End States and Subgap Structure in Proximity-Coupled Chains of Magnetic Adatoms

TLDR

Recent experiments suggest Majorana zero modes in iron chains on superconducting Pb(110). The study investigates these chains using superconducting‑tip scanning tunneling microscopy. The authors map subgap spectra with STM, compare voltage‑reversal symmetry to theory, and use model calculations to analyze higher‑energy features. High‑resolution STM reveals zero‑energy excitations, shows the putative Majorana mode overlaps with a low‑energy resonance, indicates a smaller topological gap than previously reported, and links higher‑energy subgap features to Fe d bands.

Abstract

A recent experiment [Nadj-Perge et al., Science 346, 602 (2014)] provides evidence for Majorana zero modes in iron (Fe) chains on the superconducting Pb(110) surface. Here, we study this system by scanning tunneling microscopy using superconducting tips. This high-resolution technique resolves a rich subgap structure, including zero-energy excitations in some chains. We compare the symmetry properties of the data under voltage reversal against theoretical expectations and provide evidence that the putative Majorana signature overlaps with a previously unresolved low-energy resonance. Interpreting the data within a Majorana framework suggests that the topological gap is smaller than previously extracted from experiment. Aided by model calculations, we also analyze higher-energy features of the subgap spectrum and their relation to high-bias peaks which we associate with the Fe d bands.