Non-topological zero bias peaks in full-shell nanowires induced by flux tunable Andreev states

A semiconducting nanowire core fully wrapped by a superconducting shell has been proposed as an alternative geometry for obtaining Majorana modes without the need of fine tuning the chemical potential or an external magnetic field [1]. While this robustness seems to avoid interpretation ambiguities in terms of non-topological Andreev bound states, we here demonstrate that the appearance of subgap states is actually governed by the junction region in tunneling spectroscopy measurements, not the full-shell nanowire itself [2]. Short tunneling regions never show subgap states, while longer junctions always do. This can be understood in terms of quantum dots forming in the junction and hosting Andreev levels in the Yu-Shiba-Rusinov regime. Their intricate magnetic-field dependence, both through the Zeeman and the Little-Parks effects, may result in robust zero-bias peaks, a feature that could be easily misinterpreted as originating from Majoranas, but is unrelated to topology.

[1] Vaitiekenas, S. et al. Flux-induced topological superconductivity in full-shell nanowires. Science 367, eaav3392 (2020).
[2] Valentini, M., et al. "Flux-tunable Andreev bound states in hybrid full-shell nanowires." arXiv preprint arXiv:2008.02348 (2020).