Ballistic surface channels in fully in situ defined Bi$_4$Te$_3$ Josephson junctions with aluminum contacts

TL;DR

This study investigates the electrical transport in Bi$_4$Te$_3$ Josephson junctions with aluminum contacts, revealing ballistic supercurrents and topological surface states indicative of its dual topological insulator nature.

Contribution

It demonstrates the formation of ballistic supercurrents in Bi$_4$Te$_3$ Josephson junctions, highlighting its dual topological insulator properties and the role of proximity effects.

Findings

Ballistic supercurrents observed in Bi$_4$Te$_3$ junctions.

Presence of topological surface states indicated by transport.

Superconducting gap determined via Andreev reflections.

Abstract

In this letter we report on the electrical transport properties of Bi$_4$Te$_3$ in a Josephson junction geometry using superconducting Al electrodes with a Ti interdiffusion barrier. Bi$_4$Te$_3$ is proposed to be a dual topological insulator, for which due to time-reversal and mirror symmetry both a strong topological insulator phase as well as a crystalline topological phase co-exist. The formation of a supercurrent through the Bi$_4$Te$_3$ layer is explained by a two-step process. First, due to the close proximity of the Al/Ti electrodes a superconducting gap is induced within the Bi$_4$Te$_3$ layer right below the electrodes. The size of this gap is determined by analysing multiple Andreev reflections (MARs) identified within the devices differential resistance at low voltage biases. Second, based on the Andreev reflection and reverse Andreev reflection processes a supercurrent establishes in the weak link region in between these two proximity coupled regions. Analyses of the temperature dependency of both the critical current as well as MARs indicate mostly ballistic supercurrent contributions in between the proximitized Bi$_4$Te$_3$ regions even though the material is characterized by a semi-metallic bulk phase. The presence of these ballistic modes gives indications on the topological nature of Bi$_4$Te$_3$.