Fragile topological flat bands through adatom superlattice engineering on graphene

Magic-angle twisted bilayer graphene has received a lot of interest due to its flat bands with potentially non-trivial topology that lead to intricate correlated phases. However, control over the fabrication and thus system parameters of such devices is limited.
Here, we propose a single graphene sheet with adatoms periodically placed on top as an alternative system that realizes flat bands.
Performing first principle calculations, we obtain realistic spectra for feasible transition-metal adatoms. Further group-theoretical analysis reveals the fragile nature of topology in some of the flat bands.
We study the bulk-boundary correspondence associated with the fragile topology of the flat bands by building a minimal tight-binding and numerically examine the corner-localized in-gap states, which are a consequence of the filling anomaly resulting from the nontrivial topology.
The high control over system parameters makes this system particularly interesting for experimental investigations.