Fractional Shapiro steps without fractional Josephson effect

TL;DR

This paper shows that fractional Shapiro steps can occur in trivial superconducting junctions without Majorana states, cautioning against attributing such features solely to topological effects.

Contribution

The study provides a microscopic analysis demonstrating that fractional Shapiro steps can arise in trivial junctions, challenging the assumption they indicate Majorana physics.

Findings

Fractional Shapiro steps can occur without Majorana states.

Missing Shapiro steps are not exclusive to topological junctions.

Caution is needed when interpreting experimental signatures as Majorana evidence.

Abstract

It is widely believed that superconducting junctions involving topological insulators and hosting Majorana-like bound states may exhibit unusual "fractional" ($4\pi$-periodic) ac Josephson effect. Accordingly, "fractional" Shapiro steps on the current-voltage characteristics of such junctions are expected to occur under external microwave radiation. Here, we microscopically evaluate Shapiro steps in topologically trivial highly transparent superconducting weak links. The key features recovered within our analysis -- including, e.g., the so-called "missing" Shapiro steps -- turn out to be similar to those observed in topological Josephson junctions. Our results demonstrate that caution is needed while interpreting experimental results for superconducting weak links in terms of Majorana physics.