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Discovery of broken time-reversal symmetry in the topological superconductor UPt3

March 10, 2020

Keenan Avers (lead author), William Halperin (lead PI) and Jim Sauls (PI), in collaboration with the neutron scattering team led by Morten Eskildsen at Notre Dame, report the discovery of


Broken time-reversal symmetry in the topological superconductor UPt3


  1. E. Avers, W. J. Gannon, S. J. Kuhn, W. P. Halperin, J. A. Sauls, L. DeBeer-Schmitt, C. D. Dewhurst, J. Gavilano, G. Nagy, U. Gasser & M. R. Eskildsen


Published: March 9, 2020 in Nature Physics, online at:


Topological properties of materials are of fundamental as well as practical importance. 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 computing. In striking contrast to the A phase of superfluid 3He where chiral symmetry was directly observed in electron mobility experiments, the identification of broken time-reversal symmetry of the superconducting order parameter, a key feature of chiral symmetry, has eluded detection by measurements in bulk superconducting materials. The two leading candidates for chiral superconductivity are UPt3 and Sr2RuO4. Although evidence for broken time-reversal symmetry is suggested by surface-sensitive measurements in SrRuO4 and UPt3, a long-sought demonstration of broken time-reversal symmetry in the bulk of these materials has not been achieved until now. We report experiments using vortices to probe the superconducting state in ultraclean crystals of UPt3. Small-angle neutron scattering, a bulk probe, provides a signature of the chirality. Vortices possess an internal degree of freedom in the low temperature B phase of UPt3, providing direct evidence for bulk broken time-reversal symmetry in this material.

 N. B. Experiments were carried out at ILL in Grenoble, PSI in Switzerland and Oak Ridge National Lab in the US. The idea for this experiment was developed at Northwestern. The ultra-high purity single crystals that made this experiment possible were grown and processed at Northwestern. The lead investigator making measurements was Keenan Avers, a physics PhD student in Halperin's group supported by CAPST. Morten Eskildsen from Notre Dame is a leading expert on small angle neutron scattering (SANS), the key technique to study superconducting vortices.