Imaging Kekulé Spiral Order in Graphene

Breaking the intrinsic chiral symmetry of massless Dirac fermions in graphene at the two inequivalent valleys gives rise to rich intriguing phenomena. A prototypical example of this is the Kekulé order. Here, we use STM to directly visualize the bond texture associated with Kekulé spiral order in graphene. The Kekulé order originates from the intervalley scattering at 1D grain boundaries, and its strength can be significantly suppressed when a periodic potential is applied. By mapping atomic-scale electronic wave functions, we uncover that the bond texture localized near the 1D boundaries exhibits a complex spatial dependence indicative of phase winding, and it evolves dramatically with electronic energy. Our local-probe measurements thus establish the emergence of Kekulé spiral order in graphene, highlighting topological defects as versatile building blocks for engineering valley-ordered phases.