Bimodal switching current distributions in NbTi-Bi₂Se₃-NbTi Josephson junctions

Majorana zero modes (MZMs) in topological superconductors are promising for realizing fault-tolerant quantum computing, yet their conclusive detection remains elusive. Conventional experimental signatures such as zero-bias conductance peak and Shapiro-step doubling can arise from nontopological effects, leading to ongoing controversy over MZM confirmation. Although a bimodal switching current distribution (SCD) has been theoretically predicted in topological Josephson junction (JJ) [N. Abboud et al., Phys. Rev. B 105, 214521 (2022)], experimental evidence has been lacking. Here, we report the observation of bimodal SCDs in JJs based on Sb-doped Bi₂Se₃ topological insulator nanoribbons. The bimodal SCDs emerge at low temperatures below 0.5 K and are attributed to a double-well Josephson potential formed by the coexistence of 2π-periodic conventional and 4π-periodic topological supercurrents. Furthermore, microwave-irradiated measurements reveal the absence of the first (n = 1) Shapiro step, confirming the presence of a 4π-periodic supercurrent. The simultaneous observation of bimodal SCDs and missing Shapiro steps provides a more reliable route toward verifying MZMs in topological JJs.