Chirality—or ‘handedness’—is a symmetry property crucial to fields as diverse as biology, chemistry and high-energy physics. In graphene, chiral symmetry emerges naturally as a consequence of the carbon honeycomb lattice. This symmetry can be broken by interactions that couple electrons with opposite momenta in graphene. Here we directly visualize the formation of Kekulé bond order, one such phase of broken chiral symmetry, in an ultraflat graphene sheet grown epitaxially on a copper substrate. We show that its origin lies in the interactions between individual vacancies in the copper substrate that are mediated electronically by the graphene. We show that this interaction causes the bonds in graphene to distort, creating a phase with broken chiral symmetry. The Kekulé ordering is robust at ambient temperature and atmospheric conditions, indicating that intercalated atoms may be harnessed to drive graphene and other two-dimensional materials towards electronically desirable and exotic collective phases.
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We thank G. W. Flynn, C. Marianetti, I. L. Aleiner, B. L. Altshuler and C. J. Arguello for helpful discussions and L. Zhao for sharing Gr-Cu(111) bulk crystal data. This work is supported by the Office of Naval Research (ONR) (award number N00014-14-1-0501, C.G.) and by the Air Force Office of Scientific Research (AFOSR) (award number FA9550-11-1-0010, A.N.P.). Work at Cornell University is supported by the NSF through the Cornell Center for Materials Research (NSF DMR-1120296) (J.P.). Support for synthesis and characterization was provided by ONR (N00014-12-1-0791) (K.M.S.), AFOSR (FA9550-11-1-0033 and FA2386-13-1-4118) (J.P.) and the Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (2012M3A7B4049887) (J.P.). NEXAFS data was measured at beamline 8-2 at the Stanford Synchrotron Radiation Lightsource, a National User Facility operated by Stanford University on behalf of the US Department of Energy, Office of Basic Energy Sciences (T.S., D.N.) and supported (T.S.) by the NSF MRSEC Program through Columbia in the Center for Precision Assembly of Superstratic and Superatomic Solids (DMR-1420634).
The authors declare no competing financial interests.
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Gutiérrez, C., Kim, C., Brown, L. et al. Imaging chiral symmetry breaking from Kekulé bond order in graphene. Nature Phys 12, 950–958 (2016). https://doi.org/10.1038/nphys3776
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