Ferromagnetic Impurity Induced Majorana Zero Mode in Iron-Based Superconductor

TL;DR

This paper demonstrates that ferromagnetic impurities in iron-based superconductors can induce Majorana zero modes through a quantum phase transition caused by strong exchange coupling, explaining recent experimental observations.

Contribution

It introduces a theoretical mechanism showing how ferromagnetic impurities can generate Majorana modes in topological superconductors, clarifying experimental puzzles.

Findings

Strong exchange coupling induces a quantum phase transition.

Majorana modes appear at boundaries with sign-change in the order parameter.

Theory explains experimental zero energy modes in iron-based superconductors.

Abstract

Recent experiments reported the puzzling zero energy modes associated with ferromagnetic impurities in some iron-based superconductors with topological band structures. Here, we show that the sufficiently strong exchange coupling between a ferromagnetic impurity and substrate can trigger a quantum phase transition, beyond which, the phase of the topological surface superconducting order parameter around the impurity acquires a sign-change. In such a case, we prove that a Kramers degenerate pair of Majorana modes can be induced at the boundary separating the two sign-change regimes and trapped around the impurity in the topological surface superconducting state. Furthermore, we show that our theory can explain the controversial observations and confusing features of the zero energy modes from recent experiments in some iron-based superconductors.