Triggering a global density wave instability in graphene via local symmetry-breaking

TL;DR

This paper demonstrates that a very dilute concentration of surface adatoms can induce a global Kekule9 bond density wave phase in graphene, leading to a lattice symmetry breaking and energy gap opening, revealing a new method to engineer quantum phases.

Contribution

It introduces a novel approach using dilute self-assembled adatoms to trigger a global symmetry-broken phase in graphene, distinct from moire9 superlattice methods.

Findings

Dilute adatoms (<0.3%) induce Kekule9 density wave in graphene.

The phase is confirmed by ARPES and LEED measurements.

Energy gap opens at the Dirac point due to lattice instability.

Abstract

Two-dimensional quantum materials offer a robust platform for investigating the emergence of symmetry-broken ordered phases owing to the high tuneability of their electronic properties. For instance, the ability to create new electronic band structures in graphene through moir\'e superlattices from stacked and twisted structures has led to the discovery of several correlated and topological phases. Here we report an alternative method to induce an incipient symmetry-broken phase in graphene at the millimetre scale. We show that an extremely dilute concentration ($<\!0.3\% $) of surface adatoms can self-assemble and trigger the collapse of the graphene atomic lattice into a distinct Kekul\'e bond density wave phase, whereby the carbon C-C bond symmetry is broken globally. Using complementary momentum-resolved techniques such as angle-resolved photoemission spectroscopy (ARPES) and low-energy electron diffraction (LEED), we directly probe the presence of this density wave phase and confirm the opening of an energy gap at the Dirac point. We further show that this Kekul\'e density wave phase occurs for various Fermi surface sizes and shapes, suggesting that this lattice instability is driven by strong electron-lattice interactions. Our results demonstrate that dilute concentrations of self-assembled adsorbed atoms offer an attractive alternative route towards designing novel quantum phases in two-dimensional materials.