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Séminaire spécial

Jeudi 8 décembre à 16h30,
Salle Louis Weil, E424

Orateur : Morten Eskildsen (université Notre Dame, USA)
"Direct Evidence for an Internal Degree of Freedom and Broken Time Reversal Symmetry in the B-phase of the Heavy- Fermion Superconductor UPt3"

Abstract

With three different superconducting mixed (vortex) phases the heavy-fermion material UPt3 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 E2u symmetry. This 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 UPt3 in the B-phase. Measurements of the VL scattered intensity for both H || a* and H || c allowed us to determine the temperature dependence of the London penetration depth along the principal directions of the crystal. This provides a measure of low-lying excitations in the superconducting state and reflects both the location and dispersion of the gap nodes. A comparison to calculations based on the different order parameter symmetries proposed for UPt3 showed a compelling agreement between the data and the calculation for the E2u symmetry over a wide temperature range [1].

Extending the SANS measurements with H || c we discovered 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. The two different configurations correspond to an order parameter chirality that is respectively in the same or opposite sense as the VL supercurrent circulation. Our results provide direct, bulk evidence for broken time reversal symmetry in the B-phase of UPt3 in non-zero fields.

1. W. J. Gannon et al., New J. Phys. 17, 023041 (2015).

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).

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