Temperature and angular dependence of the upper critical field of UPd2Al3 thin films

Temperature and angular resolved measurements of the upper critical field B-c2 of UPd2Al3 thin films are presented and compared with new theoretical calculations for weak-coupling s- and d-wave superconductors in the clean limit. In order to take account of the observed anisotropy 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 the phase boundaries in the temperature dependence of B-c2 near T* approximate 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''-state are also considered and incorporated in the calculations. In the region 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 field B-c2(Theta) is found experimentally. This cannot be explained by an s-wave order parameter and reduces drastically the possible d-wave functions. To explain the overall data, a mixing of s-wave and a particular d-wave order parameter is proposed.