Switching current distributions of epitaxial Al/InAs 2DEG Josephson junctions

Majorana zero modes (MZMs) are essential for realizing fault-tolerant quantum computing, yet convincing experimental evidence remains elusive. Topological Josephson junctions (JJs) exhibiting a 4π-periodic current-phase relation are expected to show missing Shapiro steps, often regarded as a potential signature of MZMs. However, similar behavior can arise from non-topological Landau–Zener transitions (LZTs). The predicted bimodal switching current distribution (SCD) in topological JJs [N. Abboud et al., PRB (2022)] offers an additional approach to distinguish these mechanisms. We fabricated highly transparent Al/InAs two-dimensional electron gas (2DEG) JJs operating in the ballistic regime and examined their Shapiro steps, SCDs, and multiple Andreev reflections (MARs). Although the first Shapiro step missing was observed at low temperatures, the SCD remained unimodal, indicating a single-well Josephson potential. From MARs analysis, we obtained a superconducting gap Δ = 156 μeV and junction transparency τ = 0.8. These results suggest that the missing Shapiro step observed in Al/InAs 2DEG JJ arises from LZTs rather than topological Josephson effect. Complementary measurements of Shapiro steps and SCDs are proposed as a more reliable method to verify MZMs in topological JJs.