Search for large topological gaps in atomic spin chains on proximitized superconducting heavy-metal layers

One-dimensional systems comprising s -wave superconductivity with meticulously tuned magnetism realize topological superconductors hosting Majorana modes whose stability is determined by the gap size. However, for atomic spin chains on superconductors, the effect of the substrate’s spin-orbit coupling on the topological gap is largely unexplored. Here, we introduce an atomic layer of the heavy metal gold on a niobium surface combining strong spin-orbit coupling and a large superconducting gap with a high crystallographic quality, enabling the assembly of defect-free iron chains using a scanning tunneling microscope tip. Scanning tunneling spectroscopy experiments and density functional theory calculations reveal ungapped Yu–Shiba–Rusinov bands in the ferromagnetic chain despite the heavy substrate. By artificially imposing a spin spiral state, the calculations indicate minigap opening and zero-energy edge state formation. The methodology enables a material screening of heavy-metal layers on elemental superconductors for ideal systems hosting Majorana edge modes protected by large topological gaps.