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 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. The absence of odd Shapiro steps in microwave-irradiated Josephson junctions (JJs) is considered to be a possible indicator of 4π-periodic supercurrents that are induced by Majorana bound states. Here, by conducting measurements on Al/WTe2 JJs, the authors suggest that the missing first Shapiro step can instead arise from the intrinsic non-linearity of the current–voltage characteristics in low-to-moderate transparency junctions.
Xu et al. (Fri,) studied this question.