End states and subgap structure in proximity-coupled chains of magnetic adatoms

TL;DR

This study uses superconducting scanning tunneling microscopy to analyze iron chains on Pb(110), revealing complex subgap structures and questioning the size of the topological gap associated with Majorana zero modes.

Contribution

It provides high-resolution experimental data and theoretical analysis of subgap states in magnetic adatom chains, challenging previous assumptions about the topological gap size.

Findings

Detection of zero-energy excitations in some chains

Evidence that the Majorana signature overlaps with a low-energy resonance

Indication that the topological gap is smaller than previously thought

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 significantly smaller than previously believed. 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.