Topological superconductivity and anti-Shiba states in disordered chains of magnetic adatoms

TL;DR

This paper investigates how vacancies in magnetic atom chains on superconductors affect topological superconductivity, revealing the formation of anti-Shiba states and their impact on the stability of the topological phase.

Contribution

It introduces theoretical approaches to analyze disordered magnetic chains, especially vacancies, and uncovers the formation of anti-Shiba states that influence topological properties.

Findings

Vacancies create low-lying anti-Shiba states below the band edge.

Proliferation of anti-Shiba states can deteriorate the topological phase.

Topological fragility is pronounced in dilute chains with vacancies.

Abstract

Regular arrays of magnetic atoms on a superconductor provide a promising platform for topological superconductivity. In this work we study effects of disorder in these systems, focusing on vacancies realized by missing magnetic atoms. We develop approaches that allow treatment of ferromagnetic dense chains as well as long-range hopping ferromagnetic and helical Shiba chains at arbitrary subgap energies. Vacancies in magnetic chains play an analogous role to magnetic impurities in a clean $s$-wave superconductor. A single vacancy in a topological chain gives rise to a low-lying "anti-Shiba" state below the band edge of a regular magnetic chain. Proliferation of the anti-Shiba band formed by a finite density of hybridized vacancy states leads to deterioration of the topological phase, which exhibits unusual fragility in a particular parameter region in dilute chains. We also consider local fluctuation in the Shiba coupling and discuss how vacancy states could contribute to experimental verification of topological superconductivity.