title: Twofold spontaneous symmetry breaking in the heavy-fermion superconductor UPt3.
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.
authors.affiliation: Department of Physics, Tokyo Institute of Technology, Meguro 152-8551, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Machida
authors.forename: A
authors.initials: A
authors.lastname: Itoh
authors.forename: Y
authors.initials: Y
authors.lastname: So
authors.forename: K
authors.initials: K
authors.lastname: Izawa
authors.forename: Y
authors.initials: Y
authors.lastname: Haga
authors.forename: E
authors.initials: E
authors.lastname: Yamamoto
authors.forename: N
authors.initials: N
authors.lastname: Kimura
authors.forename: Y
authors.initials: Y
authors.lastname: Onuki
authors.forename: Y
authors.initials: Y
authors.lastname: Tsutsumi
authors.forename: K
authors.initials: K
authors.lastname: Machida