Observation of Kekul\'e vortices induced in graphene by hydrogen adatoms

Abstract

Fractional charges are one of the wonders of the fractional quantum Hall effect, a liquid of strongly correlated electrons in a large magnetic field. Fractional excitations are also anticipated in two-dimensional crystals of non-interacting electrons under time-reversal symmetry, as bound states of a rotating bond order known as Kekul\'e vortex. However, the physical mechanisms inducing such topological defects remain elusive, preventing experimental realisations. Here, we report the observation of Kekul\'e vortices in the local density of states of graphene under time-reversal symmetry. The vortices result from intervalley scattering on chemisorbed hydrogen adatoms and have a purely electronic origin. Their 2{\pi} winding is reminiscent of the Berry phase {\pi} of the massless Dirac electrons. Remarkably, we observe that point scatterers with different symmetries such as divacancies can also induce a Kekul\'e bond order without vortex. Therefore, our local-probe study further confirms point defects as versatile building blocks for the control of graphene's electronic structure by kekul\'e order.