The Vortex Lattice of KFe$_{2}$As$_{2}$

Small angle neutron scattering (SANS) studies of the vortex lattice in the FeAs-based superconductors are currently being hindered by the strong vortex pinning that seems almost omnipresent in these materials. An investigation of Ba(Fe$_{0.93}$Co$_{0.07}$)$_{2}$As$_{2}$ [1] gave only a ring of diffraction intensity, not well-defined Bragg peaks, showing a disordered vortex structure. It is likely that local inhomogeneities in these doped compounds are responsible for the significant vortex pinning. KFe$_{2}$As$_{2}$ belongs to the 122 class of pnictide superconductors and is intrinsically superconducting (with a T$_{c}$ of 3.8 K), allowing high quality crystals to be grown that exhibit extremely low vortex lattice pinning. Our SANS measurements of KFe$_{2}$As$_{2}$ are the first to observe sharp Bragg peaks from a well-ordered vortex lattice. It is now the underlying physics, not pinning, that controls the vortex structure. We present the vortex lattice structure observed as a function of temperature and field, and discuss what the temperature dependence of the diffracted intensity implies for the superconducting order parameter. [1] M.R. Eskildsen et al. Phys. Rev. B 79, 100501(R) (2009)