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Bismuthene on a SiC substrate: A candidate for a high-temperature quantum spin Hall material

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Science  21 Jul 2017:
Vol. 357, Issue 6348, pp. 287-290
DOI: 10.1126/science.aai8142
  • Fig. 1 Bismuthene on SiC(0001) structural model.

    (A) Sketch of a bismuthene layer placed on the threefold-symmetric SiC(0001) substrate in Embedded Imagecommensurate registry. (B) Topographic STM overview map showing that bismuthene fully covers the substrate. The flakes are of ~25-nm extent, limited by domain boundaries. (C) Substrate step-height profile, taken along the red line in (B). The step heights correspond to SiC steps. (D) The honeycomb pattern is seen on smaller scan frames. (E) Close-up STM images for occupied and empty states (left and right panels, respectively). They confirm the formation of Bi honeycombs.

  • Fig. 2 Theoretical band structure and ARPES measurements.

    (A) DFT band-structure calculation (with a hybrid exchange-correlation functional) including SOC, showing the wide band gap and a substantial band splitting in the valence band (dashed line). For position of the critical points Γ, K, and M, see the surface Brillouin zone (SBZ) in (D). (B) ARPES band dispersion through the Brillouin zone. The band maximum at K and the valence-band splitting are in close agreement with the theoretical prediction (overlay). The zero of energy (EF) is aligned to the Fermi level of the spectrometer. (C) Close-up of ARPES showing a valence-band maximum at the K point with large SOC-induced splitting in a wide momentum range. The sketch in the top-right corner depicts the orientation of the cut (black lines) in the momentum space of the SBZ (blue hexagon). (D) Constant-energy surfaces from ARPES at various binding energies. The cut at low binding energies is taken at the topmost intensity corresponding to the valence-band maximum. The maps are consistent with the sixfold degeneracy of the K and K′ points of the hexagonal lattice.

  • Fig. 3 Calculated electronic structure of the low-energy effective model of Bi σ bands.

    (A) The contribution of Bi s and p orbitals to the electronic structure of bismuthene (without SOC). In each panel, the symbol size is proportional to the relative weight of the orbital. In Bi/SiC, px and py orbitals prevail around EF, demonstrating orbital decomposition. (B) Electronic structure of the low-energy effective model without SOC. (C) Inclusion of the strong atomic SOC opens a huge gap at the K point. (D) Further including the Rashba term lifts the degeneracy of the topmost valence band and induces a large splitting with opposite spin character there.

  • Fig. 4 Tunneling spectroscopy of edge states at substrate steps.

    (A) Differential conductivity dI/dV (reflecting the LDOS) at different distances to the edge. A large gap of ~0.8 eV is observed in bulk bismuthene (black curve). Upon approaching the edge, additional signal of increasing strength emerges that fills the entire gap. Inset: STS measurement locations (color-coded dots relate to spectrum color) at uphill substrate step causing the boundary. (B) Spatially resolved dI/dV data across the same step. The dI/dV signal of the in-gap states peaks at both film edges (gray dashed lines mark dI/dV maxima). (C) Topographic z(x) line profile of the step and dI/dV signal of bismuthene (integrated over the gap from +0.15 to +0.55 eV), showing an exponential decrease away from the step edge, on either side.

Supplementary Materials

  • Bismuthene on a SiC substrate: A candidate for a high-temperature quantum spin Hall material

    F. Reis, G. Li, L. Dudy, M. Bauernfeind, S. Glass, W. Hanke, R. Thomale, J. Schäfer, R. Claessen

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