Twofold spontaneous symmetry breaking in the heavy-fermion superconductor UPt3.

Department of Physics, Tokyo Institute of Technology, Meguro 152-8551, Japan.
Physical Review Letters (Impact Factor: 7.94). 04/2012; 108(15):157002. DOI: 10.1103/PhysRevLett.108.157002
Source: PubMed

ABSTRACT The field-orientation dependent thermal conductivity of the heavy-fermion superconductor UPt3 was measured down to very low temperatures and under magnetic fields throughout the distinct superconducting phases: B and C phases. In the C phase, a striking twofold oscillation of the thermal conductivity within the basal plane is resolved reflecting the superconducting gap structure with a line of node along the a axis. Moreover, we find an abrupt vanishing of the oscillation across a transition to the B phase, as a clear indication of a change of gap symmetries. We also identify extra two line nodes below and above the equator in both B and C phases. From these results together with the symmetry consideration, the gap function of UPt3 is determined as a E(1u) representation characterized by a combination of two line nodes at the tropics and point nodes at the poles.

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    ABSTRACT: We have investigated the field-angle variation of the specific heat C(H, phi, theta) of the heavy-fermion superconductor UPt3 at low temperatures T down to 50 mK, where phi and theta denote the azimuthal and polar angles of the magnetic field H, respectively. For T = 88 mK, C(H, theta=90) increases proportionally to H^{1/2} up to nearly the upper critical field Hc2, indicating the presence of line nodes. By contrast, C(H, theta=0) deviates upward from the H^{1/2} dependence for (H/Hc2)^{1/2} > 0.5. This behavior can be related to the suppression of Hc2 along the c direction, whose origin has not been resolved yet. Our data show that the unusual Hc2 limit becomes marked only when theta is smaller than 30. In order to explore the possible vertical line nodes in the gap structure, we measured the phi dependence of C in wide T and H ranges. However, we did not observe any in-plane angular oscillation of C within the accuracy of dC/C~0.5%. This result implies that field-induced excitations of the heavy quasiparticles occur isotropically with respect to phi, which is apparently contrary to the recent finding of a twofold thermal-conductivity oscillation.
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