices" - in a superconductor can contribute to this issue, because the spatial variation of magnetic field reflects the pairing. Unlike many other iron pnictides, our KFe2As2 crystals have very weak vortex pinning, allowing small-angle-neutron-scattering (SANS) observations of the intrinsic vortex lattice (VL). We observe nearly isotropic hexagonal packing of vortices, without VL-symmetry transitions up to high fields along the fourfold c-axis of the crystals, indicating rather small anisotropy of the superconducting properties around this axis. This rules out gap nodes parallel to the c-axis, and thus d-wave and also anisotropic s-wave pairing [2, 3]. The strong temperature-dependence of the intensity down to T<<Tc indicates either widely different full gaps on different Fermi surface sheets, or nodal lines perpendicular to the axis.