Superconducting tunneling spectroscopy on epitaxial UPd 2Al 3 thin films

Abstract

Results of superconducting tunneling spectroscopy experiments performed on two different types of thin film planar junctions of the heavy-fermion compound UPd 2Al 3 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 UPd 2Al 3 and reveals strong deviations from the typical BCS-result. The second type of junction consists of a thick insulater (SiO) covering the UPd 2Al 3 base electrode with a small hole (20 μm × 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.