Study of 1T-TaS2 sub-surface reconstruction via scanning tunneling microscopy and spectroscopy

1T-TaS2 is a well-known layered charge density wave-induced Mott insulator, in which electrons are localized at the centers of the Star-of-David clusters. Interlayer dimerization of these localized electrons leads to a band insulating state. In such layered systems, different stacking configurations can lead to distinct electronic structures, with corresponding signatures observable at the surface. Using scanning tunneling microscopy and spectroscopy (STM/S), we reveal the coexistence of Mott-insulating and band-insulating domains on a monatomic step edge in 1T-TaS2.  We find that variations in subsurface stacking configurations lead to a transition from Mott-insulating to band-insulating domains on the surface. STS measurements show distinct band-insulating domains characterized by variations in the band edge. Density functional theory calculations suggest these features arise from buried Mott-insulating layers, highlighting the energetic instability of Mott-insulating surfaces and their tendency to reside beneath the surface. Our insights address  questions about vertical charge order in 1T-TaS2 and open new avenues for exploring quantum phenomena in layered CDW materials.