POINT-CONTACT SPECTROSCOPY ON THE HEAVY-FERMION SUPERCONDUCTOR UPT3

Point-contact spectroscopy has been used to study the superconducting properties of the heavy-fermion system UPt3 down to about 40 mK. The differential resistance of heterocontacts between UPt3 and Pt shows, for current flow parallel to the c direction, a distinct minimum at a low bias due to Andreev reflection. From the temperature and magnetic-field dependence of this structure, we infer that Andreev reflection is observable only in the low-temperature, low-magnetic-field phase of this unconventional superconductor, giving clear evidence of different order parameters in the different superconducting phases.