Evidence of Higher Order Topology in WTe₂from Josephson Coupling through Anisotropic Hinge States

The noncentrosymmetric Td-WTe₂, previously known as a Type-II Weyl semimetal, is expected to have higher order topological phases with topologically protected, helical one-dimensional (1D) hinge states when their scarcely separated Weyl points get annihilated. However, the detection of these hinge states is difficult in the presence of the semimetallic behaviour of the bulk. Here, we spatially resolved the hinge states by analysing the magnetic field interference of supercurrent in Nb-WTe₂-Nb proximity Josephson junctions. The Josephson current along the a-axis of the WTe₂ crystal, but not along the b-axis, showed sharp enhancements at the edges of the junction; the amount of enhanced Josephson current was comparable to the upper limits of a single 1D conduction channel. Furthermore, the Josephson effect under microwave radiation, shows voltage doubling of Shapiro steps when Josephson current flows along the a-axis of WTe₂ crystal, which implies 4π periodicity of current phase relationship. Our experimental observations provide evidence of the higher order topological phase in WTe₂ and its corresponding anisotropic topological hinge states, in good agreement with theoretical calculations.