Inducing Strong Superconductivity in WTe 2 by a Proximity Effect.

The search for proximity-induced superconductivity in topological materials has generated widespread interest in the condensed matter physics community. The superconducting states inheriting nontrivial topology at interfaces are expected to exhibit exotic phenomena such as topological superconductivity and Majorana zero modes, which hold promise for applications in quantum computation. However, a practical realization of such hybrid structures based on topological semimetals and superconductors has hitherto been limited. Here, we report the strong proximity-induced superconductivity in type-II Weyl semimetal WTe ₂ , in a van der Waals hybrid structure obtained by mechanically transferring NbSe ₂ onto various thicknesses of WTe ₂ . When the WTe ₂ thickness ( t _{WTe 2} ) reaches 21 nm, the superconducting transition occurs around the critical temperature ( T _c ) of NbSe ₂ with a gap amplitude (Δ _p ) of 0.38 meV and an unexpected ultralong proximity length ( l _p ) up to 7 μm. With the thicker 42 nm WTe ₂ layer, however, the proximity effect yields T _c ≈ 1.2 K, Δ _p = 0.07 meV, and a short l _p of less than 1 μm. Our theoretical calculations, based on the Bogoliubov-de Gennes equations in the clean limit, predict that the induced superconducting gap is a sizable fraction of the NbSe ₂ superconducting one when t _{WTe 2} is less than 30 nm and then decreases quickly as t _{WTe 2} increases. This agrees qualitatively well with the experiments. Such observations form a basis in the search for superconducting phases in topological semimetals.