Abstract.– Two-dimensional materials offer a robust platform for investigating emergent behavior owing to the high tunability of their electronic properties. For instance, the ability to design electronic band structures through moiré superlattices in twisted graphene multilayers has led to the discovery of several symmetry-broken and topological phases. However, such twisted structures require exquisite care in their assembly and have micrometer dimensions that make spectroscopic measurements challenging.

In this talk, I will describe an alternative method to induce a symmetry-broken phase in graphene at the millimeter scale. I will show how an extremely dilute concentration (<0.3% coverage) of surface adatoms can self-assemble and trigger the collapse of the graphene atomic lattice into a distinct Kekulé bond density wave phase, whereby the carbon C-C bond symmetry is broken globally. By combining angle-resolved photoemission spectroscopy (ARPES) and low-energy electron diffraction (LEED) measurements, we directly probe the presence of this density wave phase and confirm the opening of an energy gap of ~200 meV at the Dirac point. We further show that this Kekulé density wave phase occurs for various Fermi surface sizes and shapes, suggesting that this lattice instability may be driven by electron-lattice interactions and not Fermi surface nesting. Our results demonstrate that superlattices of adsorbed atoms offer an attractive alternative route towards tailoring the properties of two-dimensional materials and may shed light on the nature and origin of the insulating correlated states in twisted bilayer graphene.