Semiconductor-ferromagnet-superconductor planar heterostructures for 1D topological superconductivity

TL;DR

This paper proposes a planar heterostructure of semiconductor, ferromagnetic insulator, and superconductor that can host topological superconductivity without magnetic fields, offering improved control and larger gaps over nanowire systems.

Contribution

It introduces a novel planar heterostructure design with optimized geometry for topological superconductivity, avoiding magnetic fields and enhancing gap stability.

Findings

Optimized FI layer thickness for topological phase transition.

Regular alternation of trivial and topological phases with gate tuning.

Hard topological gap reaching half of the superconductor gap.

Abstract

Hybrid structures of semiconducting (SM) nanowires, epitaxially grown superconductors (SC), and ferromagnetic-insulator (FI) layers have been explored experimentally and theoretically as alternative platforms for topological superconductivity at zero magnetic field. Here, we analyze a tripartite SM/FI/SC heterostructure but realized in a planar stacking geometry, where the thin FI layer acts as a spin-polarized tunneling barrier between the SM and the SC. We optimize the system's geometrical parameters using microscopic simulations, finding the range of FI thicknesses for which the hybrid system can be tuned into the topological regime. Within this range, and thanks to the vertical confinement provided by the stacking geometry, trivial and topological phases alternate regularly as the external gate is varied, displaying a hard topological gap that can reach half of the SC one. This is a significant improvement compared to setups using hexagonal nanowires, which show erratic topological regions with typically smaller and softer gaps. Our proposal provides a magnetic field-free planar design for quasi-one-dimensional topological superconductivity with attractive properties for experimental control and scalability.