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An unusual continuous paramagnetic-limited superconducting phase transition in 2D NbSe 2

Nature Materials volume 17, pages 504–508 (2018)Cite this article

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Abstract

Time reversal and spatial inversion are two key symmetries for conventional Bardeen–Cooper–Schrieffer (BCS) superconductivity1. Breaking inversion symmetry can lead to mixed-parity Cooper pairing and unconventional superconducting properties1,2,3,4,5. Two-dimensional (2D) NbSe2 has emerged as a new non-centrosymmetric superconductor with the unique out-of-plane or Ising spin–orbit coupling (SOC)6,7,8,9. Here we report the observation of an unusual continuous paramagnetic-limited superconductor–normal metal transition in 2D NbSe2. Using tunelling spectroscopy under high in-plane magnetic fields, we observe a continuous closing of the superconducting gap at the upper critical field at low temperatures, in stark contrast to the abrupt first-order transition observed in BCS thin-film superconductors10,11,12. The paramagnetic-limited continuous transition arises from a large spin susceptibility of the superconducting phase due to the Ising SOC. The result is further supported by self-consistent mean-field calculations based on the ab initio band structure of 2D NbSe2. Our findings establish 2D NbSe2 as a promising platform to explore novel spin-dependent superconducting phenomena and device concepts1, such as equal-spin Andreev reflection13 and topological superconductivity14,15,16.

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Fig. 1: 2D NbSe2 superconductor tunnel junction.
Fig. 2: Tunelling spectroscopy under in-plane magnetic fields.
Fig. 3: Calculations of the superconducting gap under in-plane magnetic fields.

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Acknowledgements

This research was supported by the ARO Award W911NF-17-1-0605 for the sample and device fabrication and the US Department of Energy, Office of Basic Energy Sciences contract no. DESC0013883 for the tunelling spectroscopy measurements. A portion of this work was performed at the NHMFL, which is supported by National Science Foundation (NSF) Cooperative Agreement no. DMR-1644779 and the State of Florida. The work in Hong Kong was supported by the Croucher Foundation, the Dr. Tai-chin Lo Foundation and the Hong Kong Research Grants Council through HKUST3/CRF/13 G, C6026-16W and 16324216. The work in Lausanne was supported by the Swiss National Science Foundation. We also acknowledge support from the NSF under Award nos DMR-1645901 (E.S.), DMR-1420451 (K.K.) and DMR-1410407 (Z.W.) and a David and Lucille Packard Fellowship and a Sloan Fellowship (K.F.M.).

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Authors and Affiliations

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

    Egon Sohn, Xiaoxiang Xi, Shengwei Jiang, Zefang Wang, Kaifei Kang, Jie Shan & Kin Fai Mak

  2. Department of Physics and School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA

    Egon Sohn, Shengwei Jiang, Zefang Wang, Kaifei Kang, Jie Shan & Kin Fai Mak

  3. National Laboratory of Solid State Microstructures, School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, China

    Xiaoxiang Xi

  4. Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China

    Wen-Yu He & Kam Tuen Law

  5. National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL, USA

    Ju-Hyun Park

  6. Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

    Helmuth Berger & László Forró

  7. Kavli Institute at Cornell for Nanoscale Science, Ithaca, NY, USA

    Jie Shan & Kin Fai Mak

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Contributions

E.S., J.S. and K.F.M. designed the experiments. E.S. fabricated the devices and performed the measurements with the assistance of S.J., Z.W. and K.K., and of J.-H.P. at the NHMFL. X.X. contributed to all aspects of the experiment in its early phase. W.-Y.H. and K.T.L. performed the theoretical work. H.B. and L.F. synthesized the bulk NbSe2 crystals and screened the sample quality. E.S., W.H., K.T.L., J.S. and K.F.M. analysed the data and co-wrote the paper. All the authors discussed the results and commented on the manuscript.

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Correspondence to Jie Shan or Kin Fai Mak.

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Supplementary Sections 1–6, Supplementary Figures 1–16, Supplementary Table 1, Supplementary References

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Sohn, E., Xi, X., He, WY. et al. An unusual continuous paramagnetic-limited superconducting phase transition in 2D NbSe 2 . Nature Mater 17, 504–508 (2018). https://doi.org/10.1038/s41563-018-0061-1

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