Topological Josephson Effect in Hinge State of WTe₂

Higher order topological insulators (HOTIs) are a new class of topological insulators that exhibit topological boundary modes at two or more dimensions lower than them. For example, three-dimensional HOTIs can host the topologically protected one-dimensional hinge states or zero-dimensional corner states. Although recent experimental reports suggest the existence of conducting channels at the hinges of non-centrosymmetric Td-WTe₂ crystals, the investigation of their topological nature remains an ongoing requirement. Here, we investigate Shapiro steps in Al-WTe₂-Al proximity Josephson junctions (JJs) under microwave irradiation and propose the topological nature of the hinge states in WTe₂. We have analyzed the doubling of Shapiro steps at different microwave frequencies in edge-contacted JJs, which is explained by the 4π-periodic current-phase relationship of topological Josephson junctions. Our observation reveals the topological nature of the hinge states in T_d-WTe₂ and would stimulate the investigation of topological superconductivity with topological hinge states in superconducting hybrid devices based on higher-order topological insulators, leading to the realization of realizing Majorana zero modes and topological quantum physics.