Abstract
Exotic states such as topological insulators, superconductors and quantum spin liquids are often challenging or impossible to create in a single material1,2,3. For example, it is unclear whether topological superconductivity, which has been suggested to be a key ingredient for topological quantum computing, exists in any naturally occurring material4,5,6,7,8,9. The problem can be circumvented by deliberately selecting the combination of materials in heterostructures so that the desired physics emerges from interactions between the different components1,10,11,12,13,14,15. Here we use this designer approach to fabricate van der Waals heterostructures that combine a two-dimensional (2D) ferromagnet with a superconductor, and we observe 2D topological superconductivity in the system. We use molecular-beam epitaxy to grow 2D islands of ferromagnetic chromium tribromide16 on superconducting niobium diselenide. We then use low-temperature scanning tunnelling microscopy and spectroscopy to reveal the signatures of one-dimensional Majorana edge modes. The fabricated 2D van der Waals heterostructure provides a high-quality, tunable system that can be readily integrated into device structures that use topological superconductivity. The layered heterostructures can be readily accessed by various external stimuli, potentially allowing external control of 2D topological superconductivity through electrical17, mechanical18, chemical19 or optical means20.
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Data availability
All the data supporting the findings are available from the corresponding authors upon request. The results of the DFT calculations are available on the NOMAD repository (https://doi.org/10.17172/NOMAD/2020.09.06-1).
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Acknowledgements
This research made use of the Aalto Nanomicroscopy Center (Aalto NMC) facilities and was supported by the European Research Council (ERC-2017-AdG no. 788185 “Artificial Designer Materials”), Academy of Finland (Academy professor funding no. 318995 and 320555, and Academy postdoctoral researcher no. 309975), and the Aalto University Centre for Quantum Engineering (Aalto CQE). S.G. acknowledges the support of the National Science Centre (NCN, Poland) under grant 2017/27/N/ST3/01762. Computing resources from the Aalto Science-IT project and CSC, Helsinki, are gratefully acknowledged. A.S.F. has been supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan.
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S.K., T.O. and P.L. conceived the experiment. S.K., M.N.H., M.A. and S.C.G. carried out the sample growth. S.K. did the low-temperature STM experiments. S.K. and V.V. analysed the STM data. O.J.S. and A.S.F. planned and carried out the DFT calculations. T.O. and S.G. developed the theoretical model and established its implications. S.G. carried out the numerical calculations. S.K., T.O. and P.L. wrote the manuscript with input from all co-authors.
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Extended data figures and tables
Extended Data Fig. 1 Fitting the bulk NbSe2 superconducting gap to a two-band model37.
a, c, e, The tunnelling spectra on NbSe2 (a), and in the middle (c) and at the edge (e) of an CrBr3 island fitted with the two-band model. b, d, f, The corresponding spectra after subtracting the background given by the two-band model fit. Spectra in the middle (d) and at the edge (f) of an CrBr3 island show two- and three-peak features within the superconducting gap, respectively. They were fitted by a sum of either two (positioned symmetrically with respect to zero, green lines) or three Gaussians (two of them positioned symmetrically with respect to zero, green lines; and one centred at zero, red line).
Extended Data Fig. 2 Shift of the NbSe2 states under CrBr3.
a, Blue curve, spectrum recorded on monolayer CrBr3 island; red curve, spectrum recorded on bare NbSe2 substrate. Since at these energies there are no CrBr3 states, this directly probes the states of the underlying NbSe2 substrate. The results show a clear shift of the Nb d band upwards under CrBr3. b, Spectra recorded along a line from NbSe2 to CrBr3, showing the shift of about 80 meV. Colour scale indicates value of the dI/dV signal (arbitrary units), range (0.05, 0.6).
Extended Data Fig. 3 Estimating the decay length of the edge states.
a, Experimentally measured differential tunnelling conductance maps at zero bias. Colour scale (arbitrary units) range is (0, 1.5). b, The corresponding density of states profiles across the edge of the island and corresponding Gaussian fit. ZBC, zero-bias conductance.
Extended Data Fig. 4 Robustness and reproducibility of the edge mode.
Topography (top row, extracted during grid spectroscopy) and dI/dV maps at different bias voltages (indicated in the different panels) on four different CrBr3 islands and recorded with different microscopic tip apices. Edge modes are observed in our hybrid van der Waals heterostructures on all CrBr3 islands, irrespective of their size and shape, or different microscopic tip.
Extended Data Fig. 5 Measurements in a magnetic field.
a–e, Experimentally measured differential tunnelling conductance maps as a function of the bias under an external magnetic field of 4 T. f, Corresponding dI/dV line spectra measured along the line indicated in a. Colouring as in Fig. 3a. STM feedback parameters: Vbias = +1 V, I = 10 pA, image size: 40 × 40 nm2. Zero-bias peaks (ZBPs) can occur because of the formation of a Kondo resonance that appears when many-body interactions screen magnetic impurities in metals. If the ZBPs reported here were not of topological origin but arose from the Kondo effect, they would persist beyond the superconductor-to-normal-metal transition. In our measurements, however, as soon as superconductivity is suppressed, all states (including zero-bias peaks at the edge of the island) disappear, and the spectrum becomes featureless.
Extended Data Fig. 6 Structures considered in DFT calculations.
a–e, Top and side views of the isolated CrBr3 (a) and NbSe2 (b) monolayers, as well as the heterostructures htCrSe (c), htCrNbSe (d) and htCrNb (e).
Extended Data Fig. 7 Band structures of the isolated monolayers CrBr3 and NbSe2, as well as the most stable CrBr3–NbSe2 heterostructure, htCrSe.
Blue and red lines represent spin up and down, respectively.
Extended Data Fig. 8 a, Spin-polarized band structure of the htCrSe, where the blue and lines indicate spin up and down, respectively.
b, Comparison between the bands of 2 × 2 NbSe2 and htCrSe near the Γ point. c, Unfolded spin-polarized bands. d, Band structure of htCrSe with SOC, where the blue (red) circles indicate the positive (negative) projection of the Nb electrons’ spin on the quantization axis. e, Comparison between the bands with SOC of the 2 × 2 NbSe2 and htCrSe near the Γ point. f, Unfolded bands obtained with SOC.
Supplementary information
Supplementary Information
Supplementary Notes: Description of the phenomenological model for topological superconductivity in the CrBr3-NbSe2 system and additional comparison with experimental data with 5 display items.
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Kezilebieke, S., Huda, M.N., Vaňo, V. et al. Topological superconductivity in a van der Waals heterostructure. Nature 588, 424–428 (2020). https://doi.org/10.1038/s41586-020-2989-y
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DOI: https://doi.org/10.1038/s41586-020-2989-y
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