Ladungstransport an Grenzflächen von HTSL-Rampenkontakten mit PrBa$_{2}$Cu$_{3}$O$_{7-\delta}$-Barriere

The Josephson junction is the most important basic device in active superconductor electronics. Ramp-type junctions are used in high-temperature-superconductor (HTS) devices because the fabrication process of these junctions is compatible with a complex multilayer technology and provides a high design flexibility. Most EFTS-applications require a high normal resistance of the junctions. Therefore PrBa$_{2}$Cu$_{3}$O$_{7-\delta}$ (PBCO) can be used as the barrier material with a high resistivity. In this work, the charge transport at the interfaces of the barrier to the superconducting electrodes was investigated. To compare the influence of the degradation during the ion-beam milling process of the ramp, a second fabrication process was used for an in-situ preparation of the junctions in the same deposition system. The junctions with a PBCO barrier fabricated by these two techniques showed comparable current-voltage characteristics and I$_{C}$R$_{N}$-products. This indicates a neglectable suppression of the junction properties due to the ion-beam milling process of the interface. The I$_{C}$R$_{N}$-products were in the range of 5 - 18m V at 4.2K. The junctions showed an additional conductance G$\propto$U$^{4/3}$ in the current-voltage characteristics at higher voltages and a temperature dependent normal conductance G$_{N}\propto$T$^{1/3}$. Therefore the quasiparticle current can be well described by resonant tunneling via one and two localized states in the barrier. This was already observed by other groups [Sat95][Ver96b]. The transport of the supercurrent by resonant tunneling via localized states is only possible by neglecting the Coulomb repulsion between the two electrons of the Cooper pairs at the localized state. But for barrier thicknesses of several 10nm the contribution of resonant tunneling cannot be neglected. The current-voltage characteristics showed an excess-current which cannot be described by the RCSJ-Model. This deviation can result from Andreev reflections at the interfaces. By measuring the differential conductance of the junctions, a zero-bias conductance peak could be observed. In a magnetic field the amplitude of the peak is suppressed and the width is broadened. This can be related to surface bound states, which are due to the d-wave symmetry of the superconducting order parameter. These states split in a magnetic field due to the interacting with surface currents. The splitting saturates at a magnetic field of about 1.5T and the linear dependency for larger fields cannot be extrapolated to zero splitting at zero field. This behavior was recently explained by Fogelström et al. with spontaneous surface currents generated by a sub-dominant order parameter which is stabilized by the surface [Fog97]. The conductance measurements clearly showed a subharmonic gap structure. These structure can be explained within a two gap model including multiple Andreev reflections at the interfaces. The values of the two gaps are in the range of $\Delta_{N}$=13-17meV and $\Delta_{s}$=21-26meV which is in good agreement with the results of Polturak et al. [Pol93]. The origin of the second smaller gap is not clearly identified. One possibility is a suppression of the order parameter close to the interface. The two gaps at the interface seem to be a more intrinsic property and are not caused by damaging during processing because the same gap values are observed at junctions fabricated with different processes. Another possibility is a second gap related to the crystallographic anisotropy of the YBCO superconductor and a resulting anisotropy of the order parameter along the crystallographic axes. In conclusion, the important role of Andreev reflections at the interfaces has been shown in the measurements of the stationary and the dynamic properties. A two gap model and multiple Andreev reflections can describe the subharmonic gap structure which was observed. Assuming a supercurrent transport by resonant tunneling via localized states, the linear temperature dependency of the critical current can be explained within the two gap model