Nanoscale Polymorph Engineering of Metal-Correlated Insulator Junctions in Monolayer NbSe₂

Lateral junctions composed of quantum many-body materials are highly desirable for realizing physical phenomena and device concepts. However, controllable fabrication of high-quality junctions is challenging, which greatly hinders further exploration. Here, we successfully realize monolayer heterophase homojunctions of metallic H-NbSe₂ and correlated insulating T-NbSe₂ with atomically sharp boundaries via nanoscale polymorph engineering. By applying a scanning tunneling microscopy (STM) tip pulse, T-NbSe₂ can be locally introduced from H-NbSe₂ on the side beneath the tip, thus realizing H/T-NbSe₂ heterophase homojunctions. Our in situ STM measurements, complementary by the theoretical calculations, reveal two types of atomically sharp boundaries with distinct abilities for electron transmission, owing to the structure-dependent boundary coupling effects. Moreover, there are significant electronic interactions among the metallic, correlated insulating, and charge-density-wave states at the H/T-NbSe₂ boundaries. Our results provide insight into the interacting mechanism among diverse quantum many-body states.