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[en] The node structure of the superconducting order parameter of the heavy-fermion system UPd₂Al₃ is analyzed within the weak-coupling theory. A pairing interaction induced by the exchange of antiferromagnetic spin excitations is assumed as suggested by recent inelastic neutron scattering experiments and tunneling spectroscopy. The multi-sheeted Fermi surface is taken into account. Based on a model susceptibility for the simple antiferromagnetic structure of UPd₂Al₃, line nodes result at the rim of the magnetic Brillouin zone. (orig.)

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[en] Tunneling spectroscopy on planar Giaever-type junctions is a powerful tool for the investigation of the superconducting state of metals. Since it is possible to prepare high-quality epitaxial thin films of the heavy-fermion compound UPd₂Al₃, this method can be used to examine the energy gap of this presumably unconventional superconductor. We prepared cross-junctions consisting of a UPd₂Al₃ base electrode and a metal counter electrode (Au, Al or Ag). These small area contacts without artificial barriers have only low junction resistances and suffer from irreproducibility. On the other hand, on some of those junctions we observed BCS-like tunneling conductivity. In order to increase the junction resistances AlO_x or UO_x were used as a artificial barrier with Al as a counter-electrode. As an alternative to quasi-particle tunneling, the preparation of Josephson-junctions could be an easier approach. A supercurrent between a small In-dot and an UPd₂Al₃ film was observed. (orig.)

[en] Some time ago a dependence of the resistive superconducting transition temperature on the current direction in UNi₂Al₃ thin films was observed. Although a qualitative explanation can be given, fundamental questions about this phenomenon are still open, which motivated more detailed studies. Morphological investigations by AFM and STM seem to rule out a growth mechanism induced origin. By using a specially designed photomask we were able to measure the transition down to current densities of 0.1 A/cm² and observed no vanishing of the difference in T_c. However, the magnitude of the splitting of the sc transition varies from sample to sample without any correlation to the defect density as determined by the residual resistance ratio. V(I) characteristics in the superconducting state were measured at various magnetic fields, indicating anisotropic pinning. For the current, direction I parallel c a thermally activated flux flow region could be identified. Additional measurements with higher current densities prove that the observations in the relevant range are not generated by heating effects. The shift in T_c as a function of current density agrees well with the expectation for pairbreaking critical current from Ginzburg-Landau theory. (orig.)

[en] Results of superconducting tunneling spectroscopy experiments performed on two different types of thin film planar junctions of the heavy-fermion compound UPd₂Al₃ are presented. Cross-type junctions consist of the heavy-fermion base electrode, an insulating layer of native surface oxide and a metal counter electrode (Au or Ag). The contact resistance was only weakly temperature dependent down to the superconducting transition. In the superconducting regime a strongly reduced zero bias conductivity indicated the junction being of the superconductor-insulator-normal metal type. The observed tunneling density of states is clearly due to the superconducting energy gap of UPd₂Al₃ and reveals strong deviations from the typical BCS-result. The second type of junction consists of a thick insulater (SiO) covering the UPd₂Al₃ base electrode with a small hole (20 μm x 20 μm) which enables the metal counter electrode to contact the superconductor. The temperature-dependent contact resistance including a step-like reduction at the superconducting transition shows that these junctions are essentially of the superconductor-normal metal type. Their bias dependent conductivity is discussed in the framework of Andreev-reflection supplemented by critical current and heating effects. (orig.)