  Two-dimensional (2D) magnet-superconductor hybrid systems are intensively
studied due to their potential for the realization of 2D topological
superconductors with Majorana edge modes. It is theoretically predicted that
this quantum state is ubiquitous in spin-orbit coupled ferromagnetic or
skyrmionic 2D spin-lattices in proximity to an s-wave superconductor. However,
recent examples suggest that the requirements for topological superconductivity
are complicated by the multi-orbital nature of the magnetic components and
disorder effects. Here, we investigate Fe monolayer islands grown on a surface
of the s-wave superconductor with the largest gap of all elemental
superconductors, Nb, with respect to magnetism and superconductivity using
spin-resolved scanning tunneling spectrosopy. We find three types of Fe
monolayer islands which differ by their reconstruction inducing disorder, the
magnetism and the sub-gap electronic states. All three types are ferromagnetic
with different coercive fields indicating diverse exchange and anisotropy
energies. On all three islands, there is finite spectral weight throughout the
substrate's energy gap at the expense of the coherence peak intensity,
indicating the formation of Shiba bands overlapping with the Fermi energy. The
gap filling and coherence peak reduction is strongest for the island with
largest coercive field. A strong lateral variation of the spectral weight of
the Shiba bands signifies substantial disorder on the order of the substrate's
pairing energy with a length scale of the period of the three different
reconstructions. There are neither signs of topological gaps within these bands
nor of any kind of edge modes. Our work illustrates that a reconstructed growth
mode of magnetic layers on superconducting surfaces is detrimental for the
formation of 2D topological superconductivity.
