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Designer Dirac fermions and topological phases in molecular graphene

Abstract

The observation of massless Dirac fermions in monolayer graphene has generated a new area of science and technology seeking to harness charge carriers that behave relativistically within solid-state materials1. Both massless and massive Dirac fermions have been studied and proposed in a growing class of Dirac materials that includes bilayer graphene, surface states of topological insulators and iron-based high-temperature superconductors. Because the accessibility of this physics is predicated on the synthesis of new materials, the quest for Dirac quasi-particles has expanded to artificial systems such as lattices comprising ultracold atoms2,3,4. Here we report the emergence of Dirac fermions in a fully tunable condensed-matter system—molecular graphene—assembled by atomic manipulation of carbon monoxide molecules over a conventional two-dimensional electron system at a copper surface5. Using low-temperature scanning tunnelling microscopy and spectroscopy, we embed the symmetries underlying the two-dimensional Dirac equation into electron lattices, and then visualize and shape the resulting ground states. These experiments show the existence within the system of linearly dispersing, massless quasi-particles accompanied by a density of states characteristic of graphene. We then tune the quantum tunnelling between lattice sites locally to adjust the phase accrual of propagating electrons. Spatial texturing of lattice distortions produces atomically sharp p–n and p–n–p junction devices with two-dimensional control of Dirac fermion density and the power to endow Dirac particles with mass6,7,8. Moreover, we apply scalar and vector potentials locally and globally to engender topologically distinct ground states and, ultimately, embedded gauge fields9,10,11,12, wherein Dirac electrons react to ‘pseudo’ electric and magnetic fields present in their reference frame but absent from the laboratory frame. We demonstrate that Landau levels created by these gauge fields can be taken to the relativistic magnetic quantum limit, which has so far been inaccessible in natural graphene. Molecular graphene provides a versatile means of synthesizing exotic topological electronic phases in condensed matter using tailored nanostructures.

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Figure 1: Dirac fermions in molecular graphene.
Figure 2: Dirac point engineering in a p–n–p junction.
Figure 3: Charge- and bond-density waves in molecular graphene.
Figure 4: Landau quantization and topological zero modes in a tunable pseudomagnetic field.

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Acknowledgements

This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under contract DE-AC02-76SF00515. F.G. acknowledges financial support from MICINN (Spain) through grants FIS2008-00124 and CONSOLIDER CSD2007-00010, and calculations supported by the US National Science Foundation. We thank C.-H. Park, I. Martin, A. Balatsky, T. Wehling, A. Akhmerov, E. Heller and A. Fetter for discussions.

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Authors

Contributions

K.K.G., W.M. and W.K. designed and performed experiments, analysed data and wrote the manuscript. F.G. provided the theoretical analysis. H.C.M. directed the project and wrote the manuscript.

Corresponding author

Correspondence to Hari C. Manoharan.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Text and Data, Supplementary Legend for Supplementary Movie 1, Supplementary References and Supplementary Figures 1-4. Supplementary Figure 1 shows a Graphical summary of this work; Supplementary Figure 2 shows the measurement of the surface-state density of states; Supplementary Figure 3 shows the artificial triangular lattice and Supplementary Figure 4 shows the uniaxial strain of graphene lattice. (PDF 3256 kb)

Supplementary Movie

This movie shows the nanoscale assembly sequence of an electronic honeycomb lattice by manipulating individual CO molecules on the Cu(111) two-dimensional electron surface state with the STM tip. (MOV 4353 kb)

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Gomes, K., Mar, W., Ko, W. et al. Designer Dirac fermions and topological phases in molecular graphene. Nature 483, 306–310 (2012). https://doi.org/10.1038/nature10941

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