Ubiquitous defect-induced density wave instability in monolayer graphene

Quantum materials are notoriously sensitive to their environments. Seemingly small perturbations (lattice strain, dielectric environment, atomic disorder) can tip a system in favor of one amongst several competing ground states. For example, monolayer graphene has long been predicted to host a rich assortment of competing phases, including a structural bond density wave instability ("Kekulé distortion") that couples electrons at the K/K' valleys and breaks the translational symmetry of graphene. In this talk, I will describe our observations of a ubiquitous Kekulé density wave instability in multiple millimeter-scale graphene systems that can be controllably triggered by an extremely dilute concentration of surface atoms. Combining complementary momentum-sensitive angle-resolved photoemission spectroscopy (ARPES) and low energy electron diffraction (LEED) measurements, we show that the kinetic ordering of mobile surface adatoms produces global Kekulé density wave order that opens an energy gap in graphene's energy spectrum. We further find that this Kekulé phase occurs independent of Fermi surface size and shape, suggesting that this lattice instability may be driven by strong electron-lattice interactions. Our results suggest an unexpected sensitivity of the graphene lattice to dilute surface disorder and open the door to harnessing adsorbed atoms for tailoring the properties of two-dimensional materials.