Atomically Thin Two-Dimensional Kagome Flat Band on the Silicon Surface

Kagome lattice and its non-trivial flat band have spurred the interest of many due to its potential in hosting unconventional magnetic and transport phenomena. This localized state of electrons emerges from the unique destructive inference mechanism which is allowed from the frustrated geometry of the lattice. However, the list of flat band kagome materials has been somewhat limited to a handful of series of compounds such as binary or ternary metal magnets and van der Waals Kagome materials.

Here we present a new type of kagome system in a prototypical metal-semiconductor surface state; one monolayer Ag/Si(111)√3×√3. The Ag atoms form a distorted breathing kagome lattice on the Si. Such Si surface system has not been a conventional approach to the realization of kagome flat bands. Our work suggests that surface-reconstructed systems on semiconductors hold potential as a new platform in exploring flat bands and designing ideal 2D kagome systems.

By ARPES measurement, we find that there exists a flat surface band across the Brillouin zone. We assert that the flat band is indeed a kagome flat band caused by a destructive interference resulting from a delicate balance between the hopping parameters of the in-plane d-orbitals of the Ag atoms. Also, the 'double' flat band feature and Dirac bands in the hybridized d-orbital kagome band are uncovered. Lastly, we assign a new type of compact localized state corresponding to the flat bands and rigorously unravel the exact mechanism of the quantum destructive interference.