Unexpected two-fold symmetric superconductivity in few-layer NbSe$_2$

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 journalArticle

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
JournalarXiv
StateSubmitted - Apr 6 2020

Bibliographical note

19 pages, 4 figures plus supplementary material

Keywords

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

How much support was provided by MRSEC?

  • Primary

Reporting period for MRSEC

  • Period 7

Projects

University of Minnesota MRSEC (DMR-1420013)

Lodge, T. P.

11/1/1410/31/20

Project: Research project

MRSEC SEED Projects

11/1/14 → …

Project: Research project

Cite this

Hamill, A., Heischmidt, B., Sohn, E., Shaffer, D., Tsai, K-T., Zhang, X., Xi, X., Suslov, A., Berger, H., Forró, L., Burnell, F. J., Shan, J., Mak, K. F., Fernandes, R. M., Wang, K., & Pribiag, V. S. (2020). Unexpected two-fold symmetric superconductivity in few-layer NbSe$_2$. Manuscript submitted for publication.