Graphene-capped bismuthene on SiC as a platform for correlated quantum spin Hall edge states

Abstract

Epitaxial bismuthene on SiC(0001) hosts symmetry-protected metallic edge states within a large bulk band gap, establishing it as a promising two-dimensional topological insulator for hightemperature quantum spin Hall (QSH) transport. Here we realize bismuthene islands by intercalating Bi beneath zero-layer graphene on SiC(0001) followed by hydrogen treatment, yielding well-defined edges with controlled terminations. Spectroscopic measurements demonstrate that the edge states reside inside the bulk band gap and remain charge neutral. The graphene overlayer interacts only weakly with the bismuthene, preserving its topological character while providing environmental protection. Notably, the one-dimensional edge channels exhibit signatures of enhanced electronic correlations relative to freestanding bismuthene, suggesting proximity-induced modification of the QSH edge physics. These results establish graphene-capped bismuthene as a robust and tunable platform for correlated quantum spin Hall states.

Figures (5)

• Figure 1: Intercalation and hydrogenation of Bi below ZLG. (a) SPA-LEED pattern taken at E=167 eV of Bi-intercalated ZLG/SiC showing the graphene (black circle), Bi-$(\sqrt{3}\times\sqrt{3})R30^{\circ}$ (green circle), SiC (red circle), $(6\sqrt{3} \times 6\sqrt{3})R30^{\circ}$ (purple circle) and ($6\times6$ (blue circle) periodicity. (b) Line profiles along two crystallographic directions. All diffraction experiments were performed at T = 300 K. (c) Large-scale STM topographic image (+1 V, 450 pA, 4.5 K) showing intercalated bismuthene islands (EG/Bi/SiC) surrounded by H-intercalated ZLG area. Inset: STM image showing atomically resolved HQFMLG recorded at +1.8 V, and 300 pA, T = 4.5 K. The line scan across the bismuthene islands reveals an apparent height of 2.1Å. (d) Ball and stick model of bismuthene- and H-intercalated ZLG on SiC: top view (top) and side view (bottom).
• Figure 2: Bismuthene structure and edges. (a-b) STM topographic and FFT images of a bismuthene-intercalated ZLG island measured at two different bias voltages: $V_b$ = +0.4 V (a), and $V_b$ = +2 V (b). Inset: Zoomed-in STM images (left) of the regions outlined by the green rectangles, showing the honeycomb lattice of graphene and bismuthene. The corresponding FFT images are presented in Fig. \ref{['Figure1']}a)-b) (right). (c-d) STM topographic (c) image and current map (d) of an intercalated bismuthene island exhibiting the armchair termination of bismuthene (+2 V, 450 pA). Schematic illustration of the graphene (black) and Bi-honeycomb lattice (green) is superimposed on the STM image. (e) STM topography of a small region exhibiting two intercalated bismuthene islands (EG/Bi/SiC) (+3 V, 250 pA). Inset: Zoomed-in STM image of the armchair edge. The STM images were acquired at 4.5 K.
• Figure 3: Bulk electronic structure of bismuthene. (a) DFT calculations for the band structure of bismuthene/H-passivated SiC. The indirect band gap of bismuthene is denoted as $E_g$, while $\Delta_s$ represents the energy spacing between the $S_1$ and $S_2$ bands of the valence band (VB). (b) STM topography of intercalated bismuthene islands (EG/Bi/SiC) surrounded by H-intercalated ZLG region (EG/H/SiC) (+0.8 V, 450 pA). (c) Tunneling dI/dV spectra (STS) recorded at two different positions marked by the green and gray crosses as indicated in Fig. \ref{['Figure3']}a). Tunneling conditions are $V_b$ = +1 V, $I_t$ = 450 pA (green), and $V_b$ = +0.8 V, $I_t$ = 400 pA (gray). Two tangential lines (orange) were drawn to the green STS curve to determine the conduction band (CB) edge. The Dirac point is denoted as $V_D$. (d) Average dI/dV spectrum measured at the island interior (green cross) on the top of 20 consecutive dI/dV curves concatenated into a color map ($V_b$ = +0.8 V, $I_t$ = 450 pA). In the STS map, $S_1$ and $S_2$ represent the top and next energy levels of the VB, respectively. The black-dashed line represents the zero dI/dV intensity. The STM/STS results were acquired at 4.5 K.
• Figure 4: Size- and side-effects of bismuthene islands. (a) Tunneling dI/dV spectra of three intercalated bismuthene islands with different lateral sizes. The spectra were acquired at an interior position of each island. Island 1 ($22 \times 21\,\mathrm{nm}^2$): $V_b$ = 0.8 V, $I_t$ = 500 pA. Island 2 ($33 \times 32\,\mathrm{nm}^2$): $V_b$ = 0.8 V, $I_t$ = 450 pA. Island 3 ($55 \times 32\,\mathrm{nm}^2$): $V_b$ = 0.8 V, $I_t$ = 370 pA. The spectra are shifted for better visibility. (b) STM topography of an intercalated bismuthene island measured at +1.5 V and 400 pA. (c) Line scan of dI/dV spectra ($V_b$ = +0.8 V, $I_t$ = 450 pA) measured along the gray dashed line shown in Fig. \ref{['Figure4']}b. The black and blue circles represent the starting and ending positions. The Dirac point and the phonon-induced gap are denoted as $V_D$, and $\Delta_{\mathrm{ph}}$. (d) Representative dI/dV spectra were extracted from the positions marked by the colored circles, shown in the inserted image (cut off from Fig. \ref{['Figure4']}b). Two tangential lines (orange) were drawn to the green and red STS curves to determine the CB edge. The black dashed lines represent the zero dI/dV intensity. The STM/STS data were measured at 4.5 K.
• Figure 5: Spectroscopy of the edge states. (a) STM topography of an intercalated bismuthene island boundary (+2 V and 450 pA). (b) dI/dV spectra (STS) recorded at two different positions marked by the red and blue circles in Fig. \ref{['Figure5']}a. (c-d) STM topographic (left) and conductance (dI/dV) mapping images (right) of a small region outlined by the green dashed rectangle in Fig. \ref{['Figure5']}) measured at (c) +0.5 V, and (d) +1 V. The white and blue arrows indicate the island interior and EG/H/SiC, respectively. (e) STS recorded at various positions (crosses) along the armchair edge. Inset: STM topography (+2 V, 450 pA) shows the probed positions. (f) STS measured at a H-intercalated ZLG region compared to the STS recorded at an island interior and boundary (armchair edge). For better comparison, the spectra were shifted to zero conductance. The phonon-induced gap ($\Delta_{\mathrm{ph}}$) is outlined by a blue rectangle. Inset: Zoomed-in STS of the armchair edge (green) was fitted using the TLL model with $\alpha$ = 0.41 (gray), $\alpha$ = 0.6 (brown), and $\alpha$ = 0.85 (orange). The STS data (green data points) was shifted on the energy scale to zero. The pink dashed curve indicates the phonon-induced gap contributed by the graphene overlayer. The black dashed line represents the zero dI/dV intensity. All dI/dV spectra measured at $V_b$ = +0.8 V, $I_t$ = 450 pA, T = 4.5 K.