Charge density wave order in monolayer metal dichalcogenides studied with scanning tunneling micriscopy

Monolayer metal dichalcogenides, including TiSe₂, NbSe₂, SnSe₂, as well as lateral heterojunctions of monolayer TiSe₂/NbSe₂, are synthesized with molecular beam epitaxy and characterized with low-temperature scanning tunneling microscopy and spectroscopy. These materials are particularly interesting due to the emergence of complex electronic orders at low temperatures, e.g. charge density wave (CDW) and superconductivity, even at the 2D limit. Here we report that seamless high-quality lateral heterojunctions can be grown between the 2D monolayer NbSe₂ and TiSe₂ by molecular beam epitaxy. Scanning tunneling microscopy reveals that the monolayer NbSe₂ and TiSe₂ in the heterostructure remains the (3×3×3) and (2×2×2) charge-density-wave orders of their bulk counterparts respectively at low temperature, leading to a first example of lateral CDW heterojunctions. More strikingly, we observe an unexpectedly unidirectional CDW proximity effect at the NbSe₂ vicinity of the heterojunctions, implying a complicate interplay between the two CDW orders.