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.     

Enhancement of the critical current density of YBa2Cu3Ox superconductors under hydrostatic pressure

The dependence of the critical current density Jc on hydrostatic pressure to 0.6 GPa is determined for a single 25 degrees [001]-tilt grain boundary in a bicrystalline ring of nearly optimally doped melt-textured YBa2Cu3Ox. Jc is found to increase rapidly under pressure at +20%/GPa. A new diagnostic method is introduced (pressure-induced Jc relaxation) which reveals a sizable concentration of vacant oxygen sites in the grain boundary region. Completely filling such sites with oxygen anions should lead to significant enhancements in Jc.     

Contributions to photodoping in oxygen-deficient YBa2Cu3Ox films

We report our studies on the superconducting and normal-state properties of metallic thin films ( 52 K) exposed to long-term white-light illumination (photodoping). It was observed that the effects of photoexcitation strongly depended on the temperature at which the photodoping was performed. At low temperatures, both the Hall mobility and the Hall number were photoenhanced, whereas, at temperatures slightly below room temperature, the Hall mobility initially showed an abrupt increase followed by a long-term decrease, and the Hall number increased even stronger than at low temperatures. The enhancement of the film's superconducting transition temperature T_c, caused by photodoping, exhibited the same temperature dependence as the enhancement of the Hall number, being largest ( 2.6 K) at high temperatures. From the asynchronous behavior of the Hall quantities, we conclude that both the photoassisted oxygen ordering and charge transfer mechanisms contribute to photodoping. The relative contributions of both mechanisms and, thus, the electronic properties of the photoexcited state are strongly temperature dependent. Studies of the relaxation of the photoexcited state at 290 K showed an unexpectedly short relaxation time of the Hall mobility after termination of the illumination. The relaxation saturated somewhat below the initial, undoped value, similarly to the decrease of the Hall mobility, observed upon long illumination. These latter findings give evidence for a competition between the oxygen ordering and thermal disordering processes during and after the photoexcitation in the high-temperature range. Received: 13 October 1997 / Accepted: 19 November 1997     

Magnetic field dependence of critical current density in YBa2Cu3O7

We have measured the transport critical current densityJ _cof sintered YBa₂Cu₃O₇, in various applied fields up to 185 Oe at 77 K. We find a sharp decay ofJ _cwith magnetic field. We show that this sharp decay is consistent with the low field hysteresis results of Groveret al. We argue that the observed field dependence is not caused by intragranular weak links.     

Oxygen dependence of the fermi-level and electron-phonon coupling constant in YBa2Cu3Ox films

We present a detailed investigation of the dynamics of laser-excited carriers in YBa₂Cu₃O_x thin films with various oxygen contents x. We observe a Fermi level shift due to oxygen reduction and determine the energy gap between the Fermi level and the upper Hubbard band. The electron-phonon coupling constant is determined as a function of transition temperature.     

Photo-induced increase of the superconducting coherence lengths in oxygen-deficient YBa2Cu3Ox thin films

() thin films were photodoped with white light at various temperatures from 70 K to 290 K. Before and after the excitation, the magnetoconductivity was measured in a magnetic field B = 0.5 T, and the experimental results were fitted to the Aslamazov-Larkin theory of superconducting order-parameter fluctuations to determine the superconducting coherence lengths, and . We observed that the photodoping process enhanced and and that the amount increased with the photodoping temperature increase. On the other hand, the superconducting anisotropy / decreased with increasing temperature. The photodoping effect enhances superconducting properties of partially oxygen-deficient samples and is considerably increased by high doping temperatures. Received 15 December 1999 and Received in final form 24 May 2000     

Andreev states and the Josephson effect in superconducting heterojunctions on thin YBa2Cu3Ox films

Conductance anomalies at low bias voltages and superconducting currents in Au/YBa₂Cu₃O_x and Nb/Au/YBa₂Cu₃O_x heterojunctions in which the c axis of the YBa₂Cu₃O_x (YBCO) epitaxial film is rotated in the (110) YBCO plane through 11° with respect to the normal to the substrate plane were studied experimentally. The films were prepared by laser deposition onto (7 2 10)-oriented NdGaO₃ substrates. The current-voltage characteristics of the heterojunctions exhibit conductance anomalies at low voltages. The behavior of these anomalies is studied at various temperatures and in various magnetic fields. The critical current and Shapiro steps observed in the current-voltage characteristics of Nb/Au/YBa₂Cu₃O_x were evidence of the Josephson effect in these heterojunctions. The experimental results are analyzed in terms of the model of the arising of bound states caused by Andreev reflection in superconductors with d-type symmetry of the superconducting order parameter.     

Anisotropy of a vortex instability observed at high current densities in YBa2Cu3O7δ superconducting films

Experimental data are provided for YBaCuO films that an instability of the vortex system, which manifests itself by a voltage jump at a critical current I*, exhibits strong anisotropy if the magnetic field is tilted from parallel to perpendicular to the c-axis. The angular dependence of I* can be well described by a model emphasizing the component of the magnetic field parallel to the c-axis. If the current range is restricted to values close to I*, the current-voltage characteristics below the instability show a satisfactory agreement with the prediction of the theory of ‘Self-Organized Criticality’ (SOC). In terms of this theory it is possible to relate the critical vortex velocity v* to the temperature and field dependent characteristic size of the underlying vortex avalanches. If, however, standard Larkin-Ovchinnikov theory is applied to describe the instability, this critical velocity is related to the scattering rate of quasiparticles. Analyzed in this way and assuming an isotropic diffusion constant of the quasiparticles, an anisotropic scattering rate and its temperature dependence can be extracted.