Measurement of the Josephson effect of heavy-fermion superconductor UPt ₃ as a test of the odd-parity order parameter

The Josephson critical current I _c between a single-crystal UPt3 and Al has been measured for the junctions on the UPt3 surface perpendicular to the hexagonal a[1120], b[1010], and c[0001] axes. The magnetic field dependence of I _c shows a Fraunhofer diffraction pattern for a current flow parallel to the c-axis, suggesting that the Josephson coupling is uniform throughout the junction. Small modifications applied to the Fraunhofer pattern have been observed for a current flow parallel to the a- and b-axes. These results contradict the E _2u scenario for the odd-parity order parameter in UPt ₃, in which the Josephson effect is forbidden in the three directions, whereas the recently proposed E1 _u scenario permits, at least, the a- and c-axis Josephson couplings.