Transparent Josephson junctions in higher-order topological insulator WTe₂ via Pd diffusion

Highly transparent superconducting contacts to a topological insulator (TI) remain a persistent challenge on the route to engineer topological superconductivity. Recently, the higher-order TI WTe2 was shown to turn superconducting when placed on palladium (Pd) bottom contacts, demonstrating a promising material system in pursuing this goal. Here, we report the diffusion of Pd into WTe2 and the formation of superconducting PdTex as the origin of observed superconductivity. We find an atomically sharp interface in the direction vertical to the van der Waals layers between the diffusion crystal and its host crystal, forming state-of-the-art superconducting contacts to a TI. The diffusion is discovered to be nonuniform along the width of the WTe2 crystal, with a greater extent along the edges compared to the bulk. The potential of this contacting method is highlighted in transport measurements on Josephson junctions by employing external superconducting leads.