We have fabricated interface-engineered junctions with YbBa2Cu3Oy as the counterelectrode. The junctions fabricated on YBa2Cu3Oy base electrodes exhibited excellent Josephson characteristics with the 1σ-spread in Ic as low as 5.4% for 16 junctions with an average Ic of around 1 mA. We also confirmed that the 1σ in Ic correlates with the surface morphology of the base-electrode layer, indicating that further improvements in 1σ would be possible by advancing the thin-film growth technology.
The mechanisms of current transport in interface-engineered junctions (IEJs) with ramp-edge geometry were investigated to clarify the possible origin of the statistical fluctuation of the Josephson critical current. More than 1000 junctions with a ramp edge aligned either along the [100] or [110] axis of the high-temperature superconductor electrode were fabricated under various process conditions. These junctions exhibited a critical current density ranging from 10 2 to 10 6 A/cm 2 at 4.2 K while maintaining a magnetic field modulation of the critical current exceeding 80% without any indication of the peculiar effect of d-wave pairing symmetry. The junctions with a critical current density exceeding 10 4 A/cm 2 exhibited an appreciable amount of excess current that grew rapidly within an approximate voltage range of less than 5 mV. The critical current versus temperature characteristics of these junctions were found to be explained reasonably well by a superconductornormal-superconductor (SNS) junction model in the diffusive regime. This model is also consistent with our observation of a weak subharmonic gap structure due to multiple Andreev reflections. In addition, we found that the Josephson critical current ͑I c ͒ exhibited a good correlation with the differential resistance near 0 V, while the normal resistance defined at a current level of two to three times I c varied appreciably even for junctions with a similar I c . This indicates that another conduction channel with little contribution to the Josephson current coexists within the junctions. The dI / dV measurement for high resistance junctions revealed that resonant tunneling of quasiparticles through localized states in an insulating barrier constitutes this second conduction channel. All these results suggest that IEJs should be regarded as an array of microscopic SNS contacts embedded in an insulating barrier with random orientation. The fluctuation in the number of SNS contacts in a junction restricts the attainable minimum spread of the I c value.
We have developed a novel multilayer process for HTS-SFQ circuits adopting a NBCO groundplane and tin-oxide isolation layers, and fabricated an HTS-SFQ ring oscillator circuit including 21 Josephson junctions. The junctions on the buried groundplane exhibited excellent Josephson characteristics with a magnetic modulation of exceeding 90%, and an product of 0.75 mV at 30 K. The sheet inductance of the wiring layer at 30 K was evaluated to be 1.1 pH for the counter-electrode layer, and 1.3 pH for the base-electrode layer. We confirmed the correct operation of the 10-stage ring oscillator at 20-30 K. The maximum dc output voltage of the ring oscillator was 0.06 mV at 30 K and 0.12 mV at 20 K, indicating the signal delay per stage of 3.4 ps and 1.8 ps, respectively.
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