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Ising pairing in superconducting NbSe2 atomic layers

Nature Physics volume 12, pages 139–143 (2016)Cite this article

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Abstract

The properties of two-dimensional transition metal dichalcogenides arising from strong spin–orbit interactions and valley-dependent Berry curvature effects have recently attracted considerable interest1,2,3,4,5,6,7. Although single-particle and excitonic phenomena related to spin–valley coupling have been extensively studied1,3,4,5,6, the effects of spin–valley coupling on collective quantum phenomena remain less well understood. Here we report the observation of superconducting monolayer NbSe2 with an in-plane upper critical field of more than six times the Pauli paramagnetic limit, by means of magnetotransport measurements. The effect can be interpreted in terms of the competing Zeeman effect and large intrinsic spin–orbit interactions in non-centrosymmetric NbSe2 monolayers, where the electron spin is locked to the out-of-plane direction. Our results provide strong evidence of unconventional Ising pairing protected by spin–momentum locking, and suggest further studies of non-centrosymmetric superconductivity with unique spin and valley degrees of freedom in the two-dimensional limit.

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Figure 1: Spin–momentum and spin–layer locking in NbSe2.
Figure 2: Layer number dependence of superconductivity in NbSe2.
Figure 3: Superconductivity of bulk, trilayer and monolayer NbSe2 under a magnetic field.
Figure 4: H–T superconducting phase diagram for atomically thin NbSe2.

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Acknowledgements

We thank M. H. W. Chan for fruitful discussions. This work was supported by the US Department of Energy, Office of Basic Energy Sciences (contract No. DESC0013883 (K.F.M.) and DESC0012635 (J.S.)). Optical spectroscopy was supported by the National Science Foundation (NSF) under Award No. DMR-1410407. The NHMFL is supported by the NSF Cooperative Agreement No. DMR-1157490 and the State of Florida. K.T.L. is supported by HKUST3/CRF/13G and the Croucher Innovation Grant. The work in Lausanne was supported by the Swiss National Science Foundation. We also acknowledge support from the NSF MRSEC under Award No. DMR-1420451 (Z.W.) and the MRI-2D Center at Penn State University (X.X.).

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Author notes

  1. Xiaoxiang Xi and Zefang Wang: These authors contributed equally to this work.

Authors and Affiliations

  1. Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802-6300, USA

    Xiaoxiang Xi, Zefang Wang, Weiwei Zhao, Jie Shan & Kin Fai Mak

  2. National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA

    Ju-Hyun Park

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

    Kam Tuen Law

  4. Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland

    Helmuth Berger & László Forró

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Contributions

J.S. and K.F.M. designed the experiments. X.X. and Z.W. performed the experiments with the assistance of W.Z. at the Penn State Physics Low Temperature Laboratory, and of J.-H.P. at the National High Magnetic Field Laboratory. X.X., Z.W., J.S. and K.F.M. analysed the data and co-wrote the paper. K.T.L. contributed to the interpretation of the results. H.B. and L.F. contributed NbSe2 crystals. All 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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Xi, X., Wang, Z., Zhao, W. et al. Ising pairing in superconducting NbSe2 atomic layers. Nature Phys 12, 139–143 (2016). https://doi.org/10.1038/nphys3538

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