ANDREEV REFLECTION IN POINT CONTACTS BETWEEN THE HEAVY-FERMION SUPERCONDUCTOR UPT3 AND ORDINARY SUPERCONDUCTORS

The differential resistance dV/dI versus applied voltage V of UPt3-Zn point contacts at very low temperatures shows the double-minimum struc ture around V = 0 characteristic for Andreev reflection due to superco nducting Zn. In magnetic fields above B-c of Zn the dV/dI curves exhib it only a shallow minimum at V = 0 which is attributed to superconduct ing UPt3. A fit of the dV/dI curves on the basis of the Blonder-Tinkha m-Klapwijk theory taking into account a lifetime broadening Gamma of t he quasiparticle density of states yields for Zn an energy gap Delta(T ) perfectly following the BCS theory, a barrier strength Z similar to 0.45 and Gamma greater than or equal to 1/4 Delta(0). The usual Z valu e contradicts the notion of extremely high reflection coefficient of e lectrons at the interface with heavy-fermiom compounds due to their he avy masses. All this suggest that UPt3 behaves in the contacts as norm al metal, possibly with strong pair breaking taking into account the h igh Gamma value. This might be the reason for the failure to observe a Josephson coupling in contacts UPt3 with Zn, Pb, and NbTi.