Observation of Kekulé vortices induced in graphene by hydrogen adatoms

Fractional charges are one of the wonders of the fractional quantum Hall effect. 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 Kekulé vortex. However, the physical mechanisms  inducing such topological defects remain elusive, preventing experimental realisations.
Here, we report the observation of Kekulé 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 2pi 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 Kekulé 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 Kekulé order.

DOI : https://doi.org/10.48550/arXiv.2307.10024

Authors : Y. Guan, C. Dutreix, H. Gonzales-Herrero, M. M. Ugeda, I. Brihuega, M. I. Katsnelson, O. V. Yazyev, V. T. Renard