Influence of multiband sign-changing superconductivity on vortex cores and vortex pinning in stoichiometric high-Tc CaKFe4As4

We use a scanning tunneling microscope to study the superconducting density of states and vortex lattice of single crystals of CaKFe4As4. This material has a critical temperature of T-c = 35 K, one of the highest among stoichiometric iron based superconductors (FeBSCs), and is comparable to T-c found near optimal doping in other FeBSCs. We observe quasiparticle scattering from defects with a pattern related to interband scattering between zone centered hole sheets. We measure the tunneling conductance in vortex cores and find a peak due to Caroli-de Gennes-Matricon bound states. The peak is located above the Fermi level, showing that CaKFe4As4 is a clean superconductor with vortex core bound states close to the so-called extreme quantum limit. We identify locations where the superconducting order parameter is strongly suppressed due to pair breaking. Vortices are pinned at these locations, and the length scale of the suppression of the order parameter is of order of the vortex core size. As a consequence, the vortex lattice is disordered up to 8 T.