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 d ifferential resistance of heterocontacts between UPt3 and Pt shows, fo r current flow parallel to the c direction, a distinct minimum at a lo w bias due to Andreev reflection. From the temperature and magnetic-fi eld dependence of this structure, we infer that Andreev reflection is observable only in the low-temperature, low-magnetic-field phase of th is unconventional superconductor, giving clear evidence of different o rder parameters in the different superconducting phases.