Twofold spontaneous symmetry breaking in a heavy fermion superconductor   UPt$_3$
  The field-orientation dependent thermal conductivity of the heavy-fermion superconductor UPt$_3$ was measured down to very low temperatures and under magnetic fields throughout three distinct superconducting phases: A, 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 UPt$_3$ is conclusively determined as a $E_{1u}$ representation characterized by a combination of two line nodes at the tropics and point nodes at the poles. 
Y. Machida, A. Itoh, Y. So, K. Izawa, Y. Haga, E. Yamamoto, N. Kimura,   Y. Onuki, Y. Tsutsumi, K. Machida
cond-mat.supr-con cond-mat.str-el