Andreev reflections on the heavy-fermion superconductor UPt3

The superconducting heavy fermion system UPt3 has been studied by means of point contacts adjusted with a bimorph stepping motor working in the mixing chamber of a dilution refrigerator (T ≥ 30 mK). The measured differential resistance curves of Ag/UPt3 point contacts are compared with the usual isotropic BTK model describing the Andreev-reflection process of quasiparticles in normal/superconductor point contacts. For this heavy-fermion superconductor, the experimental data do not fit at all with the predictions of this model. An adaptation of the BTK theory to the case of anisotropic superconductors has been performed and demonstrates that our data can be interpreted in terms of the hybrid symmetry Δk = Δ0kz(kx + iky), if one assumes a preferential injection of the electrons in the forward direction. (orig.)