Coexistence of Robust Edge States and Superconductivity in Few-Layer Stanene

High-quality stanene films have been actively pursued for realizing not only quantum spin Hall edge states without backscattering, but also intrinsic superconductivity, two central ingredients that may further endow the systems to host topological superconductivity. Yet to date, convincing evidence of topological edge states in stanene remains to be seen, let alone the coexistence of these two ingredients, owing to the bottleneck of growing high-quality stanene films. Here we fabricate one- to five-layer stanene films on the Bi(111) substrate and observe the robust edge states using scanning tunneling microscopy/spectroscopy. We also measure distinct superconducting gaps on different-layered stanene films. Our first-principles calculations further show that hydrogen passivation plays a decisive role as a surfactant in improving the quality of the stanene films, while the Bi substrate endows the films with nontrivial topology. The coexistence of nontrivial topology and intrinsic superconductivity renders the system a promising candidate to become the simplest topological superconductor based on a single-element system.

Figure 1

(a) Schematics of a freestanding monolayer stanene (left) and a sample structure of four-layer $\text{stanene}/\mathrm{Bi}(111)/\mathrm{Si}$ with the top surface of stanene passivated by hydrogen (right). (b),(c) Topographies of the Bi(111) substrate before [(b): ${\mathrm{V}}_s=2.00\mathrm{V}$, ${\mathrm{I}}_{\mathrm{set}}=100\mathrm{pA}$] and after [(c): ${\mathrm{V}}_s=2.27\mathrm{V}$, ${\mathrm{I}}_{\mathrm{set}}=50\mathrm{pA}$] depositing Sn. (d) Topography of the first-layer stanene films on Bi(111) after annealing, with a profile shown at the bottom depicting the height along the black dotted line. ${\mathrm{V}}_s=2.0\mathrm{V}$, ${\mathrm{I}}_{\mathrm{set}}=70\mathrm{pA}$. Inset: atomically resolved image taken on the stanene film. (e) Topography of multilayer stanene ($\sim 2.5$ layers) on Bi(111), with the corresponding height profile shown below. ${\mathrm{V}}_s=2.7\mathrm{V}$, ${\mathrm{I}}_{\mathrm{set}}=80\mathrm{pA}$.

Figure 2

(a) Calculated band structure (shifted) of a four-layer stanene film on Bi(111) with contributions from different Sn orbitals highlighted. (b) Comparison between calculated DOS and experimentally detected $\mathrm{dI}/\mathrm{dV}$ spectrum of the four-layer stanene film on Bi(111). The dotted lines highlight the well-matched low-energy range around the Fermi level. (c) Topography of a fourth-layer stanene island with a hexagonal shape. ${\mathrm{V}}_s=2.0\mathrm{V}$, ${\mathrm{I}}_{\mathrm{set}}=80\mathrm{pA}$. (d) $\mathrm{dI}/\mathrm{dV}$ spectra taken in the interior and at the A or B edge of the island shown in (c). ${\mathrm{I}}_{\mathrm{set}}=250\mathrm{pA}$, ${\mathrm{V}}_{\mathrm{mod}}=3\mathrm{mV}$ (961 Hz). (e) 3D plots of the $\mathrm{\Delta }\mathrm{LDOSs}$ as a function of energy and spatial distance along the black arrows in (c) crossing the A (left) and B (right) edges. The $\mathrm{\Delta }\mathrm{LDOSs}$ at the respective edges with positions marked by the black triangles are projected on the sidewalls, with the gray shadowed areas showing the energy windows dominated by the edge states ($\mathrm{\Delta }\mathrm{LDOS}>0$). The spatial variations of the edge states are projected by the blue curves. (f) $\mathrm{dI}/\mathrm{dV}$ mappings of the stanene island shown in (c) at different energies. (g) $\mathrm{dI}/\mathrm{dV}$ mappings of different-layered stanene islands taken at the energy of the respective bulk dip minimum. White scale bars: 5 nm. ${\mathrm{I}}_{\mathrm{set}}=250\mathrm{pA}$, ${\mathrm{V}}_{\mathrm{mod}}=3\mathrm{mV}$ (961 Hz) for (f) and (g). $\mathrm{dI}/\mathrm{dV}$ spectra were taken at 4.2 K.

Figure 3

(a) Spatial variation of the superconducting gap crossing the A edge along the dotted arrow in Fig. 2. The contour map is projected underneath. ${\mathrm{I}}_{\mathrm{set}}=250\mathrm{pA}$, ${\mathrm{V}}_{\mathrm{mod}}=20\mu \mathrm{V}$ (961 Hz). (b) Layer-dependent superconducting gap of stanene films. ${\mathrm{I}}_{\mathrm{set}}=250\mathrm{pA}$, ${\mathrm{V}}_{\mathrm{mod}}=20\mu \mathrm{V}$ (961 Hz). (c) Variations of the superconducting gap ($\mathrm{\Delta }$) and the ZBC with the layer thickness. (d) Attenuation of superconductivity under an out-of-plane magnetic field, taken on a third-layer stanene island. All the superconducting gaps in this figure are symmetrized and normalized by the intensity of the normal states at the starting bias.