ARPES and STM studies of monolayer 1<i>T</i>-NbSe₂

Recently, monolayer transition-metal dichalcoenides (TMDs) are attracting particular attention due to realization of exotic physical properties distinct from the bulk counterpart. One of the most intriguing features of TMDs is variety of physical properties depending on the crystal structure such as trigonal prismatic 2H or octahedral 1T phases. Among the TMDs, 2H-NbSe₂ has been intensively studied since it simultaneously exhibits charge density wave and superconductivity. In contrast, 1T-NbSe₂ has not been well studied due to its structural instability. Here we report ARPES and STM studies of monolayer NbSe₂ epitaxially grown on bilayer graphene. We have succeeded for the first time in selectively fabricating monolayer 2H- and 1T-NbSe₂, and revealed electronic structure by using ARPES. We found that monolayer 2H-NbSe₂ exhibits metallic behavior as in bulk, while monolayer 1T-NbSe₂ shows insulating behavior in contrast to the band theory which predicts the metallic nature with half-filled bands. In the presentation, we will show how to selectively fabricate 2H- and 1T-NbSe₂, and discuss the origin of unexpected insulating state in monolayer 1T-NbSe₂.