Ferromagnetically coupled chains of magnetic adsorbates on a superconductor have recently been suggested to host Majorana bound states. Scanning tunneling microscopy has been used to detect these states in Fe on Pb(110) as localized zero-energy modes at the ends of the chain.

Here, we start by characterizing the subgap-bound states of individual magnetic adsorbates on superconducting Pb surfaces by scanning tunneling spectroscopy with superconducting tips. We show that individual Manganese (Mn) atoms give rise to a distinct number of Yu-Shiba-Rusinov (YSR) states, depending on the crystal field imposed by the adsorption site. The spatial extension of these states directly reflects their origin as the singly occupied d-states.

In the limit of densely arranged chains of magnetic adatoms, the YSR-states hybridize and give rise to extended bands. Motivated by the results of Fe chains, we test whether Co chains exhibit similar characteristics on a Pb(110) surface. Using Co-coated STM tips, we detect spin-polarized d-bands reflecting ferromagnetic coupling along the chain in analogy to the Fe chains. The YSR-bands inside the superconducting energy gap also exhibit spin contrast. However, the rich subgap structure does not provide evidence of Majorana end states. In a simple model of tight-binding calculations, we ascribe the absence of Majorana end states to an even number of d-bands crossing the Fermi level. This is a major difference to the Fe case.