Zero-bias conductance peaks at zero applied magnetic field due to stray fields from integrated micromagnets in hybrid nanowire quantum dots

TL;DR

This study demonstrates that stray magnetic fields from integrated micromagnets induce zero-bias conductance peaks in hybrid nanowire devices, offering a low-field alternative to external magnetic fields for exploring topological superconductivity.

Contribution

It introduces the use of micromagnets to locally break time-reversal symmetry in superconductor-semiconductor nanowires, providing a new approach to study zero-bias peaks without external magnetic fields.

Findings

Zero-bias conductance peaks observed at zero external magnetic field.

Hysteretic Josephson supercurrent due to micromagnet switching.

Stray fields produce trivial zero-bias peaks, not Majorana modes.

Abstract

Many recipes for realizing topological superconductivity rely on broken time-reversal symmetry, which is often attained by applying a substantial external magnetic field. Alternatively, using magnetic materials can offer advantages through low-field operation and design flexibility on the nanoscale. Mechanisms for lifting spin degeneracy include exchange coupling, spin-dependent scattering, spin injection-all requiring direct contact between the bulk or induced superconductor and a magnetic material. Here, we implement locally broken time-reversal symmetry through dipolar coupling from nearby micromagnets to superconductor-semiconductor hybrid nanowire devices. Josephson supercurrent is hysteretic due to micromangets switching. At or around zero external magnetic field, we observe an extended presence of Andreev bound states near zero voltage bias. We also show a zero-bias peak plateau of a non-quantized value. Our findings largely reproduce earlier results where similar effects were presented in the context of topological superconductivity in a homogeneous wire, and attributed to more exotic time-reversal breaking mechanisms [1]. In contrast, our stray field profiles are not designed to create Majorana modes, and our data are compatible with a straightforward interpretation in terms of trivial states in quantum dots. At the same time, the use of micromagnets in hybrid superconductor-semiconductor devices shows promise for future experiments on topological superconductivity.