Reversible Switching of the Environment-Protected Quantum Spin Hall Insulator Bismuthene at the Graphene/SiC Interface

Abstract

Quantum Spin Hall Insulators (QSHI) have been extensively studied both theoretically and experimentally because they exhibit robust helical edge states driven by spin-orbit coupling and offer the potential for applications in spintronics through dissipationless spin transport. However, to realize devices, it is indispensable to gain control over the interaction of the active layer with the substrate, and to protect it from environmental influences. Here we show that a single layer of elemental Bi, formed by intercalation of an epitaxial graphene buffer layer on SiC(0001), is a promising candidate for a QSHI. This layer can be reversibly switched between an electronically inactive precursor state and a ``bismuthene state'', the latter exhibiting the predicted band structure of a true two-dimensional bismuthene layer. Switching is accomplished by hydrogenation (dehydrogenation) of the sample, i.e., a partial passivation (activation) of dangling bonds of the SiC substrate, causing a lateral shift of Bi atoms involving a change of the adsorption site. In the bismuthene state, the Bi honeycomb layer is a prospective QSHI, inherently protected by the graphene sheet above and the H-passivated substrate below. Thus, our results represent an important step towards protected QSHI systems beyond graphene.

Figures (3)

• Figure 1: Electronic and geometric structure of the bismuthene precursor phase:a LEED pattern with the reciprocal lattice vectors of graphene (G), SiC, and Bi highlighted ($E =$100eV). b Energy-momentum cuts along the $\Gamma-\text{K}_{\text{G}}$ and $\Gamma-\text{M}_{\text{G}}$ directions of graphene. The weakly dispersing low-energy state originating from the bismuthene precursor is marked by a black line derived from fitting the corresponding energy distribution curves. c Energy-momentum cut at K$_{\text{G}}$ in the direction perpendicular to the map shown in b. The linear dispersion of the -band is indicated in black, as obtained by fitting the maxima of the momentum distribution curves using a nearest-neighbour tight-binding approximation for graphene. d-f NIXSW imaging results for bulk-Si (d), bulk-C (e), and Bi (f). The Fourier-reconstructed electron densities in a plane spanned by the [1$\overline{\text{1}}$00] and [0001] crystallographic directions are shown, white maxima correspond to the positions of the atoms. A ball-and-stick model of the bulk structure is superimposed on d and e and fits to the experimental finding very well. In f, the clearest maxima are located at $\text{T}_\text{4}$ sites, indicating the adsorption sites of Bi. Orange dashed lines in d and f indicate the two possible SiC surface terminations S2 and S2* (A and C planes). g Structure of the graphene-protected bismuthene precursor sample, as derived from NIXSW imaging. The maxima from maps d, e and f are shown in red, gray and blue, indicating the positions of bulk-Si, bulk-C and Bi, respectively. Note that in order to mimic the ball-and-stick model from h, maxima from the panels d-f and from another adjacent parallel plane, offset by half a lattice vector, are shown for the bulk species. Graphene, since it is incommensurate with the bulk structure, is shown as a horizontal black bar at its corresponding adsorption height. h Top view and side view of the proposed atomic arrangement for the bismuthene precursor structure on SiC(0001) in a ($\sqrt{3} \times \sqrt{3}$)$R30^{\circ}$ superstructure. Bi is located on the $\text{T}_\text{4}$ hollow sites, while the $\text{H}_\text{3}$ hollow and $\text{T}_\text{1}$ on-top sites are unoccupied. The graphene layer has been omitted in the top view for clarity.
• Figure 2: Geometric and electronic structure of the 2D bismuthene layer on SiC(0001) (after hydrogenation):a NIXSW imaging result for Bi, shown for the plane spanned by the [1$\overline{\text{1}}$00] and [0001] crystallographic directions. White maxima correspond to the positions of the atoms. b Structure of the graphene-protected bismuthene sample, as derived from NIXSW imaging. Red, gray, and blue circles represent the maxima of the Fourier-reconstructed electron densities for bulk-Si, bulk-C and Bi, respectively. Graphene, since it is incommensurate with the bulk structure, is shown as a horizontal black bar at its corresponding adsorption height. c Top view and side view of a ball-and-stick model showing the determined arrangement of the bismuthene honeycomb on the SiC(0001) surface in a ($\sqrt{3} \times \sqrt{3}$)$R30^{\circ}$ superlattice. The graphene layer has been omitted in the top view for clarity. d Energy-momentum band maps along the $\Gamma-\text{K}_{\text{G}}$ and $\Gamma-\text{M}_{\text{G}}$ directions of graphene. The white dashed lines represent the valence band structure of bismuthene as calculated by DFT. e Energy-momentum map around K$_{\text{G}}$ of graphene in the direction perpendicular to d. Two energetically shifted Dirac cones of graphene are observed, due to the coexistence of graphene areas on bismuthene ($\text{G}_{\text{Bi}}$) and hydrogen intercalated graphene areas ($\text{G}_{\text{H}}$). Black lines were obtained by fitting the maxima of momentum distribution curves with the nearest-neighbor tight-binding band structure of graphene. f Close-up in the vicinity of K$_{\sqrt{3}}$ as marked in d by a white dashed rectangle. The inset shows the Fermi contour of the bismuthene in a $k_x$-$k_y$ map. Brillouin zone boundaries are drawn as white lines. The left part of the contour, indicated by a black dashed line, corresponds to the bismuthene contribution, in the right part it is superimposed by the graphene replica cones (marked "r"). g, h$E$-$k_y$ energy-momentum maps in the direction perpendicular to the map shown in f, at the position indicated by a black vertical dashed line in f and at the K$_{\sqrt{3}}$ point of bismuthene, respectively.
• Figure 3: Energy-momentum band maps of bismuthene at different doping levels. Black dashed lines indicate the DFT calculated dispersion of the Bi Dirac cone. A rigid band shift was applied to the calculated bands to match the experimental results. Different doping levels were obtained by the use of different substrate polytypes 4H-SiC and 6H-SiC, which imply different spontaneous polarisation strengths, and by additional adsorption of the alkali metal Cs on top of graphene, inducing a charge transfer into the bismuthene layer. a 4H-SiC substrate, no Cs. b 6H-SiC substrate, no Cs. c 4H-SiC substrate, with Cs. d 6H-SiC substrate, with Cs. In d the valence band maximum is positioned significantly below the Fermi level, indicating the presence of a band gap.