Ballistic 1D channels in multilayer-WTe₂Josephson junctions.

WTe₂, a transition metal dichalcogenide, is predicted to have striking topological properties that combine type II Weyl semimetal character with second-order 3D topological insulator (SOTI) character. SOTIs are characterized by topologically protected (insensitive to disorder) helical 1D states at their hinges. 1D states located at certain edges of multilayer WTe₂ have indeed been demonstrated in Josephson interferometry experiments. However, their ballistic nature was not tested.

We have designed a WTe₂-based Superconducting Quantum Interference Device (SQUID) in which the supercurrent through one edge of the crystal interferes with the supercurrent far from the edge. The critical current of this asymmetric SQUID yields the supercurrent-versus-phase relation of the edge states. Its sawtooth shape is a tell-tale sign that the supercurrent through the edge flows ballistically over 600 nm (which is ten times the estimated normal state mean free path). This is due to the SOTI character of WTe2.