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. 6–8) 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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|Published - May 1 2020
Bibliographical noteFunding Information:
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.
© 2020, The Author(s), under exclusive licence to Springer Nature Limited.
ASJC Scopus subject areas
- Physics and Astronomy (all)