Mottness collapse in 1T-TaS_2-xSe_x transition-metal dichalcogenide: An interplay between localized and itinerant orbitals

The layered transition-metal dichalcogenide 1T-TaS₂ has been recently found to undergo a Mott-insulator-to-superconductor transition induced by high pressure, charge doping, or isovalent substitution. By combining scanning tunneling microscopy measurements and first-principles calculations, we investigate the atomic scale electronic structure of the 1T-TaS₂ Mott insulator and its evolution to the metallic state upon isovalent substitution of S with Se. We identify two distinct types of orbital textures-one localized and the other extended-and demonstrate that the interplay between them is the key factor that determines the electronic structure. In particular, we show that the continuous evolution of the charge gap visualized by scanning tunneling microscopy is due to the immersion of the localized-orbital-induced Hubbard bands into the extended-orbital-spanned Fermi sea, featuring a unique evolution from a Mott gap to a charge-transfer gap. This new mechanism of Mottness collapse revealed here suggests an interesting route for creating novel electronic states and designing future electronic devices.