The quantum spin Hall insulator is characterized by a bandgap in the two-dimensional (2D) interior and helical 1D edge states1,2,3. Inducing superconductivity in the helical edge state results in a 1D topological superconductor, a highly sought-after state of matter at the core of many proposals for topological quantum computing4. In the present study, we report the coexistence of superconductivity and the quantum spin Hall edge state in a van der Waals heterostructure, by placing a monolayer of 1T′-WTe2, a quantum spin Hall insulator1,2,3, on a van der Waals superconductor, NbSe2. Using scanning tunnelling microscopy and spectroscopy (STM/STS), we demonstrate that the WTe2 monolayer exhibits a proximity-induced superconducting gap due to the underlying superconductor and that the spectroscopic features of the quantum spin Hall edge state remain intact. Taken together, these observations provide conclusive evidence for proximity-induced superconductivity in the quantum spin Hall edge state in WTe2, a crucial step towards realizing 1D topological superconductivity and Majorana bound states in this van der Waals material platform.
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Tang, S. et al. Quantum spin Hall state in monolayer 1T2-WTe2. Nat. Phys. 13, 683–687 (2017).
Fei, Z. et al. Edge conduction in monolayer WTe2. Nat. Phys. 13, 677–682 (2017).
Wu, S. et al. Observation of the quantum spin Hall effect up to 100 kelvin in a monolayer crystal. Science 359, 76–79 (2018).
Alicea, J. New directions in the pursuit of majorana fermions in solid state systems. Rep. Prog. Phys. 75, 076501 (2012).
Kitaev, A. Y. Unpaired majorana fermions in quantum wires. Phys. Uspekhi 44, 131–136 (2001).
Fu, L. & Kane, C. L. Superconducting proximity effect and majorana fermions at the surface of a topological insulator. Phys. Rev. Lett. 100, 096407 (2008).
DasSarma, S., Freedman, M. & Nayak, C. Majorana zero modes and topological quantum computation. npj Quantum Inf. 1, 15001 (2015).
Sato, M. & Ando, Y. Majorana zero modes and topological quantum computation. Rep. Prog. Phys. 80, 076501 (2017).
Maeno, Y., Kittaka, S., Nomura, T., Yonezawa, S. & Ishida, K. Evaluation of spin-triplet superconductivity in Sr2 RuO4. J. Phys. Soc. Jpn 81, 011009 (2012).
Fu, L. & Kane, C. L. Josephson current and noise at a superconductor/quantum-spin-Hall-insulator/superconductor junction. Phys. Rev. B 79, 161408 (2009).
Wang, M.-X. et al. The coexistence of superconductivity and topological order in the Bi2Se3 thin films. Science 336, 52–55 (2012).
Sun, H.-H. et al. Majorana zero mode detected with spin selective Andreev reflection in the vortex of a topological superconductor. Phys. Rev. Lett. 116, 257003 (2016).
Hart, S. et al. Induced superconductivity in the quantum spin Hall edge. Nat. Phys. 10, 638–643 (2014).
Bocquillon, E. et al. Gapless Andreev bound states in the quantum spin Hall insulator HgTe. Nat. Nanotechnol. 12, 137–143 (2016).
Jia, Z.-Y. et al. Direct visualization of a two-dimensional topological insulator in the single-layer 1T2-WTe2. Phys. Rev. B 96, 041108 (2017).
Peng, L. et al. Observation of topological states residing at step edges of WTe2. Nat. Commun. 8, 659 (2017).
Shi, Y. et al. Imaging quantum spin Hall edges in monolayer WTe2. Sci. Adv. 5, eaat8799 (2019).
Qian, X., Liu, J., Fu, L. & Li, J. Quantum spin Hall effect in two-dimensional transition metal dichalcogenides. Science 346, 1344–1347 (2014).
Fatemi, V. et al. Electrically tunable low-density superconductivity in a monolayer topological insulator. Science 362, 926–929 (2018).
Sajadi, E. et al. Gate-induced superconductivity in a monolayer topological insulator. Science 362, 922–925 (2018).
Zeng, Y. et al. High-quality magnetotransport in graphene using the edge-free Corbino geometry. Phys. Rev. Lett. 122, 137701 (2019).
Cucchi, I. et al. Microfocus laser angle-resolved photoemission on encapsulated mono-, bi- and few-layer 1T2-WTe2. Nano Lett. 19, 554–560 (2019).
Garoche, P., Veyssié, J. J., Manuel, P. & Molinié, P. Experimental investigation of superconductivity in 2H-NbSe2 single crystal. Solid State Commun. 19, 455–460 (1976).
Huang, C. et al. Inducing strong superconductivity in WTe2 by a proximity effect. ACS Nano 12, 7185–7196 (2018).
Li, Q. et al. Proximity-induced superconductivity with subgap anomaly in type II Weyl semi-metal WTe2. Nano Lett. 18, 7962–7968 (2018).
Reeg, C. R. & Maslov, D. L. Hard gap in a normal layer coupled to a superconductor. Phys. Rev. B 94, 020501 (2016).
Jäck, B. et al. Observation of a Majorana zero mode in a topologically protected edge channel. Science 364, 1255–1259 (2019).
Khestanova, E. et al. Unusual suppression of the superconducting energy gap and critical temperature in atomically thin NbSe2. Nano Lett. 18, 2623–2629 (2018).
Li, G., Luican, A. & Andrei, E. Y. Self-navigation of a scanning tunneling microscope tip toward a micron-sized graphene sample. Rev. Sci. Instrum. 82, 073701 (2011).
We thank D. Xiao, D. Cobden and X. Xu for helpful discussions and N. Speeney and N. Iskos for assistance in the laboratory. B.M.H. was supported by the Department of Energy under the Early Career award programme (DE-SC0018115). Crystal growth and characterization at ORNL were supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering. We thank the Pennsylvania State University Two-Dimensional Crystal Consortium—Materials Innovation Platform (2DCC-MIP), which is supported by NSF DMR-1539916, for supplying further 2D materials. F.L. and D.W. were supported by the NSF DMR-1809145 for STM measurements. We acknowledge NSF DMR-1626099 for acquisition of the STM instrument. S.C.d.l.B. was supported by the Department of Energy (DE-SC0018115) for fabrication of proximity-effect van der Waals heterostructures. Density functional theory calculations were supported by the Department of Energy under grant no. DE-SC0014506.
The authors declare no competing interests.
Peer review information Nature Physics thanks Feng Miao and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Lüpke, F., Waters, D., de la Barrera, S.C. et al. Proximity-induced superconducting gap in the quantum spin Hall edge state of monolayer WTe2. Nat. Phys. 16, 526–530 (2020). https://doi.org/10.1038/s41567-020-0816-x
ACS Nano (2020)
Physical Review Research (2020)