Spectroscopic evidence for surface reconstruction in the layered charge density wave material 1T-TaS₂

Vertical charge order plays a crucial role in shaping the electronic properties of layered charge density wave (CDW) materials. Utilising scanning tunnelling microscopy and spectroscopy (STM/STS), we observe the coexistence of Mott-insulating and band-insulating domains on a monatomic step edge of a prototypical CDW Mott insulator, 1T-TaS₂. Our findings reveal that surface reconstruction, driven by changes in subsurface stacking configurations, leads to a transition from Mott-insulating to band-insulating domains. On flat surfaces, STS measurements on a flat surface show distinct band-insulating domains characterised by variations in the band edge. Density functional theory (DFT) 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. Notably, these buried layers may explain the quantum spin signatures observed in this material. Our insights address long-standing questions about vertical charge order in 1T-TaS₂ and open new avenues for exploring quantum phenomena in layered CDW materials.