TEMPERATURE AND ANGULAR-DEPENDENCE OF THE UPPER CRITICAL-FIELD OF UPD2AL3 THIN-FILMS

Temperature and angular resolved measurements of the upper critical fi eld B-c2 of UPd2Al3 thin films are presented and compared with new the oretical calculations for weak-coupling s- and d-wave superconductors in the clean limit. In order to take account of the observed anisotrop y of the initial slopes near T-c, B-c2' = -4.9 T/K for B parallel to c and B-c2' = -5.8 T/K for B perpendicular to c, and the crossing of th e phase boundaries in the temperature dependence of B-c2 near T appro ximate to 0.75T/T-c, the calculations include anisotropic orbital and paramagnetic pair breaking. Recent speculations upon the occurrence of a non-uniform superconducting ''Fulde-Ferrell-Larkin-Ovchinnikov''-st ate are also considered and incorporated in the calculations. In the r egion around T a minimum between B parallel to c and B perpendicular to c in the out-of-plane angular dependence of the upper critical fiel d B-c2(Theta) is found experimentally. This cannot be explained by an s-wave order parameter and reduces drastically the possible d-wave fun ctions. To explain the overall data, a mixing of s-wave and a particul ar d-wave order parameter is proposed.