Fractional AC Josephson effect in a topological insulator proximitized by a self-formed superconductor

TL;DR

This study demonstrates the fractional AC Josephson effect in a topological insulator device with a self-formed superconductor, showing suppressed Shapiro steps indicative of Majorana fermions, but emphasizes the need for further verification.

Contribution

It reports the first observation of fractional AC Josephson effect in a topological insulator with a self-formed superconductor, advancing the experimental evidence for Majorana states.

Findings

Suppressed first and third Shapiro steps observed in specific devices.

Simulations support the role of the self-formed superconductor in step suppression.

Other device configurations do not show suppression, indicating device-specific effects.

Abstract

A lateral Josephson junction in which the surface of a 3D topological insulator serves as the weak link should support topologically protected excitations related to Majorana fermions. The resulting $4\pi$-periodic current-phase relationship could be detected under high-frequency excitation by the suppression of odd Shapiro steps. Here, we demonstrate such devices through the self-formation of a Pd-Te superconducting layer from a telluride topological insulator, and observe suppressed first and third Shapiro steps. Other devices, including those where the Pd-Te layer is bolstered by an additional Al layer, show no suppression of Shapiro steps, a difference supported by simulations. Though we rule out the known trivial causes of suppressed Shapiro steps in our devices, we nevertheless argue that corroborating measurements and disorder-aware theoretical descriptions of these systems are needed before confidently claiming the observation of Majorana states.