Interface microstructure in epitaxial YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox ramp-type Josephson junctions

The interfaces in ramp-type YBa₂Cu₃O_x/PrBa₂Cu₃O_x/YBa₂Cu₃O_x Josephson junctions with different ramp slopes were characterized electrically and structurally. From the cross-sectional TEM images, it has been found that the epitaxy remains through all layers even on the ramp surface, providing the ramp angle is less than 45°. No big defects such as grain boundaries and secondary phases occurred at the interface. The major defects which appeared at the interface are antiphase boundaries which were formed by lattice shifts with c/3 unit between PrBa₂Cu₃O_x and YBa₂Cu₃O_x. The interface between YBa₂Cu₃O_x base-layer and PrBa₂Cu₃O_x barrier is more defective in comparison with that between PrBa₂Cu₃O_x barrier and top-layer. Alteration of the ramp slope from 10° to 45° leads to a significant increase in the density of defects near the base-YBa₂Cu₃O_x/PrBa₂Cu₃O_x interface. The average roughness of the ramp surface created by ion milling is about 10 nm, which seems insensitive to the ramp slope. The damaged ramp surface could be repaired during the subsequent deposition process, leading to a negligible interface resistance.     

Vortex-creep activation energy in YBa2Cu3O7/PrBa2Cu3O7 superlattices

YBa₂Cu₃O₇/PrBa₂Cu₃O₇ (YBCO/PBCO) superlattices with a different ratio of the superconducting and insulating layer thicknesses were prepared by high pressure dc sputtering. The vortex-creep activation energy U₀ was determined by analyzing the in-plane resistive transition of 200 μm wide bridges with the external magnetic field B oriented along the c axis. It was found that U₀ is proportional to the thickness of the YBCO layers, and does only weakly depend on the PBCO layer thickness, when the latter exceeds two unit cells. We observed a change in the variation of U₀ with the current I in the specimen: U₀ exhibits a plateau in the low-I region, then decreases significantly with increasing I. This behaviour is explained in terms of a crossover plastic vortex creep – elastic (collective) creep induced by the transport current.     

Ac-Josephson effect in YBa2Cu3O7/YBa2Cu3O7 point contact junctions

The ac-Josephson effect could be demonstrated up to dc voltages of 20 mV on the I–V characteristic of adjustable YBa₂Cu₃O₇/YBa₂Cu₃O₇ point contacts at 4.2 K. A detailed analysis of the rf power dependence of the size of microwave-induced constant voltage steps is given for microwave frequencies of 70 and 90 GHz. The results prove that electron pairing is responsible for the superconductivity in YBa₂Cu₃O₇.     

Macroscopic quantum tunneling in d-wave YBa2Cu3O7-delta Josephson junctions

The escape rate from the zero voltage state in a superconducting Josephson junction (JJ) is determined by the temperature, but it saturates at low temperature due to macroscopic quantum tunneling (MQT). Complications due to d-wave symmetry in a high temperature superconductor, like low energy quasiparticles and an unconventional current-phase relation, may influence the escape rate. We report, for the first time to our knowledge, the observation of MQT in a YBa(2)Cu(3)O(7-delta) grain boundary biepitaxial JJ. This proves that dissipation can be significantly reduced by a proper junction configuration, which is of significance for quantum coherence.     

Ac-Josephson effect in YBa2Cu3O7/PbSn point contact junctions

Adjustable YBa₂Cu₃O₇/PbSn point contact Josephson junctions are exposed to a microwave field. The current voltage characteristics of the junctions show microwave-induced constant voltage steps. The power dependence of the current width of the steps is well described by the Bessel function behaviour. This clearly demonstrates the presence of macroscopic quantum effects in the new highT _c material.