Moiré superlattices of antimonene on a Bi(111) substrate with van Hove singularity and Rashba-type spin polarization

doi:10.21203/rs.3.rs-4233126/v1

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Moiré superlattices of antimonene on a Bi(111) substrate with van Hove singularity and Rashba-type spin polarization

https://doi.org/10.21203/rs.3.rs-4233126/v1

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published 26 Aug, 2024

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Moiré superlattices consisting of two-dimensional (2D) materials have attracted immense attention because of emergent phenomena such as flat band-induced Mott insulating states and unconventional superconductivity. However, the effects of spin-orbit coupling (SOC) on these materials have not yet been fully explored. Here,we show that single- and double-bilayer (BL) Sb honeycomb lattices, referred to as antimonene, form moiré superlattices on a Bi(111) substrate due to lattice mismatch. Scanning tunnelling microscopy (STM) measurements reveal the presence of spectral peaks near the Fermi level, which are spatially modulated with the moiré period. Angle-resolved photoemission spectroscopy (ARPES) combined with density functional theory (DFT) calculations clarify the surface band structure with saddle points near the Fermi level, which allows us to attribute the observed STM spectral peaks to the van Hove singularity. Moreover, spin-resolved ARPES measurements reveal that the observed surface states are Rashba-type spin-polarized. The present work has significant implications in that Fermi surface instability and symmetry breaking may emerge at low temperatures, where the spin degree of freedom and electron correlation also play important roles.

Physical sciences/Physics/Condensed-matter physics/Surfaces, interfaces and thin films

Physical sciences/Materials science/Nanoscale materials/Two-dimensional materials

The advent of artificially stacked two-dimensional (2D) materials with moiré superlattices, which are induced by a lattice mismatch and/or a twist angle, has led to the development of new paradigms in condensed matter physics and materials science ^1-3. The successful fabrication of graphene/hexagonal boron nitride heterostructures and, more recently, twisted double layers of graphene have spawned a series of exciting discoveries, e.g., Hofstadter’s butterfly and fractal quantum Hall effect ^4-6, correlated insulating states and unconventional superconductivity ^7-9, charge order ¹⁰, ferromagnetism and the quantum anomalous Hall effect ^11-14. The introduction of spin-orbit coupling (SOC) into moiré superlattices can lead to even richer emergent phenomena such as topological superconductivity, but these studies have been limited to transition metal dichalcogenides thus far ^14-18. The application of elemental 2D materials with strong SOCs may greatly expand the potential of moiré superlattices, but the exploration of such a possibility has been scarce thus far ¹⁹.

Bi and Sb are heavy VA elements with strong atomistic SOC and hence important ingredients in topological materials. The most stable form of bulk crystals is the A7 rhombohedral crystal structure, which consists of covalently bonded buckled honeycomb 2D layers (conventionally called “bilayers”) stacked by weak interlayer bonding ^20-22. This feature makes Bi and Sb atomic layers promising 2D materials beyond graphene ^23-25. They were theoretically predicted to become 2D topological insulators (quantum spin Hall insulators), and their topological edge states were found experimentally ^26-32. They are also reported to be resilient against air exposure and chemical processes and thus can potentially be used for various practical applications ^23,24,33.

Here, we show by scanning tunneling microscopy (STM) that single- and double-bilayer (BL) Sb honeycomb lattices, referred to as antimonene ^23,25,34-38, form moiré superlattices on a Bi(111) substrate due to lattice mismatch. They exhibit clear spectral peaks located near the Fermi level, which show distinctive behaviors regarding the moiré periodicity. While the peaks found for single-BL antimonene show only weak modulations in height, those for double-BL antimonene are split by ~100 mV and exhibit strong spatial modulations, suggesting localization due to the moiré superlattice. Angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES (SARPES) combined with density functional theory (DFT) calculations clarify the presence of saddle points near the Fermi level. This allows us to identify the origin of the STM spectral peaks as the van Hove singularity. These surface states are found to be Rashba-type spin polarized. The present work has significant implications for demonstrating that Fermi surface instability and symmetry breaking may emerge at low temperatures, where the spin degree of freedom and electron correlation are intimately involved.

All the experiments were carried out in ultrahigh vacuum (UHV) chambers with a base pressure of ~ 1×10^− 10 mbar (see Methods). First, a Bi(111) thin film was grown on a clean Si(111) surface to 10 BLs by molecular beam epitaxy (MBE) ^39,40. For some of the ARPES experiments, a 250 BL-thick Bi(111) film was grown on a clean Ge(111) surface ^41,42. Here, we adopt the rhombohedral crystallographic notation to describe the plane index of the film ^20,21. The crystallinity of the prepared samples was confirmed with scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). The surface of the Bi(111) film was then covered by 1–2 BL Sb (for LEED patterns, see Supplementary Information A). Since Bi(111) and Sb(111) bilayers share the same buckled honeycomb structure with similar lattice constants of 0.454 nm and 0.431 nm, respectively, a Sb(111) film could grow epitaxially on a Bi(111) film ^43,44. However, the lattice constant of the free-standing Sb bilayer (1BL antimonene), which is predicted to be 0.408–0.412 nm ^45,46, is significantly smaller than that of bulk Bi(111). This allows antimonene to grow nonepitaxially on Bi(111) and to form a moiré superlattice, which we indeed observe as follows.

The main panel of Fig. 1a shows a representative STM image of a Bi(111) surface covered with more than 1 BL of Sb. The lower terrace in the image (Region I) features a triangular lattice structure, which is a moiré superlattice made of 1BL antimonene (1BL Sb) on a Bi(111) surface. Although this superlattice includes defects and local deformations, the presence of a well-defined periodicity is clear from its fast Fourier transform (FFT) image (inset of Fig. 1a). From repeated experiments with different surface regions and samples, we determined the moiré lattice constant to be 4.70 ± 0.30 nm (Supplementary Information B). On the upper terrace, there exists another region of the moiré superlattice with a longer periodicity (Region II). Since the height difference of ~ 0.4 nm between Regions I and II (Fig. 1b) is approximately equal to the height of the Sb bilayer (0.374 nm for bulk) ²⁰, Region II is identified as 2BL antimonene (2BL Sb) on a Bi(111) surface. Its moiré lattice constant was determined to be 6.59 ± 0.89 nm. The relatively large uncertainty is due to variations throughout different surface regions, presumably reflecting very small differences in energy. This 2BL Sb layer is bordered by another 1BL Sb layer (Region III), which is located in the upper-right corner of the image. Since they have almost the same topographic heights ²⁰, the boundary (indicated by the dashed line) is identified as the location of a buried atomic step of the Bi(111) surface. Our repeated experiments indicate that antimonene layers grow from the step edges of Bi(111) surfaces (Supplementary Information C).

Magnified STM images of 1BL and 2BL Sb are displayed in Fig. 1c and Fig. 1d, respectively, where the Sb atomic lattices are clearly resolved. The moiré unit cells are indicated by the dashed parallelograms. We determined the lattice constant of 1BL Sb to be 0.415 ± 0.004 nm (Supplementary Information B). The fact that this value is greater than that of free antimonene (0.408–0.412 nm) ^45,46 is attributed to the tensile strain exerted from the Bi(111) surface. Likewise, the lattice constant of 2BL Sb was determined to be 0.423 ± 0.005 nm. This value is closer to that of bulk Sb(111) (0.431 nm) than that of 1BL Sb, suggesting lattice relaxation toward the bulk crystal. Combined with the moiré lattice constant determined above, the ratio of the numbers of Bi and Sb atoms can be calculated. We find (N_Bi:N_Sb) = (10:11) for 1BL Sb and (N_Bi:N_Sb) = (13:14) − (17:18) for 2BL Sb, where N_Bi and N_Sb are the number of Bi and Sb atoms within the unit cells along the principal axis, respectively. Our FFT analysis of STM images over an extended area reveals that there is no twisting between the moiré and Sb lattices on average (Supplementary Information B), although there are some local deviations due to deformations. Furthermore, Fig. 1c, d shows that the surface is divided into three characteristic regions in terms of topographic height. By comparing these observations to previous reports on related moiré superstructures ^47–51, we can safely assign them to the regions of the AA, AB, and AC stacking sequences (Fig. 1e). In the AA stacking, all atoms in the two layers are vertically overlapped, while only half of them are in the AB and AC stacking layers. Because of the significant buckling of the honeycomb lattice, the vertical distance between the overlapping atoms in the AB stacking is greater than that in the AA stacking. This leads to the lowering of the top layer by an attractive force. Conversely, in the AC stacking, the vertical distance between the overlapped atoms is smaller than that in the AA stacking. This leads to the raising of the top layer by a repulsive force. As a result of structural relaxation, the areas corresponding to the AB and AC stacking regions expand and shrink, respectively ^47,48. These features are clearly observed in Fig. 1c, d.

The electronic states of the moiré superlattices and their spatial modulations were investigated by scanning tunnelling spectroscopy (STS). First, for 1BL Sb on Bi(111), dI/dV spectra were taken at the center of the AA stacking region at five locations, and this process was repeated for the AB and AC stackings. The selected spectral sites are shown in the topographic STM image in Fig. 2a with red (AA), blue (AB), and green (AC) squares. Figure 2b shows the results of the STS measurements. The broken lines show individual dI/dV spectra, and the solid lines show the average values for the same stacking sequences, with their colors corresponding to those in Fig. 2a. The average of all the measured spectra are also shown by the solid black line. For all of these spectra, clear peak structures are noticeable near the zero bias voltage, with the full width at half maximum of 80 − 100 mV. The spectral peaks at the AB sites are particularly conspicuous, while those at the AA and AC sites are relatively suppressed. More detailed information was obtained through line spectroscopy; dI/dV spectra were taken along a straight line connecting the centers of the AC, AB, AA, and AC sites in this sequence. The right panel of Fig. 2c shows a 2D plot of color-coded dI/dV spectra as a function of bias voltage and lateral distance from the starting point. In the left panel of Fig. 2c, the topographic profile along the line is also shown. We find that the spectral peak is fixed at approximately 0 − 30 mV, while its intensity varies. This result strongly suggests the presence of delocalized states around the Fermi level that are weakly modulated by the moiré superlattice.

Figure 2d-f displays site-dependent dI/dV spectra for 2BL Sb on Bi(111) obtained in the same manner. The symbols and colors used in the figure follow the conventions of Fig. 2a-c. Figure 2e shows that spectral peaks are shifted from zero bias by 48 mV (AA site), -48 mV (AB site), and 144 mV (AC site) on average. The right panel of Fig. 2f shows that clear peak structures at approximately 40 mV and − 100 mV are confined within the AA and AB regions, respectively. This result indicates the presence of multiple states near the Fermi level that are localized due to the moiré superlattice ⁵².

To clarify the origin of the spectral peaks observed by STS, we performed laser-based high-resolution ARPES/SARPES measurements ⁵³. For simplicity, the data were analyzed based on the Brillouin zone of Bi(111) (Fig. 3a). First, we focused on the results for 1BL of Sb on a Bi(111) surface. Figure 3b shows a 2D plot of the ARPES intensity along the Γ-M direction and as a binding energy E_B (dark: high, bright: low). We can recognize two bands, denoted S₁ and S_2, starting from an E_B ≅ 0.2 eV and dispersing upward. These bands disappear at approximately k_x = 0.05–0.1 Å⁻¹ by crossing the Fermi level but seem to disperse downward and reappear at approximately k_x = 0.4–0.5 Å⁻¹. The band dispersions determined from the plot are highlighted with red dashed curves. The signals are better visualized by the SARPES signal plotted for the range of -0.22 Å⁻¹ < k_x < + 0.22 Å⁻¹, where the intensity and the spin polarization in the y direction are indicated by brightness (dark: high, bright: low) and color (red: positive, blue: negative), respectively. The maximum spin polarization of the photoelectron is ~ 0.6. The spin polarizations of the S₁ and S₂ bands are opposite to each other and are antisymmetric with respect to k_x = 0 Å⁻¹. This is characteristic of Rashba-type spin-polarization, which will be discussed later. Figure 3c shows a similar 2D plot of the ARPES intensity along the \(\stackrel{-}{{\Gamma }}-\stackrel{-}{\text{K}}\) (k_y) direction. The S₁ and S₂ bands are also noticeable (highlighted with red dashed lines), but the S₂ band reaches a local maximum near the Fermi level at approximately k_y = 0.1 Å⁻¹ and then disperses downward.

Figure 3e shows the 2D plot of the ARPES intensity measured near the Femi level (E_B = 0.02 eV) in the k_x - k_y space, which gives the Fermi surface contour. The central ring and a surrounding star-like structure are clearly noticeable and can be identified as the S₁ and S₂ bands, respectively. Notably, some parts of the S₂ band appear very weak due to the anisotropic transfer of matrix elements during the photoemission process. By referring to the band dispersions in Fig. 3b-d, we can identify the areas indicated by the red ellipsoids as saddle points, where the S₂ band takes a local maximum in the \(\stackrel{-}{{\Gamma }}-\stackrel{-}{\text{K}}\) direction and a local minimum in the orthogonal direction. This means that the van Hove singularity exists at the Fermi level ⁵⁴ and explains the origin of the zero bias peak for 1BL Sb/Bi(111) described above. To confirm this result, we also performed Fermi surface mapping with an imaging-type ARPES instrument, which allows us to access a larger momentum space at a faster speed (Fig. 3f) ⁴². The acquired Femi surface well reproduces the features observed in Fig. 3e while better reflecting the sixfold symmetry expected from the C₃ and time-reversal symmetries of the present system. We note that the band structure and the Fermi surface resemble those of Bi(111) and Sb(111) surfaces ^{21,40,41,55,56}, while saddle points are absent near the Fermi level in the latter cases.

The distribution of spin polarization in momentum space was investigated with the same imaging-type instrument for 1BL Sb/Bi(111). Figure 3g shows a 2D plot of the SARPES intensity and spin polarization in the y direction (P_y) measured near the Fermi level (E_B = 0.03 eV). The observed signal is mostly attributed to the S₂ band, the location of which is reproduced from Fig. 3f (red solid lines). Along the k_x axis (white dashed line), the distribution of the P_y signal is antisymmetric with respect to k_x = 0. This is consistent with a Rashba-type spin polarization, as mentioned above. We should note that the actual distribution of spin polarization deviates from the ideal vortical form, as indicated by the reversal of P_y with respect to the lines at ±60° to the k_x axis (black dashed lines). An analogous behavior was also predicted and observed for a clean Bi(111) surface with a giant Rashba splitting ^42,57,58.

We also performed ARPES/SARPES measurements of 2BL Sb on a Bi(111) surface (Supplementary Information D). These results are nearly identical to those for 1BL Sb/Bi(111) (Fig. 3), but the observed ARPES signals are clearer than those for 1BL Sb/Bi(111). This difference may be attributed to the better moiré periodicity observed with STM (Fig. 1a).

Ab initio calculations of the electronic structure of Sb/Bi(111) moiré superlattices are difficult because of the large number of heavy atoms involved within a moiré unit cell. To circumvent this problem, we carried out DFT calculations based on an epitaxial model consisting of 1BL Sb(111) on 5BL Bi(111) (Methods). Although this model does not include the effect of moiré periodicity, it can account for the overall band structures within the Bi(111) Brillouin zone. The structural relaxation within each stacking region in the actual moiré structure (Fig. 1c,d) rationalizes this treatment. Figure 4a shows the band diagrams calculated for the AB stacking sequence. The orange and blue rectangles correspond to the same marked areas in Fig. 3b,c. The sizes of the purple (light blue) circles represent the contributions of the top Sb (Bi) BL. Overall, the two bands starting from the \(\stackrel{-}{{\Gamma }}\) point below the Fermi level (designated by the red dashed lines) are mostly derived from the top Sb BL, indicating that they can be preferentially detected in surface-sensitive STM and ARPES measurements. Judging from their dispersions, they can be assigned to the S₁ and S₂ bands identified above (Fig. 3b-d). The same calculations for the AA and AC stackings give very similar band structures near the \(\stackrel{-}{{\Gamma }}\) point and around the Fermi level (Supplementary Information E, Fig. E1). Therefore, the presence of saddle points near the Fermi level is theoretically confirmed. These features are reflected in the projected density of states (PDOS) on the top Sb BL (Fig. 4c). The three sharp peaks indicated by the arrows at E – E_F = 0.02–0.05 eV, corresponding to the AA, AB and AC stackings, are due to the van Hove singularity of the saddle point. The peak energies are very close to one another, reproducing the STS results shown in Fig. 2b,c. Assuming that the Fermi level is aligned near these peaks in real samples, we plot the Fermi surface contour of 1BL Sb(111)/5BL Bi(111) with AB stacking at E – E_F = -0.02 eV (Fig. 4b). The central rings and a surrounding star-like structure are consistent with the ARPES results (Fig. 3e, f).

The same calculations were also conducted based on an epitaxial model of 2BL Sb(111) on 5BL Bi(111). The band structures and the Fermi surface (E – E_F = -0.02 eV) obtained for the AB stack (Fig. 4d, e) resemble those obtained for the 1BL Sb(111) model (Fig. 4a, c) as well as the ARPES results (Supplementary Information D). This is also the case for the AA and AC stackings (Supplementary Information E, Fig. E2). However, the PDOSs around the Fermi level calculated for the AA, AB, and AC stacks exhibit more separated energies (Fig. 4f). Qualitatively, these results are in line with the STS data (Fig. 2e, f), but there are some clear discrepancies; e.g., the sharp peak at E – E_F = 0 eV for the AC stacking has no corresponding structure in the STS data (Fig. 2e). This difference may be attributed to incomplete structural optimization of the 2BL Sb model, which results from our simplified models of fixing the locations of the Bi atom to those of the bulk Bi crystal (see Methods).

We calculated the band structures of antimonene on a Bi(111) surface based on epitaxial models of 1BL Sb(111)/5BL Bi(111) and 2BL Sb(111)/5BL Bi(111), which successfully explained our STM and ARPES data. Obviously, the adopted models are rather crude and do not include the effects of moiré modulations. The fact that all stacking sequencies AA, AB, and AC result in qualitatively identical electronic band structures may also explain the success of the present models. We also note that Bi is a topologically nontrivial semimetal and that its (111) surface states are protected ⁴¹. Since Sb and Bi are isovalent, the surface states of antimonene on Bi(111) can be topologically equivalent to those on Bi(111). The absence of energy gaps due to the moiré superlattice, at least within the experimental resolution, may be due to topological protection against potential modulations. In this case, (high-order) van Hove singularities are predicted to emerge at the \(\stackrel{-}{\text{K}}\) point of the moiré Brillouin zone ⁵⁹, which calls for a future study.

Finally, we discuss the implications of the findings in the present work. Generally, the presence of the van Hove singularity means a logarithmic divergence of the density of states and an enhancement of Coulomb interactions. Tuning the Fermi level to a van Hove singularity point can lead to a variety of symmetry-broken phases at low temperatures, such as unconventional superconductivity, charge density waves, ferromagnetism, charge order, and nematicity; these phases have been discussed within the context of high-T_c cuprates, graphene, kagome metals, etc. ^54,60–63. Since the van Hove singularities in our systems are located close to the Fermi level, their tuning must be technically viable through gate voltage or molecular doping ⁶⁴. Among the possible low-temperature phases, superconductivity is the most likely because of the presence of Rashba-type spin polarization and the resulting spin-momentum locking ⁵⁹. In this case, the absence of space inversion symmetry should lead to a spin singlet-triplet mixed state ⁶⁵. We note that electron‒phonon coupling, which is responsible for superconductivity, is likely to be enhanced here, as discussed for granular Bi films ^66,67. Regarding 2BL Sb/Bi(111), moiré-induced electron confinement and enhanced electron correlation can further enrich the low-temperature physics beyond the simple van Hove singularity scenario. The inclusion of Rashba-type spin polarization in the moiré system is an open and intriguing problem. Thus, the present Sb/Bi(111) moiré superlattices will offer a new playground for investigating the role of the spin degree of freedom in van der Waals materials.

Sample preparation

The Si(111) substrates were cleaned by direct current heating at 1250°C for 10 seconds. After repeating the cycle several times, 7\(\times\)7 clean surfaces were obtained. The Ge(111) substrates were cleaned by repeated Ar⁺ sputtering and annealing several times to obtain 2×1 surfaces. Both surfaces were confirmed by observing sharp LEED spots. Bi(111) films were then grown by MBE on Si(111)-7\(\times\)7 surfaces to 10 BLs or on Ge(111)-2×1 surfaces to 250 BLs at room temperature. To improve the flatness of the film, the Bi films were annealed at approximately 190°C for 5 min. Subsequently, Sb was deposited on the Bi(111) surfaces at room temperature to form moiré antimonene. The crystallinity of the sample was improved by mild annealing at approximately 100 °C.

STM measurements

The STM measurements were conducted at 78 K and 4.6 K with a Nanonis controller Mimea BP5e. Topographic images were obtained in constant current mode. The lateral scale of the STM images was calibrated through observation of Bi(111) surfaces by assuming that the lattice constant is equivalent to that of a bulk crystal (0.454 nm). STS measurements were conducted with a built-in lock-in amplifier with a typical bias voltage modulation of 20 mV at 477 Hz.

ARPES measurements

ARPES and SARPES measurements were performed at National Institute for Materials Science (NIMS) and at the Institute for Solid State Physics (ISSP), University of Tokyo. For the NIMS measurements, we employed a momentum microscope equipped with an imaging spin detector ^42,68. A 10.9-eV laser was used as the excitation light. For the measurements at the ISSP, the photoelectrons excited by a 6.994-eV laser were analyzed by a hemispherical photoelectron analyzer equipped with an ultralow-speed electron diffraction spin detector ⁵³. For both measurements, samples were prepared in situ. The sample temperature during the measurements was 30 K.

DFT calculations

To calculate the electronic band structures, we adopted epitaxial models of 1BL Sb(111)/5BL Bi(111) and 2BL Sb(111)/5BL Bi(111). The locations of the Bi atoms and those of the Sb atoms in the in-plane directions were fixed to those of the bulk Bi crystal. The out-of-plane Bi atom positions follow the structure for the Bi(111) thin film used in the literature ⁶⁹. Here, the Bi atom locations in the top Bi BL (adjacent to the Sb BLs) are modified, while the other Bi atoms follow the bulk Bi structure. In contrast, the locations of the Sb atoms in the out-of-plane direction were set equal to those at the centers of the AA, AB and AC stacking regions of the numerically optimized moiré superlattices (indicated by the red, blue and green circles, respectively, in Fig. E1a and Fig. E2a; see Supplementary Information E). To optimize the 1BL Sb(111)/5BL Bi(111), we prepared a single 11×11 supercell of the Sb(111) BL on five vertically stacked 10×10 supercells of the Bi(111) BL. While the locations of the Bi atoms are fixed as in the Bi(111) thin film ⁶⁹, the locations of the Sb atoms are optimized by utilizing a neural network potential, PFP (without U) version 5.0.0, on Matlantis (https://matlantis.com/) ⁷⁰. Similarly, to optimize the 2BL Sb(111)/5BL Bi(111), we prepared two 14×14 supercells of the Sb BL on five 13×13 supercells of the Bi BL and optimized the positions of the Sb atoms.

The noncollinear DFT calculations for the epitaxial models are performed by OpenMX version 3.9 ^71–74 with the PBE exchange correlation functional and spin–orbit coupling. We use the 15×15×1 k-point grid in the first Brillouin zone for self-consistent field (SCF) calculations. After the SCF calculation, the density of states is obtained on a 256×256×1 k-point grid by the tetrahedron method. The energy cutoff is set to 100 or 200 Ry. The dependence of the band structure and density of states on the k-point grid and energy cutoff was examined to ensure convergence.

Data availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors thank S. Yoshizawa, F. Arai, S. Takezawa, H. Tanaka, A. Harasawa, and T. Iimori for their technical support during the STM and ARPES experiments. This work was supported financially by JSPS KAKENHI (Grant Numbers 20H05621, 22H01961, 20K15133, 22H01183, 23H03818, 23H04524), the World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics, MEXT, Japan and the Innovative Science and Technology Initiative for Security Grant Number JPJ004596, ATLA, Japan.

Author contributions

T.N. and T.U. conceived the experiment, and T.N., T.U., K.Y. and Y.Y. wrote the manuscript. T.N., R.N. and Q.W. carried out the STM measurements, and T.N. analyzed the data. T.N., K.Y., Y.F., K.K., and R.M. carried out (S)ARPES measurements at ISSS under the supervision of T.K. Y.C., K.Y., and S.T. carried out (S)ARPES measurements at NIMS. K.Y. and S.T. analyzed the ARPES/SARPES data. Y.Y. performed the DFT calculations. All the authors discussed the results and contributed to finalizing the manuscript.

Competing interests

The authors declare no competing interests.

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Moiré superlattices of antimonene on a Bi(111) substrate with van Hove singularity and Rashba-type spin polarization

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