Charge transport at interfaces of HTS ramp-type junctions with PrBa2Cu3O7-δ barrier

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 HTS-applications require a high normal resistance of the junctions. Therefore PrBa2Cu3O7-δ (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 ICRN-products. This indicates a neglectable suppression of the junction properties due to the ion-beam milling process of the interface. The ICRN-products were in the range of 5-18 mV at 4.2 K. The junctions showed an additional conductance G∝U4/3 in the current-voltage characteristics at higher voltages and a temperature dependent normal conductance GN∝T4/3. 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. 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 10 nm the contribution of resonant tunneling cannot be neglected. (orig.)