Bound states of a ferromagnetic wire in a superconductor

TL;DR

This paper investigates bound states in ferromagnetic wires on superconductors, showing that strong magnetization can produce zero-energy peaks similar to Majorana modes observed experimentally.

Contribution

It extends the theory of impurity bound states to wire-like geometries and demonstrates the formation of zero-energy peaks near wire ends, mimicking Majorana signatures.

Findings

Bound states form bands with van Hove singularities.

Strong magnetization induces zero-energy peaks at wire ends.

Results resemble experimental Majorana signatures.

Abstract

We consider the problem of bound states in strongly anisotropic ferromagnetic impurities in a superconductor, motivated by recent experiments that claim to observe Majorana modes at the ends of ferromagnetic wires on a superconducting substrate [S. Nadj-Perge et al., Science 346, 602 (2014)]. Generalizing the successful theory of bound states of spherically symmetric impurities, we consider a wire-like potential using both analytical and numerical approaches. We find that away from the ends of the wire the bound states form bands with pronounced van Hove singularities, giving rise to subgap peaks in the local density of states. For sufficiently strong magnetization of the wire, we show that this process generically produces a sharp peak at zero energy in the local density of states near the ends of the wire. This zero-energy peak has qualitative similarities to the claimed signature of a Majorana mode observed in the aforementioned experiment.