Edge states in mesoscopic Bi islands on superconducting Nb(110)

Abstract

Islands of Bi showing (110)- and (111)-orientated facets were grown on a Nb(110) single crystal and were studied with a scanning tunneling microscope at low temperatures. On the (111) facets, several states localized at step edges are identified from maps of the local differential conductance. We find edge states on both types of zig-zag step edges that decay exponentially over a length scale of several nanometers into the terrace. These states agree with the necessary condition for topologically nontrivial edge states. Further, we find states localized at the atomic scale at the step edge, which we interpret as dangling bond states. To investigate the nature of these states, we studied the Bi(110) surface, where the sharply localized states form stripe patterns, while the more extended edge state does not show a strong lattice-periodic pattern. Thus, our findings indicate a causal link between the strongly localized edge states on Bi(111) and the stripe pattern on Bi(110) as they are both related to the zig-zag termination of the honeycomb structure of Bi. Finally, we observed a decay of the superconducting proximity gap of the Bi islands with island thickness.