Gap in KFe2As2 studied by small-angle neutron scattering observations of the magnetic vortex lattice

By neutron scattering, we have observed a well-ordered magnetic vortex lattice (VL) in KFe2As2 single crystals. With the field along the c axis, a nearly isotropic hexagonal VL is formed, with no symmetry transitions up to high fields, indicating a small anisotropy of the superconducting state around this axis. This rules out line nodes parallel to the c axis, and thus d-wave or anisotropic s-wave pairing. However, the strong temperature dependence of the signal at T << T-c shows that extremely small gap values exist; these may arise from nodal lines perpendicular to the c axis.