Topological Insulator nano-SQUID: Flux-tunable platform for topological superconductivity

TL;DR

This paper proposes a novel topological insulator nano-SQUID platform that is flux-tunable and robust against disorder, enabling the realization of topological superconductivity with potential applications in quantum computing.

Contribution

The authors introduce a new TI-based nano-SQUID structure with asymmetric junctions that supports a robust topological phase, confirmed by experimental behavior matching theoretical predictions.

Findings

Device behaves as a columnar nano-SQUID with supercurrent on top and bottom surfaces

Top/bottom asymmetry can be tuned via back gate

Topological phase occurs periodically over a wide flux range

Abstract

Many efforts have been made in the past decade to realize topological superconductivity using superconducting proximity effect, but an ideal platform is still lacking. A 3D topological insulator (TI) is promising for this purpose due to the spin-momentum-locked surface state. Here we propose a novel yet simple TI platform which gives rise to a topological phase that is robust against disorder. It consists of a bulk-insulating rectangular TI nanowire laterally sandwiched by two superconductors. In this structure, the top and bottom surfaces individually work as SNS line junctions, forming a nanometer-scale columnar SQUID in which the nanowire cross-section defines the threading magnetic flux $\Phi$ in axial magnetic fields. We theoretically show that, when the two junctions are asymmetric, a robust topological phase occurs periodically for a wide range of $\Phi$, independently of the chemical potential. Our experiment found that a TI device of this structure indeed behaves as a columnar nano-SQUID where the supercurrent flows only through the top and bottom surfaces with vanishing bulk contribution. Furthermore, the top/bottom asymmetry can be tuned by a back gate, a key ingredient for the topological phase.