Detecting topological superconductivity using low-frequency doubled Shapiro steps

TL;DR

This paper proposes that low-frequency doubled Shapiro steps can serve as a more reliable indicator of topological superconductivity, distinguishing it from non-topological effects that can mimic fractional Josephson signatures.

Contribution

It introduces a nontopological mechanism for fractional Josephson effects and demonstrates that low-frequency doubled Shapiro steps uniquely identify topological superconductivity.

Findings

Low-frequency doubled Shapiro steps are a robust signature of topological superconductivity.

Nontopological impurity states can produce fractional Josephson effects at higher frequencies.

The proposed method improves the reliability of detecting Majorana modes in experiments.

Abstract

The fractional Josephson effect has been observed in many instances as a signature of a topological superconducting state containing zero-energy Majorana modes. We present a nontopological scenario which can produce a fractional Josephson effect generically in semiconductor-based Josephson junctions, namely, a resonant impurity bound state weakly coupled to a highly transparent channel. We show that the fractional ac Josephson effect can be generated by the Landau-Zener processes which flip the electron occupancy of the impurity bound state. The Josephson effect signature for Majorana modes become distinct from this nontopological scenario only at low frequency. We prove that a variant of the fractional ac Josephson effect, namely, the low-frequency doubled Shapiro steps, can provide a more reliable signature of the topological superconducting state.