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

Temperature and angular resolved measurements of the upper critical field Bc2 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 Tc, B'c2 = -4.9 T/K for B parallel 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 Bc2 near T* ∼ 0.75T/Tc, the calculations include anisotropic orbital and paramagnetic pair breaking. Recent speculations upon the occurence 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 c and B perpendicular to c in the out-of-plane angular dependence of the upper critical field Bc2(θ) 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. (orig.)