In a comparative Andreev-reflection study of normal-superconductor point contacts on the c plane of ${\mathrm{UPt}}_3$, ${\mathrm{CeCu}}_2$${\mathrm{Si}}_2$, and ${\mathrm{URu}}_2$${\mathrm{Si}}_2$ crystals at temperatures down to T=30 mK and in a magnetic field up to B=10 T, differences are found in the current-voltage characteristics. The behavior of the differential resistance dV/dI(V) of ${\mathrm{UPt}}_3$ is attributed to the presence of nodes in the superconducting order parameter Δ(k^) and is compared with theoretical predictions for the symmetry of the pairing state. The data for ${\mathrm{CeCu}}_2$${\mathrm{Si}}_2$ and ${\mathrm{URu}}_2$${\mathrm{Si}}_2$ point to the absence of zeros in Δ(k^).

• Received 13 October 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.2278