Twofold spontaneous symmetry breaking in the heavy-fermion superconductor UPt

Machida, Yo*; Ito, Atsushi*; So, Yoshinao*; Izawa, Koichi*; Haga, Yoshinori   ; Yamamoto, Etsuji  ; Kimura, Noriaki*; Onuki, Yoshichika; Tsutsumi, Yasumasa*; Machida, Kazushige*

The field-orientation dependent thermal conductivity of the heavy-fermion superconductor UPt was measured down to very low temperatures and under magnetic fields throughout the distinct superconducting phases: and phases. In the 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 axis. From this result together with the symmetry consideration, the gap function of UPt is determined as a representation characterized by a combination of two line nodes at the tropics and point nodes at the poles.