Two-fold symmetric superconductivity in few-layer NbSe2

Alex Hamill, Brett Heischmidt, Egon Sohn, Daniel Shaffer, Kan-Ting Tsai, Xi Zhang, Xiaoxiang Xi, Alexey Suslov, Helmuth Berger, László Forró, Fiona J. Burnell, Jie Shan, Kin Fai Mak, Rafael M. Fernandes, Ke Wang, Vlad S. Pribiag

Research output: Contribution to journalArticlepeer-review

Abstract

Two-dimensional transition metal dichalcogenides (TMDs) have been attracting significant interest due to a range of properties, such as layer-dependent inversion symmetry, valley-contrasted Berry curvatures, and strong spin-orbit coupling (SOC). Of particular interest is niobium diselenide (NbSe2), whose superconducting state in few-layer samples is profoundly affected by an unusual type of SOC called Ising SOC. Combined with the reduced dimensionality, the latter stabilizes the superconducting state against magnetic fields up to ~35 T and could lead to other exotic properties such as nodal and crystalline topological superconductivity. Here, we report transport measurements of few-layer NbSe$_2$ under in-plane external magnetic fields, revealing an unexpected two-fold rotational symmetry of the superconducting state. In contrast to the three-fold symmetry of the lattice, we observe that the magnetoresistance and critical field exhibit a two-fold oscillation with respect to an applied in-plane magnetic field. We find similar two-fold oscillations deep inside the superconducting state in differential conductance measurements on NbSe$_2$/CrBr$_3$ superconductor-magnet junctions. In both cases, the anisotropy vanishes in the normal state, demonstrating that it is an intrinsic property of the superconducting phase. We attribute the behavior to the mixing between two closely competing pairing instabilities, namely, the conventional s-wave instability typical of bulk NbSe$_2$ and an unconventional d- or p-wave channel that emerges in few-layer NbSe2. Our results thus demonstrate the unconventional character of the pairing interaction in a few-layer TMD, opening a new avenue to search for exotic superconductivity in this family of 2D materials.
Original languageUndefined/Unknown
JournalNature Physics
DOIs
StatePublished - Apr 15 2021

Bibliographical note

Funding Information:
We thank E.-A. Kim for useful discussions. B.H. and A.H. thank D. Graf and S. Maier for their discussions and support related to work done at the National High Magnetic Field Laboratory. Special thanks also go to Z. Jiang for all of the support associated with the Physical Property Measurement System at UMN. The work at the University of Minnesota (UMN) was supported primarily by the National Science Foundation through the University of Minnesota MRSEC, under Awards DMR-2011401 and DMR-1420013 (iSuperSeed). Portions of the UMN work were conducted in the Minnesota Nano Center, which is supported by the National Science Foundation through the National Nano Coordinated Infrastructure Network (NNCI) under award no. ECCS-1542202. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative agreement no. DMR-1644779 and the State of Florida. The research at Cornell was supported by the Office of Naval Research (ONR) under award no. N00014-18-1-2368 for the tunnelling measurements, and the National Science Foundation (NSF) under award no. DMR-1807810 for the fabrication of tunnel junctions. The work in Lausanne was supported by the Swiss National Science Foundation. K.F.M. also acknowledges support from a David and Lucille Packard Fellowship.

Publisher Copyright:
© 2021, The Author(s), under exclusive licence to Springer Nature Limited.

Keywords

  • cond-mat.supr-con
  • cond-mat.str-el

How much support was provided by MRSEC?

  • Primary

Reporting period for MRSEC

  • Period 7
  • Period 1

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