Anisotropic distributions of electrical currents in high-T-c grain-boundary junctions

We developed a self-consistent method for the calculation of spatial current distributions in high-T c grain-boundary junctions. It is found that crystallographic anisotropy of high-T c superconducting electrodes results in the effects, which previously were not taken into account for interpretations of experimental data. Among them is a significant redistribution of electrical currents in superconducting electrodes in the vicinity of a grain boundary. In particular in the case of [100]-tilt bicrystal junctions, this current redistribution results in a substantial focusing to the top or bottom part of a thickness of the grain boundary, depending on “roof”- or “valley”-type of the grain boundary. This redistribution is accompanied by generation of vortex currents around the grain boundary, which leads to self-biasing of grain-boundary junctions by magnetic field nucleated by these vortex currents. It is shown that twinning or variation of geometrical shape of the high-T c electrode may also result in intensive redistribution of electrical currents and nucleation of local magnetic fields inside a high-T c superconducting electrode

Evolution of electrical and electrodynamic properties of YBa2Cu3O7-x bicrystal Josephson junctions with oxygen loading

The IV-curves and dynamic resistances R-d(V) of [001]-tilt YBa2Cu3O7-x bicrystal Josephson junctions have been experimentally studied at various levels of oxygen loading. Oxygen content was modified by annealing in ozone atmosphere, in vacuum or in air. Deviations of the IV-curves from those predicted by the RSJC model in the form of additional nonlinearities at the middle and high voltages were found to decrease after enhancement of oxygen content in junctions. Electrodynamic properties of Josephson junctions were analyzed using log-periodic YBa2Cu3O7-x antennas, integrated with the junctions. Fine structures on the IV-curves, related to antenna resonances, were modified with oxygen content due to better antenna-junction coupling and lower electromagnetic losses in YBa2Cu3O7-x film

S-c-S junctions as nonlinear elements of microwave receiving devices

The analysis of possible phenomena in the superconductor-constriction-superconductor (S-c-S) junctions inserted into microwave circuits is carried out. The results of experiments in which properties of the junctions making them different in comparison with usual nonlinear microwave elements were studied are discussed. The characteristics of receiving devices such as amplifiers, detectors and converters operating on the basis of these phenomena are obtained. The limit characteristics of these devices in different regimes of operation are analysed and compared with experimentally achieved characteristics. It is shown that the noise properties of the receiving devices utilizing the S-c-S junctions close to ideal can be obtained for the frequencies lower than characteristic frequency of the junction. This frequency for known realizations of the S-c-S junctions (point contacts and thin film bridges of small dimensions) can reach of order of 500-1 000 GHz or in other words of the middle of the submillimeter waveband range.On analyse les phénomènes qui peuvent se produire dans une jonction S-c-S (supraconducteur-constriction-supraconducteur), couplée à un circuit hyperfréquence. On étudie et on discute les propriétés des jonctions, comparées à celles des éléments non linéaires usuels en hyperfréquence. On obtient les caractéristiques des récepteurs (amplificateurs, détecteurs, ou changeurs de fréquence) utilisant ces phénomènes. Les caractéristiques limites de ces dispositifs dans différents régimes de fonctionnement sont analysées et comparées avec les caractéristiques obtenues expérimentalement. On montre que les propriétés de bruit de ces dispositifs utilisant des jonctions S-c-S quasi idéales, peuvent être obtenues pour des fréquences inférieures à la fréquence caractéristique de la jonction. Cette fréquence est de l'ordre de 500 à 1 000 GHz pour les contacts à pointes ou les microponts en couche mince

Trimming of high-Tc bicrystal Josephson junctions by ozone and vacuum annealing

AbstractSpecific values of characteristic voltages IcRn and normal-state resistances Rn of high-Tc Josephson junctions (JJ) are often required for their THz applications in detection and spectroscopy. Recently, evolution of these electrical parameters of [001]-tilt YBa2Cu3O7-x bicrystal JJ after low-temperature ozone annealing has been studied. Here, we present the effect of ozone and vacuum annealing on the IV-curves of [100]-tilt YBa2Cu3O7-x bicrystal JJ. Annealing temperature Ta was varied from 100°C to 225°C. It was demonstrated, that annealing in ozone-oxygen mixture and in vacuum has an opposite effect on JJ parameters, and the changes were reversible at Ta up to 145°C. Ozone annealing at Ta=100°C resulted in up to 1.5 times decrease of Rn. After annealing in vacuum at higher Ta, a transition temperature Tc of JJ electrodes was reduced from 90K to 52K, and significant, multifold changes of IcRn and Rn were observed. Further annealing in ozone at Ta up to 225°C restored Tc and Rn almost to their initial values, but IcRnvalues couldn’t be increased above 20% of the initial value. The obtained results can be used for trimming of junction parameters after main fabrication procedure