Nonreciprocal charge-density-wave proximity effect in a lateral heterojunction of NbSe2/TiSe2

Lateral heterostructures between two-dimensional (2D) transition metal dichalcogenides are highly interesting since they have potential applications for constructing monolayer electronic devices. Such in-plane heterostructures are also ideal systems for exploring the interfacial effects on quantum phases, such as charge-density wave (CDW) order in the true 2D limit. Here, we report on a lateral CDW heterojunction made of monolayer NbSe2 and TiSe2 with atomically sharp interface. Scanning tunneling microscopy reveals an unexpectedly nonreciprocal CDW proximity effect at the NbSe2 vicinity of the heterojunctions: the (2 × 2) CDW order of TiSe2 is found to propagate into the NbSe2 side of the heterostructure, while the (3 × 3) CDW order of NbSe2 resides in the NbSe2 side. Such a nonreciprocal CDW proximity effect indicates that the quantum phase of electrons in NbSe2 can be easily tuned by the adjacent TiSe2. Our study highlights the significance of quantum interfacial effect in lateral heterojunctions, which may help for constructing tunable 2D in-plane quantum devices based on proximity effects or in-plane interfaces.