Observation of zero-energy edge states in rare-earth atomic chains on superconducting Nb(110) surface

The concept of generating topologically protected edge states by coupling Yu-Shiba-Rusinov (YSR) states in a 1D chain of magnetic adsorbates has gained significant attention, sparking increased exploration of YSR states. While extensive research has focused on 3d transition metals on various superconducting substrates, experimental investigations of 4f-shell rare-earth metals remain limited. Rare-earth elements like Gd, Tb, and Eu, when coupled with superconductors, present exciting opportunities due to their strongly localized 4f orbitals, which are primarily shielded by outer electronic shells. In this experimental study, we examine YSR states induced by these rare-earth adatoms on a Nb(110) superconducting surface. By constructing Gd and Tb atomic chains along the substrate's [1-10] and [001] directions, we uncover distinct behaviors based on chain orientation. Chains along the [1-10] direction exhibit spectroscopic features at their ends, corresponding to trivial edge states, whereas [001]-oriented chains reveal zero-energy edge states, indicating a non-trivial nature.