Inducing superconductivity by proximity in materials with nontrivial band topology has become a method in the search for unconventional forms of superconducting pairing. At the one-dimensional (1D) edges of two-dimensional (2D) topological insulators the presence of non-Abelian parafermions has been predicted, which could have important applications in topological quantum computation. Here, we investigate heterostructures of the quantum spin Hall candidate 1T’-WTe₂, grown by in situ van der Waals epitaxy on a superconductor 2H-NbSe₂ by scanning tunneling spectroscopy down to 500 mK, which enables us to resolve the superconducting local density of states (LDOS) across the interface between the two materials.

Figure 1 shows scanning tunneling microscopy (STM) data of 1T’-WTe₂/2H-NbSe₂ heterostructures. In agreement with previous studies of WTe₂ on bilayer graphene, we observe islands with disordered boundaries and sizes up to a few tens of nanometer in diameter that are polycrystalline on NbSe₂ substrates. Lattice parameters estimated from FFT patterns of the STM images are a ~ a₁ = 3.5 ± 0.2 Å, and b~2a₂cos(30°) = 6.2 ± 0.3 Å, in good agreement with the lattice parameters of WTe₂ (a = 3.48 Å and b = 6.28 Å) and NbSe₂ (a₁ = a₂ = 3.45 Å). Local lattice matching implies a 5% compressive lattice strain along b, which has previously been shown to further stabilize the WTe₂ bulk gap.

Fig. 1. (left panels) STM topography of monolayer 1T’-WTe₂ islands grown on a single crystal of 2H-NbSe₂ by van der Waals epitaxy. Upper right panel shows height profile corresponding to the dashed white line in the lower right panel, showing a monolayer height of 8.2 Å. The inset shows the respective 1T’-WTe₂ and 2H-NbSe₂ crystal structures. Atomic-resolution close-up of the 1T’-WTe₂ edge, showing detail of the atomic alignment.

In Fig. 2, tunneling spectra taken on the monolayer WTe₂ and the NbSe₂ substrate at 500 mK are presented, demonstrating the clear signature of a superconducting energy gap in the 2D monolayer WTe₂. The measured superconducting LDOS within a self-consistent multiband framework that considers a superconducting gap of each band and an interband Cooper-pair tunneling rate. It has been already known that the gap of NbSe₂ is well described with a scheme of intrinsic two-band superconductor, and here we consider a third order parameter Δ_WTe2, coupled to the two NbSe₂ bands, assuming intrinsic Δ_WTe2 = 0. Fits to both a two-band (NbSe₂, red line) and a three-band (WTe₂/NbSe₂, blue line) model explain the data well (Fig. 2) and reproduce the known NbSe₂ order parameters. Self-consistently obtained Δ_WTe2 = (0.57 ± 0.02) meV, reflecting induced pairing, which suggests strong hybridization between the overlayer and substrate.

Fig. 2. tunneling spectra comparing the WTe₂/NbSe₂ spectrum with that measured on the bare NbSe₂ substrate. Solid red and blue lines are fits to our two-band (NbSe₂) and three-band (WTe₂) models, respectively.

Our tight-binding calculations revealed that the strong hybridization in WTe₂/NbSe₂ heterostructure substantially weakens the topological edge state signature compared with WTe₂/HOPG, which is only weakly hybridized. In spite of the suppressed edge states, however, we observed enhanced order parameter at the edge as shown in Fig. 3, where a spatial profile of the extracted order parameter Δ_WTe2(x) based on the three-band superconductivity analysis is shown. The plot also shows spatial oscillations with a period of ~2 nm. Oscillations of comparable period are also observed in the order parameter extracted from a simple single-band BCS model. This suggests that these arise from Friedel-like oscillations in the local density of states due to scattering of 2D bulk states at the WTe2 edge. The exponential decaying 1D edge state, with decay length (1.1 ± 0.2) nm is comparable to the normal-state LDOS of the edge.

Fig. 3. (upper panel) STM-height profile measured across a clean edge of a WTe₂ monolayer crystal. The inset shows the corresponding STM topograph indicating position and direction of the line profile (red arrow). (middle) A series of tunneling spectra measured T = 500 mK for points across the edge. (lower) Extracted spatial profile of the induced order parameter Δ_WTe2(x) (blue), compared with a single-band BCS-Dynes model (black).

References

• [1] W. Tao et al., Phys. Rev. B 105, 094512 (2022).

Authors

• W. Tao^a, Z. J. Tong^a, A. Das^b, D.-Q. Ho^a, Y. Sato, M. Haze, J. Jia^a, Y. Que^a, F. Bussolotti^c, K. E. J. Goh^a,c, B. Wang^d, H. Lin^e, A. Bansil^d, S. Mukherjee^b, Y. Hasegawa, and B. Weber^a,f
• ^aNanyang Technological University, Singapore
• ^bIndian Institute of Technology Madras
• ^cAgency for Science Technology and Research (A*STAR)
• ^dNortheastern University
• ^eInstitute of Physics, Academia Sinica
• ^fMonash Universit