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

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.

References

  1. 1.

    Bauer, E. & Sigrist, M. Non-centrosymmetric Superconductors: Introduction and Overview (Springer, Berlin, Heidelberg, 2012).

    Book  Google Scholar 

  2. 2.

    Bauer, E. et al. Heavy fermion superconductivity and magnetic order in noncentrosymmetric CePt3Si. Phys. Rev. Lett. 92, 027003 (2004).

    Article  Google Scholar 

  3. 3.

    Yogi, M. et al. Evidence for novel pairing state in noncentrosymmetric superconductor CePt3Si: 29Si-NMR knight shift study. J. Phys. Soc. Jpn. 75, 013709 (2006).

    Article  Google Scholar 

  4. 4.

    Yip, S. Noncentrosymmetric superconductors. Annu. Rev. Condens. Matter Phys. 5, 15–33 (2014).

    Article  Google Scholar 

  5. 5.

    Smidman, M., Salamon, M. B., Yuan, H. Q. & Agterberg, D. F. Superconductivity and spin–orbit coupling in non-centrosymmetric materials: a review. Rep. Progress. Phys. 80, 036501 (2017).

    Article  Google Scholar 

  6. 6.

    Cao, Y. et al. Quality heterostructures from two-dimensional crystals unstable in air by their assembly in inert atmosphere. Nano Lett. 15, 4914–4921 (2015).

    Article  Google Scholar 

  7. 7.

    Tsen, A. W. et al. Nature of the quantum metal in a two-dimensional crystalline superconductor. Nat. Phys. 12, 208–212 (2016).

    Article  Google Scholar 

  8. 8.

    Ugeda, M. M. et al. Characterization of collective ground states in single-layer NbSe2. Nat. Phys. 12, 92–97 (2016).

    Article  Google Scholar 

  9. 9.

    Xi, X. et al. Ising pairing in superconducting NbSe2 atomic layers. Nat. Phys. 12, 139–143 (2016).

    Article  Google Scholar 

  10. 10.

    Meservey, R. & Tedrow, P. M. Spin-polarized electron-tunneling. Phys. Rep. 238, 173–243 (1994).

    Article  Google Scholar 

  11. 11.

    Meservey, R., Tedrow, P. M. & Bruno, R. C. Tunneling measurements on spin-paired superconductors with spin–orbit scattering. Phys. Rev. B 11, 4224–4235 (1975).

    Article  Google Scholar 

  12. 12.

    Adams, P. W., Herron, P. & Meletis, E. I. First-order spin-paramagnetic transition and tricritical point in ultrathin Be films. Phys. Rev. B 58, R2952–R2955 (1998).

    Article  Google Scholar 

  13. 13.

    Zhou, B. T., Yuan, N. F., Jiang, H.-L. & Law, K. T. Ising superconductivity and Majorana fermions in transition-metal dichalcogenides. Phys. Rev. B 93, 180501 (2016).

    Article  Google Scholar 

  14. 14.

    Yuan, N. F., Mak, K. F. & Law, K. T. Possible topological superconducting phases of MoS2. Phys. Rev. Lett. 113, 097001 (2014).

    Article  Google Scholar 

  15. 15.

    He, W.-Y. et al. Magnetic field driven nodal topological superconductivity in monolayer transition metal dichalcogenides. Preprint at https://arxiv.org/abs/1604.02867 (2016).

  16. 16.

    Hsu, Y.-T., Vaezi, A., Fischer, M. H. & Kim, E.-A. Topological superconductivity in monolayer transition metal dichalcogenides. Nat. Commun. 8, 14985 (2017).

    Article  Google Scholar 

  17. 17.

    Tinkham, M. Introduction to Superconductivity (McGraw-Hill, New York, NY, 2004).

    Google Scholar 

  18. 18.

    Liu, C. X. Unconventional superconductivity in bilayer transition metal dichalcogenides. Phys. Rev. Lett. 118, 087001 (2017).

    Article  Google Scholar 

  19. 19.

    Frigeri, P. A., Agterberg, D. F. & Sigrist, M. Spin susceptibility in superconductors without inversion symmetry. New J. Phys. 6, 115 (2004).

    Article  Google Scholar 

  20. 20.

    Gor’kov, L. P. & Rashba, E. I. Superconducting 2D system with lifted spin degeneracy: mixed singlet–triplet state. Phys. Rev. Lett. 87, 037004 (2001).

    Article  Google Scholar 

  21. 21.

    Yip, S. K. Two-dimensional superconductivity with strong spin–orbit interaction. Phys. Rev. B 65, 144508 (2002).

    Article  Google Scholar 

  22. 22.

    Wakatsuki, R. & Law, K.T. Proximity effect and Ising superconductivity in superconductor/transition metal dichalcogenide heterostructures. Preprint at https://arxiv.org/abs/1604.04898 (2016).

  23. 23.

    Lu, J. M. et al. Evidence for two-dimensional Ising superconductivity in gated MoS2. Science 350, 1353 (2015).

    Article  Google Scholar 

  24. 24.

    Saito, Y. et al. Superconductivity protected by spin–valley locking in ion-gated MoS2. Nat. Phys. 12, 144–149 (2015).

    Article  Google Scholar 

  25. 25.

    Nam, H. et al. Ultrathin two-dimensional superconductivity with strong spin–orbit coupling. Proc. Natl Acad. Sci. USA 113, 10513–10517 (2016).

    Article  Google Scholar 

  26. 26.

    Xi, X. et al. Strongly enhanced charge–density–wave order in monolayer NbSe2. Nat. Nanotec. 10, 765–769 (2015).

    Article  Google Scholar 

  27. 27.

    Daghero, D. & Gonnelli, R. S. Probing multiband superconductivity by point-contact spectroscopy. Supercond. Sci. Technol. 23, 043001 (2010).

    Article  Google Scholar 

  28. 28.

    Webb, G. W., Marsiglio, F. & Hirsch, J. E. Superconductivity in the elements, alloys and simple compounds. Phys. C 514, 17–27 (2015).

    Article  Google Scholar 

  29. 29.

    Bianchi, A. et al. First-order superconducting phase transition in CeCoIn5. Phys. Rev. Lett. 89, 137002 (2002).

    Article  Google Scholar 

  30. 30.

    Radovan, H. A. et al. Magnetic enhancement of superconductivity from electron spin domains. Nature 425, 51–55 (2003).

    Article  Google Scholar 

  31. 31.

    Lortz, R. et al. Calorimetric evidence for a Fulde–Ferrell–Larkin–Ovchinnikov superconducting state in the layered organic superconductor κ-(BEDT-TTF)2Cu(NCS)2. Phys. Rev. Lett. 99, 187002 (2007).

    Article  Google Scholar 

  32. 32.

    Zocco, D. A., Grube, K., Eilers, F., Wolf, T. & von Löhneysen, H. Pauli-limited multiband superconductivity in KFe2As2. Phys. Rev. Lett. 111, 057007 (2013).

    Article  Google Scholar 

  33. 33.

    Kiss, T. et al. Charge-order-maximized momentum-dependent superconductivity. Nat. Phys. 3, 720–725 (2007).

    Article  Google Scholar 

Download references

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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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 Information

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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