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Understanding the mechanism and symmetry of electron pairing in iron-basedsuperconductors represents an important challenge in condensed matter physics[1-3]. The observation of magnetic flux lines - "vortices" - in asuperconductor can contribute to this issue, because the spatial variation ofmagnetic field reflects the pairing. Unlike many other iron pnictides, ourKFe2As2 crystals have very weak vortex pinning, allowingsmall-angle-neutron-scattering (SANS) observations of the intrinsic vortexlattice (VL). We observe nearly isotropic hexagonal packing of vortices,without VL-symmetry transitions up to high fields along the fourfold c-axis ofthe crystals, indicating rather small anisotropy of the superconductingproperties 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 strongtemperature-dependence of the intensity down to T<<Tc indicates either widelydifferent full gaps on different Fermi surface sheets, or nodal linesperpendicular to the axis.

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