AC Josephson effect in higher-order topological insulator WTe₂

WTe₂ is a layered transitional metal dichalcogenide, known for its topological properties. In the form of a monolayer it is a 2D topological insulator and as a bulk material it is a type-II Weyl semimetal. Recently, another topological phase in WTe₂ has been predicted.  The new phase is a higher-order topological insulator with helical hinge states. The evidence of such 1D states along certain crystallographic directions in thin WTe₂ has been obtained in a number of experiments employing Josephson junctions.

Here, we study the AC Josephson effect in the few-layer thick WTe₂. Our measurements indicate a strong contribution of the multi Cooper pair tunnelling events to the supercurrent. This observation suggests non-sinusoidal current-phase relation of the studied Josephson junctions due to the presence of superconducting modes with high transparency. We connect these modes with the hinge states in WTe₂.