Valley dependent superconducting proximity effect in a twisted van der Waals heterojunction

Leakage of Cooper pairs through heterointerfaces leads to the superconducting proximity effect, which has been utilized extensively in building functional quantum devices and inducing novel superconductivity. While an atomically sharp interface in real-space is known to be crucial for effective Cooper pair proximity transfer, the influence of electronic valleys in the momentum space has not been sufficiently investigated. Here, we report the observation of valley dependent superconducting proximity effect in a heterostructure with twisted overlapping. The heterostructure is realized by growth of multidomain Bi(111) films on a single-crystal NbSe2 substrate with molecular beam epitaxy. With spectroscopic imaging scanning tunneling spectroscopy, we identified different types of atomic overlapping in the Bi films, and measured drastic changes of proximity-induced superconducting gap sizes on the differently oriented Bi domains. Based on our theoretical model calculation, this phenomenon can be interpreted as valley dependent superconducting proximity coupling between the Bi film and NbSe2. We also investigated the lateral proximity effect between two adjacent Bi domains, which determines a significant reduction of the mean free path of electrons, associated with interfacial scattering. Our study expands the scope of tunable physical properties with the valley degree of freedom.