Intrinsic Non-linearity of Josephson Junctions as an Alternative Origin of the Missing First Shapiro Step

TL;DR

This study shows that the intrinsic non-linearity of Josephson junctions can cause the missing first Shapiro step, challenging the interpretation that it solely indicates Majorana bound states, and introduces zigzag boundary features as diagnostic markers.

Contribution

The paper demonstrates that intrinsic non-linearity in Josephson junctions can suppress the first Shapiro step, providing an alternative explanation to Majorana-related interpretations and identifying distinctive zigzag features.

Findings

Intrinsic non-linearity can suppress the first Shapiro step.

Zigzag boundaries between steps serve as diagnostic signatures.

Numerical simulations confirm switching jumps cause step collapse.

Abstract

The missing first Shapiro step in microwave-irradiated Josephson junctions has been widely interpreted as a hallmark of Majorana bound states. However, conventional mechanisms like junction underdamping or Joule heating can produce similar signatures. Here, we demonstrate that the intrinsic non-linear current-voltage characteristic of low-to-moderate transparency junctions can also suppress the first step, accompanied by distinctive zigzag boundaries between the zeroth and first step at intermediate driving frequencies. Microwave measurements on Al/WTe2 junctions and numerical simulations of a non-linear resistively and capacitively shunted junction model reveal the first step collapse induced by switching jumps of current, together with zigzag features absent in scenarios solely driven by finite \b{eta} or Joule heating. This zigzag signature therefore provides a crucial diagnostic tool, emphasizing the necessity of comprehensive analysis of microwave spectra before attributing the absence of the first Shapiro step to Majorana physics.