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Broken time-reversal symmetry in the topological superconductor UPt3

Nature Physics volume 16, pages 531–535 (2020)Cite this article

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Abstract

Topological properties of materials are of fundamental as well as practical importance1,2. Of particular interest are unconventional superconductors that break time-reversal symmetry, for which the superconducting state is protected topologically and vortices can host Majorana fermions with potential use in quantum computing3,4. However, in striking contrast to the unconventional A phase of superfluid 3He where chiral symmetry was directly observed5, identification of broken time-reversal symmetry of the superconducting order parameter, a key component of chiral symmetry, has presented a challenge in bulk materials. The two leading candidates for bulk chiral superconductors are UPt3 (refs. 6,7,8) and Sr2RuO4 (ref. 9), although evidence for broken time-reversal symmetry comes largely from surface-sensitive measurements. A long-sought demonstration of broken time-reversal symmetry in bulk Sr2RuO4 is the observation of edge currents, which has so far not been successful10. The situation for UPt3 is not much better. Here, we use vortices to probe the superconducting state in ultraclean crystals of UPt3. Using small-angle neutron scattering, a strictly bulk probe, we demonstrate that the vortices possess an internal degree of freedom in one of its three superconducting phases, providing direct evidence for bulk broken time-reversal symmetry in this material.

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Fig. 1: A UPt3 phase diagram indicating the three vortex phases (A, B and C) for H ∥ c.
Fig. 2: VL diffraction patterns.
Fig. 3: The field dependence of the VL configuration.
Fig. 4: The B–C phase transition determined from the onset of VL disordering.

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Data availability

The SANS data obtained at ILL that support the findings of this study are available in the ILL repository44. All SANS data were analysed using the GRASP software package45. The data represented in Figs. 3 and 4 and Extended Data Fig. 2 are available as source data. All other data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This work was supported by the US Department of Energy, Office of Basic Energy Sciences, under Award Nos. DE-SC0005051 (M.R.E.: University of Notre Dame; neutron scattering) and DE-FG02-05ER46248 (W.P.H.: Northwestern University; crystal growth and neutron scattering), and the Center for Applied Physics and Superconducting Technologies (J.A.S.: Northwestern University; theory). The research of J.A.S. is supported by National Science Foundation Grant DMR-1508730. A portion of this research used resources at the High Flux Isotope Reactor, a US DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. Part of this work is based on experiments performed at the Institut Laue–Langevin, Grenoble, France and at the Swiss Spallation Neutron Source SINQ, Paul Scherrer Institute, Villigen, Switzerland.

Author information

Author notes

  1. W. J. Gannon

    Present address: Department of Physics and Astronomy, University of Kentucky, Lexington, KY, USA

  2. S. J. Kuhn

    Present address: Center for Exploration of Energy & Matter, Indiana University, Bloomington, IN, USA

  3. G. Nagy

    Present address: Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

Authors and Affiliations

  1. Department of Physics and Astronomy, Northwestern University, Evanston, IL, USA

    K. E. Avers, W. J. Gannon, W. P. Halperin & J. A. Sauls

  2. Center for Applied Physics & Superconducting Technologies, Northwestern University, Evanston, IL, USA

    K. E. Avers, W. P. Halperin & J. A. Sauls

  3. Department of Physics, University of Notre Dame, Notre Dame, IN, USA

    S. J. Kuhn & M. R. Eskildsen

  4. Large Scale Structures Group, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, TN, USA

    L. DeBeer-Schmitt

  5. Institut Laue-Langevin, Grenoble, France

    C. D. Dewhurst

  6. Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, Villigen, Switzerland

    J. Gavilano, G. Nagy & U. Gasser

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Contributions

W.J.G., W.P.H., J.A.S. and M.R.E. conceived the experiment. W.J.G. grew the crystals. K.E.A., W.J.G., S.J.K., W.P.H. and M.R.E. performed the SANS experiments with assistance from L.D.-S., C.D.D., J.G., G.N. and U.G. W.P.H., J.A.S. and M.R.E. wrote the paper with input from all authors.

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Correspondence to M. R. Eskildsen.

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Extended data

Extended Data Fig. 1 Vortex lattice diffraction patterns.

a-f, Vortex lattice diffraction pattern for a range of different field magnitudes following a field reduction. g-l, Same, but following a field reversal. The VL domain splitting (ω) is indicated in each panel. Measurements were performed at a single angular setting, satisfying the Bragg condition for VL reflections at the top of the detector. Zero field background scattering is subtracted, and the detector center near Q = 0 is masked off.

Extended Data Fig. 2 Vortex density determined from the magnitude of the VL scattering vectors.

a, Magnetic induction vs applied magnetic field. The inset shows VL Bragg peaks, their corresponding scattering vectors (Q1∕2), and the opening angle (β). Peaks associated with the two different VL domain orientations are indicated by full and dashed lines respectively. Only Bragg peaks indicated by solid symbols were imaged by SANS. The line is a linear fit to the data. b, Ratio of magnetic induction to applied field (B/H). The center and width of the gray bar indicates the average and standard deviation of the values for fields ≥ 0.4 T.

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Supplementary Figs. 1 and 2 and discussion.

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Avers, K.E., Gannon, W.J., Kuhn, S.J. et al. Broken time-reversal symmetry in the topological superconductor UPt3. Nat. Phys. 16, 531–535 (2020). https://doi.org/10.1038/s41567-020-0822-z

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