SUPERCONDUCTING TUNNELING SPECTROSCOPY ON EPITAXIAL UPD2AL3 THIN-FILMS

Results of superconducting tunneling spectroscopy experiments performe d on two different types of thin film planar junctions of the heavy-fe rmion compound UPd2Al3 are presented. Cross-type junctions consist of the heavy-fermion base electrode, an insulating layer of native surfac e oxide and a metal counter electrode (Au or Ag). The contact resistan ce was only weakly temperature dependent down to the superconducting t ransition. In the superconducting regime a strongly reduced zero bias conductivity indicated the junction being of the superconductor-insula tor-normal metal type. The observed tunneling density of states is cle arly due to the superconducting energy gap of UPd2Al3 and reveals stro ng deviations from the typical BCS-result. The second type of junction consists of a thick insulater (SiO) covering the UPd2Al3 base electro de with a small hole (20 mu m x 20 mu m) which enables the metal count er electrode to contact the superconductor. The temperature-dependent contact resistance including a step-like reduction at the superconduct ing transition shows that these junctions are essentially of the super conductor-norma metal type. Their bias dependent conductivity is discu ssed in the framework of Andreev-reflection supplemented by critical c urrent and heating effects.