pi and 4 pi Josephson Effects Mediated by a Dirac Semimetal

Cd3As2 is a three-dimensional topological Dirac semimetal with connected Fermi-arc surface states. It has been suggested that topological superconductivity can be achieved in the nontrivial surface states of topological materials by utilizing the superconductor proximity effect. Here we report observations of both pi and 4 pi periodic supercurrents in aluminum-Cd3As2-aluminum Josephson junctions. The pi period is manifested by both the magnetic-field dependence of the critical supercurrent and the appearance of half-integer Shapiro steps in the ac Josephson effect. Our macroscopic theory suggests that the pi period arises from interference between the induced bulk superconductivity and the induced Fermi-arc surface superconductivity. The 4 pi period is manifested by the missing first Shapiro steps and is expected for topological superconductivity.