Event Details

With three different superconducting mixed (vortex) phases the heavy-fermion material **UPt_3** can be considered a paradigm for unconventional superconductivity. Despite more than three decades of study, a definitive understanding of the superconducting state in this material has remained elusive. The order parameter structure that is consistent with a number of experiments is an odd-parity, f-wave orbital state of **E_{2u}** symmetry. Here the order parameter is chiral and breaks time reversal symmetry in the low-temperature superconducting B-phase. We have performed small-angle neutron scattering (SANS) studies of the vortex lattice (VL) in **UPt_3** in the B- and C-phases with H // c. This led to the discovery of a previously unknown field-induced VL rotation in the B-phase. Furthermore, the magnitude of the VL rotation show a subtle magnetic field history dependence; VLs prepared with the field parallel or anti-parallel with respect to initial direction with which one enters the B-phase are rotated by different amounts. This demonstrates an internal degree of freedom associated with the vortex cores, and provides direct evidence for broken time reversal symmetry in the B-phase of UPt3 in non-zero fields by a bulk measurement. We propose that this correspond to an order parameter chirality that is respectively in the same or opposite sense as the VL supercurrent circulation. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Awards DE-FG02-10ER46783 (University of Notre Dame; neutron scattering) and DE-FG02-05ER46248 (Northwestern University; crystal growth and characterization).