Surface structure and stacking of the commensurate ( 13 × 13 ) R 13 . 9 ∘ charge density wave phase of 1 T − TaS 2 ( 0001 )

Layered materials forming charge-density-wave (CDW) states exhibit an intriguing interplay between crystallographic structure and electronic properties. Using quantitative low-energy electron diffraction, the authors determine with picometer accuracy the complex surface structure of the transition-metal dichalcogenide 1$T$-TaS${}_{2}$ in its commensurate CDW phase. The key findings are the vertical CDW stacking between first and second trilayer and the broken inversion symmetry in the first S-Ta-S trilayer, due to an expansion of the topmost S layer.