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Crossover from Ising- to Rashba-type superconductivity in epitaxial Bi2Se3/monolayer NbSe2 heterostructures

Nature Materials volume 21, pages 1366–1372 (2022)Cite this article

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Abstract

A topological insulator (TI) interfaced with an s-wave superconductor has been predicted to host topological superconductivity. Although the growth of epitaxial TI films on s-wave superconductors has been achieved by molecular-beam epitaxy, it remains an outstanding challenge for synthesizing atomically thin TI/superconductor heterostructures, which are critical for engineering the topological superconducting phase. Here we used molecular-beam epitaxy to grow Bi2Se3 films with a controlled thickness on monolayer NbSe2 and performed in situ angle-resolved photoemission spectroscopy and ex situ magnetotransport measurements on these heterostructures. We found that the emergence of Rashba-type bulk quantum-well bands and spin-non-degenerate surface states coincides with a marked suppression of the in-plane upper critical magnetic field of the superconductivity in Bi2Se3/monolayer NbSe2 heterostructures. This is a signature of a crossover from Ising- to Rashba-type superconducting pairings, induced by altering the Bi2Se3 film thickness. Our work opens a route for exploring a robust topological superconducting phase in TI/Ising superconductor heterostructures.

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Fig. 1: MBE-grown Bi2Se3/monolayer NbSe2 heterostructures on epitaxial bilayer graphene.
Fig. 2: ARPES band maps of Bi2Se3/monolayer NbSe2 heterostructures.
Fig. 3: Crossover from Ising- to Rashba-type pairing symmetry in Bi2Se3/monolayer NbSe2 heterostructures.
Fig. 4: Theoretical calculations of Bi2Se3/monolayer NbSe2 heterostructures.

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Data availability

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

Code availability

The codes used in the theoretical simulations and calculations are available from the corresponding author upon reasonable request.

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Acknowledgements

We thank Y. Cui, K. T. Law and D. Xiao for helpful discussions. This work is primarily supported by the Penn State MRSEC for Nanoscale Science (DMR-2011839) (H.Y. and C.-Z.C.). The electrical transport measurements and sample characterization are partially supported by the NSF CAREER award (DMR-1847811) (C.-Z.C.). The theoretical calculations and simulations are partially supported by a DOE grant (DE-SC0019064) (C.-X.L.). Y.W. acknowledges support from a startup grant from the University of North Texas. The MBE growth and ARPES measurements were performed in the NSF-supported 2DCC MIP facility (DMR-2039351) (N.S. and C.-Z.C.). The dilution refrigerator transport measurements at University of Washington are supported by the AFOSR award (FA9550-21-1-0177) (X.X.) and acknowledge the usage of the millikelvin optoelectronic quantum material laboratory supported by the M. J. Murdock Charitable Trust. D.R.H. and N.A. acknowledge support from the NSF CAREER award (DMR-1654107). C.-Z.C. also acknowledges support from the Gordon and Betty Moore Foundation’s EPiQS Initiative (grant GBMF9063 to C.-Z.C.).

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Author notes

  1. These authors contributed equally: Hemian Yi, Lun-Hui Hu, Yuanxi Wang.

Authors and Affiliations

  1. Department of Physics, The Pennsylvania State University, University Park, PA, USA

    Hemian Yi, Lun-Hui Hu, Yuanxi Wang, Run Xiao, Yi-Fan Zhao, Ling-Jie Zhou, Ruoxi Zhang, Anthony R. Richardella, Moses H. W. Chan, Nitin Samarth, Chao-Xing Liu & Cui-Zu Chang

  2. Department of Physics, University of North Texas, Denton, TX, USA

    Yuanxi Wang

  3. Department of Physics, University of Washington, Seattle, WA, USA

    Jiaqi Cai & Xiaodong Xu

  4. Department of Chemistry, The Pennsylvania State University, University Park, PA, USA

    Danielle Reifsnyder Hickey

  5. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, USA

    Danielle Reifsnyder Hickey, Chengye Dong, Nasim Alem & Joshua A. Robinson

  6. Department of Materials Science and Engineering, University of Washington, Seattle, WA, USA

    Xiaodong Xu

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Contributions

C.-Z.C. conceived and designed the experiment. H.Y. performed the MBE growth and ARPES measurements and used the physical property measurement system with the help of Y.-F.Z., L.-J.Z., R.Z., A.R.R., M.H.W.C., N.S. and C.-Z.C. R.X. and H.Y. performed the 3He transport measurements with the help of N.S. and C.-Z.C. J.C. and X.X. performed the dilution transport measurements. Y.W. performed the first-principles calculations. C.D. and J.A.R. prepared the bilayer graphene-terminated 6H-SiC(0001) substrates. D.R.H. and N.A. carried out the TEM measurements. L.-H.H. and C.-X.L. performed the theoretical simulations. H.Y., L.-H.H., Y.W. and C.-Z.C. analysed the data and wrote the manuscript with inputs from all the authors.

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Correspondence to Cui-Zu Chang.

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Extended data

Extended Data Fig. 1 RHEED patterns during MBE growth.

(a) Bilayer graphene terminated 6H-SiC(0001) substrate. (b) Monolayer NbSe2 film grown on bilayer graphene. (c) 5 QL Bi2Se3/monolayer NbSe2 heterostructure grown on bilayer graphene.

Extended Data Fig. 2 Cross-sectional ADF-STEM images and EDS maps of the MBE-grown Bi2Se3/monolayer NbSe2 heterostructure.

(a–c) ADF-STEM images of the Bi2Se3/monolayer NbSe2 heterostructure along different orientations. (d,e) Large-scale ADF-STEM image and corresponding EDS maps of Si, Nb, Bi, and Se of the Se layer-capped 6QL Bi2Se3/monolayer NbSe2 heterostructure grown on bilayer graphene terminated 6H-SiC(0001).

Extended Data Fig. 3 Electronic band structures of monolayer NbSe2 and 1QL Bi2Se3/monolayer NbSe2.

(a) Fermi surface of monolayer NbSe2. (b) Fermi surface of 1QL Bi2Se3/monolayer NbSe2. The red dashed lines in (a) and (b) locate the hole pocket near the Γ point from monolayer NbSe2. The blue dashed line in (b) locates the electron pocket near the Γ point from 1QL Bi2Se3. (c, d) ARPES band maps of monolayer NbSe2 and 1QL Bi2Se3/monolayer NbSe2. (e) Comparison of the energy distribution curves at the Γ point in monolayer NbSe2 (red line) and 1QL Bi2Se3/monolayer NbSe2(blue line).

Extended Data Fig. 4 Rashba-type bulk QW bands and Dirac SSs in mQL Bi2Se3/monolayer NbSe2 heterostructures.

(a-e) The second derivative spectra of the ARPES data in Fig. 2b–f of the main text. (f) Rashba splitting parameter αR of QW1 and QW2 states as a function of m.

Extended Data Fig. 5 Ising-type superconductivity in monolayer NbSe2.

(a) R-T curves of monolayer NbSe2 under different out-of-plane magnetic fields \(\mu _0H_ \bot\). (b) R-T curves of monolayer NbSe2 under different in-plane magnetic fields \(\mu _0H_\parallel\). (c) Hc2-T phase diagram of monolayer NbSe2.

Extended Data Fig. 6 R-μ0H curves of mQL Bi2Se3/monolayer NbSe2 heterostructures under in-plane magnetic fields at different temperatures.

(a) m=1. (b) m=2. (c) m=3. (d) m=4. (e) m=5. (f) m=6.

Extended Data Fig. 7 R-μ0H curves of monolayer NbSe2 and mQL Bi2Se3/NbSe2 heterostructures under out-of-plane magnetic fields at different temperatures.

(a) Monolayer NbSe2. (b) m=1. (c) m=2. (d) m=6.

Extended Data Fig. 8 Estimation of the charge transfer between monolayer NbSe2 and BiSe.

(a) Side view of calculation supercell for \(\sqrt 2 \times 4\sqrt 2\) BiSe on \(\sqrt 3 \times 7\) NbSe2. (b) Projected band structures of BiSe/monolayer NbSe2 (left) and freestanding monolayer NbSe2 (right). The projections of total wavefunction onto Nb orbitals and Bi orbitals are shown in red and blue, respectively.

Extended Data Fig. 9 (m–1) bulk QW states in mQL Bi2Se3 films.

The calculated electronic band structures of freestanding mQL Bi2Se3 films with 2 ≤ m ≤ 6.

Extended Data Fig. 10 Bulk QW states in a freestanding 4QL Bi2Se3 film.

Partial charge densities for QW1, QW2, and QW3 in freestanding 4QL Bi2Se3 films, accompanied with their xy-plane-averaged representations ρxy(z) plotted in the same way as Fig. 4g in the main text. Black curves are schematics for the charge densities of the envelope wavefunctions.

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Supplementary Information

Supplementary text for Extended Data Figs. 1–10, Figs. 1–6 and accompanying text and references.

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Yi, H., Hu, LH., Wang, Y. et al. Crossover from Ising- to Rashba-type superconductivity in epitaxial Bi2Se3/monolayer NbSe2 heterostructures. Nat. Mater. 21, 1366–1372 (2022). https://doi.org/10.1038/s41563-022-01386-z

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