Surface superconductivity in the heavy-fermion superconductor UPt3

We present a study of the surface critical field H-c3(Phi,Theta,T) measured for two needlelike whiskers of UPt3. Dominant surface effects were observed in the angular dependence of the critical field by means of ac-resistivity measurements. These surface superconductivity effects show a surprisingly nonlinear thermal variation of H-c3 contrary to behavior expected from conventional theory, where H-c3/H-c2 = 1.69 is predicted. The ratio H-c3/H-c2 is strongly depressed from its initial value 1.7 when going from the A to the C phase as the temperature is decreased. It seems to remain constant in the C phase for even lower T. Nevertheless, for temperatures close to T-c+ it is possible to describe the angular behavior of H-c3(Theta,Phi) with a standard model by introducing an effective-mass anisotropy of the heavy quasiparticles. These results are compared to recent H-c3 calculations for different representations of the order parameter and seem to provide a direct evidence for the suppression of one component of the order parameter at the surface. The restrictions imposed by these measurements on the choice of the representations of the unconventional order parameter will be discussed by also taking into account the limitations imposed due to the temperature dependence of the basal plane H-c2 modulation.