Tunable proximity effects and topological superconductivity in ferromagnetic hybrid nanowires

TL;DR

This paper demonstrates that topological superconductivity can be achieved in ferromagnetic hybrid nanowires through tunable proximity effects, with device geometry and gating controlling the emergence of the topological phase.

Contribution

It provides numerical evidence that geometrical constraints and gate tuning enable the realization of topological phases in hybrid nanowires with superconducting and magnetic layers.

Findings

Topological phase occurs when Al and EuS layers overlap on the wire.

Gate voltages can tune magnetic and superconducting proximity effects.

Local direct induced spin polarization is crucial for topological states.

Abstract

Hybrid semiconducting nanowire devices combining epitaxial superconductor and ferromagnetic insulator layers have been recently explored experimentally as an alternative platform for topological superconductivity at zero applied magnetic field. In this proof-of-principle work we show that the topological regime can be reached in actual devices depending on some geometrical constraints. To this end, we perform numerical simulations of InAs wires in which we explicitly include the superconducting Al and magnetic EuS shells, as well as the interaction with the electrostatic environment at a self-consistent mean-field level. Our calculations show that both the magnetic and the superconducting proximity effects on the nanowire can be tuned by nearby gates thanks to their ability to move the wavefunction across the wire section. We find that the topological phase is achieved in significant portions of the phase diagram only in configurations where the Al and EuS layers overlap on some wire facet, due to the rather local direct induced spin polarization and the appearance of an extra indirect exchange field through the superconductor. While of obvious relevance for the explanation of recent experiments, tunable proximity effects are of interest in the broader field of superconducting spintronics.