Vladimir N. Gubankov

Josephson effects in YBa2Cu3O7−x grain-boundary junctions on (001) NdGaO3 bicrystal substrates

Abstract Grain-boundary YBa2Cu3O7−x Josephson junctions were fabricated on (001) NdGaO3 bicrystal substrates with a misorientation angle of 2×18.4°. Both DC and AC Josephson effects were studied in these high-Tc junctions in the range from liquid-helium up to liquid-nitrogen temperatures. The values of characteristic voltages were up to 0.34 mV at a temperature of 10 K. The I-V curves of the junctions and their response to low-intensity millimeter-wave radiation were shown to be described by the RSJ model with thermal fluctuations. Values of responsivity up to 106 V/W and a noise equivalent power as low as 10−14W/ Hz 1 2 were obtained for these junctions operating both in wideband and selective detection modes at a frequency of 78 GHz. The applicability of YBa2Cu3O7−x Josephson junctions made on NdGaO3 bicrystal substrates for detection and spectral analysis of millimeter-wave and far-infrared radiation is emphasized.

Terahertz Characterization of External Resonant Systems by High-

High-Tc Josephson technology looks promising for THz applications. One of the remaining problems, important both for detection and emission of THz radiation, is an optimum coupling of high-Tc junctions with environment. Recently, we have demonstrated a possibility to characterize electromagnetic systems integrated with high-Tc junctions using dc I-V curves of these junctions. Here, we report in details on modification of the dc I-V curves of YBa2Cu3O7-x bicrystal junctions, related to interaction of Josephson oscillations with terahertz resonance antennas. Thin-film log-periodic antennas with various resonance structures were fabricated on the same substrate as Josephson junctions and were excited by Josephson oscillations. Frequency-dependent admittances of the antennas were derived from fine log-periodic structures on the dc I-V curves of the junctions and compared with the results of numerical simulations. Data obtained can be used for optimization of coupling between the junctions and antennas.

T_{c}

We developed a self-consistent method for the calculation of spatial current distributions in high-Tc grain-boundary junctions. It is found that crystallographic anisotropy of high-Tc 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-Tc electrode may also result in intensive redistribution of electrical currents and nucleation of local magnetic fields inside a high-Tc superconducting electrodes.

Josephson Junctions

Oscillations of the differential resistance of a superconductor-two-dimensional electron gas contact were observed at low temperatures. These oscillations reflect the quantum nature of the electric charge transport perpendicular to the two-dimensional electron gas.

Anisotropic distributions of electrical currents in high-Tc grain-boundary junctions

Abstract The motion of Abrikosov vortices from one pinning center to another by the impact of both α-particles and γ-rays on a superconducting film is observed with the aid of the Josephson tunnel junction. This motion is explained on the basis of a heating model of the interaction of α-particles and γ-rays with superconducting film. The Josephson tunnel junction with trapped A-vortices is put forward as a new type of nuclear radiation detector.

Quantum oscillations of the differential resistance of superconductor-two-dimensional electron gas contacts

The influence of Abrikosov vortices (A-vortices) on the dynamics of fluxons (Josephson vortices) in Nb-AlO/sub x/Nb long Josephson tunnel junctions (LJTJs) has been investigated experimentally. Trapping of Abrikosov vortices was obtained by cooling of the junction through the superconducting transition temperature in magnetic fields applied perpendicularly to the plane of the junction (H/sub perpendicular to /). The dependences of the Josephson critical current on an external magnetic field applied parallel to the plane of the junction were measured after cooling at different values of H/sub perpendicular to /. Changes of both the maximum Josephson critical current and the critical magnetic field of LJTJ H/sub c1/ have been observed for an LJTJ with an irregular structure of A-vortices trapped in the electrodes of the junction. The effects are explained by the pinning of Josephson vortices at the effective misaligned A-vortices.<<ETX>>