Article: Nontopological Zero-bias Peaks in Full-shell Nanowires Induced by Flux-tunable Andreev States published in Science.
Hybrid superconductor-semiconductor nanowires are a system of choice to try to create the necessary conditions for one-dimensional topological superconductivity, thanks to their relative conceptual simplicity and the impressive know-how in the semiconducting labs nowadays. Over the last decade there has been an enormous effort, both theoretical and experimental, to look for the elusive Majorana quasiparticles at the ends of such wires. The improvements in the quality of the materials, device fabrication and measurement techniques over these years have been outstanding, as well as the accompanying theoretical understanding of those devices. However, the experimental reality of these systems has turned to be more complex than previously anticipated. The influence of the electrostatic environment, the introduction of disorder at the device fabrication process or the need for large magnetic fields to drive the wire into the topological regime, are some factors that prevent a clear detection and moreover plague the wires with trivial zero energy states. Unfortunately, these are difficult to distinguish from true Majoranas with local-probe measurements.
Recently, a new hybrid nanowire design, called a full-shell nanowire, has been put forward that presents several advantages with respect to previous designs. In these wires, the parent superconductor is a thin epitaxially-grown shell that covers all around the wire. In a first round of experiments lead by Charlie Marcus in Copenhagen, they find very compelling zero-bias anomalies compatible with the presence of Majorana bound states at small magnetic fields, according to theory developed by the group of Roman Lutchyn.
In our work, we utilize the same kind of full-shell wires, provided by Peter Krogstrup, and perform similar differential conductance experiments led by Georgios Katsaros of the IST in Austria, but we challenge the above Majorana interpretation. Specifically, we demonstrate that the appearance of subgap states in such structures is actually governed by the junction region in tunneling spectroscopy measurements and not the full-shell nanowire itself. Short tunneling regions never show subgap states, whereas longer junctions always do. The theoretical modeling, carried out by Ramón Aguado, Fernando Peñaranda and Pablo San-Jose from the ICMM-CSIC, and the IFIMAC researcher Elsa Prada (who later moved to the ICMM), shows that the phenomenology of these experiments can be understood in terms of quantum dots forming in the junction and hosting Andreev levels in the Yu-Shiba-Rusinov regime. The intricate magnetic field dependence of the Andreev levels, through both the Zeeman and Little-Parks effects, may result in robust zero-bias peaks of trivial origin—features that could be easily misinterpreted as originating from Majorana zero modes but are unrelated to topological superconductivity. [Full article]