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Chinese Journal of Physics

Physics of High-T, Josephson Junctions and SQUIDs

H. C. Yang, Jiazhen Lu, J. D. Lee, J. F. Jian, Hsiao-Mei Cho, H. E. Horng

Published Apr 1, 1996• 0 citations
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Abstract

Effects of magnetic fields and microwave radiation on I-V characteristics of YBa2Cu3Oy high-T, Josephson junctions, SQUIDS and Josephson arrays were reported in this experiment, with special emphasis focused on the effects of microwave radiation. In addition to the ordinary constant-voltage steps resulting from the ac Josephson effect, noninteger constant voltage steps are occasionally observed under microwave radiation. We attribute these noninteger constant-voltage steps to the flux coupled to junctions or SQUIDS. Resistive SQUIDS disclose V-Φmodulation without a biased current. The peak to peak value AV of a V-Φ curve is enhanced by microwave radiation.

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Effects of magnetic fields and microwave radiation on I-V characteristics of YBa2Cu3Oy high-T, Josephson junctions, SQUIDS and Josephson arrays were reported in this experiment, with special emphasis focused on the effects of microwave radiation. In addition to the ordinary constant-voltage steps resulting from the ac Josephson effect, noninteger constant voltage steps are occasionally observed under microwave radiation. We attribute these noninteger constant-voltage steps to the flux coupled to junctions or SQUIDS. Resistive SQUIDS disclose V-Φmodulation without a biased current. The peak to peak value AV of a V-Φ curve is enhanced by microwave radiation.
Fabrication and characterization of YBa2Cu3Oy Josephson junctions with step-edge, bi-epitaxial, and sandwich geometries are performed in this study to investigate the effects of microwave irradiation on the I–V characteristics. I–V curves under microwave irradiation reveal Shapiro steps. The width of the constant voltage steps were modulated by the microwave power. The period of the magnetic induction ΔB detected from the V–Φ curves for superconducting quantum interference devices is temperature dependent. Additionally, Tc and Ic of the proximity sandwich junctions are enhanced by the microwave irradiation.
A high temperature superconducting YBa2Cu3Oy array of five Josephson junctions designed with additional coupling lines has been developed to demonstrate the effects of frequency locking and impedance matching for applications such as oscillators, mixers, and detectors. The Josephson self-radiation power was directly detected by a superheterodyne receiver, and Shapiro steps were also measured. The Josephson self-radiation properties reveal good quality of phase locking and microwave coupling with external circuits. The maximum self-radiation power of our array is about 50 pW which is several ten times higher than that of a single Josephson junction, and its peak point exactly satisfies the Josephson current-voltage relation. The Shapiro-step measurements show that the behavior of current-voltage curve depends on the effective inductance of coupling lines which affects the total impedance of Josephson junction array and microwave coupling. The Josephson oscillation frequency was obtained up to about 880 GHz which is 73% of the maximum available frequency calculated from the characteristic voltage of the Josephson junctions. Experimental results show that this type of Josephson junction array can improve the Josephson self-radiation power and increase the maximum detectable frequency.
Quantized voltage across the unbiased terminals of a bicrystal YBa 2 Cu 3 O 7 dc SQUID exposed to microwave radiation has been observed at 77 K. Using this voltage as an indication of the microwave power coupled to the junctions, we identified three distinct regions in the I – V curves as the microwave power was tuned, similar to those reported for low- T c SQUID. Pronounced double period steps were observed when the microwave radiation power was high enough to suppress the dc Josephson current completely. Half-integer steps also appeared as the applied dc-magnetic flux was close to half-integers of flux quanta. The experimental phenomenon was explained based on the resistively shunted Josephson junction model.
Chapter deals with Josephson effects, Feynman's derivation of DC and AC Josephson effects, Josephson effects in superfluid helium, calibration invariance, influence of DC magnetic field, temperature dependence of critical current, phase difference electrodynamics in the junction, RCSJ model, voltage-current characteristics, microwave irradiated junction, macroscopic quantum interference, technology, DC voltage standard, sub mm radiation receivers, microwave generators, SQUIDS and its applications.
Experimental and theoretical studies are made of the effect of a microwave on dc current-voltage characteristics of long Josephson junctions. In the case of zero external dc current, dc voltages were observed when the microwave was applied to the junctions. By comparing the observed magnetic-field and microwave-power dependences with theoretical results, it is concluded that these dc voltages resulted from vortex motion which was induced by the applied microwave. When the junctions were in a flux-flow state caused by the external current, a good agreement between theory and experiment concerning the microwave-power and magnetic-field dependences of Shapiro steps demonstrates that an almost ideal traveling-wave type coupling between the applied microwave and a moving vortex array took place in the long Josephson junctions.
We report on the influence a preferential magnetic vortices motion has on the magnitude of the inverse ac Josephson effect (the appearance of dc current Shapiro steps) and the coherent operation of asymmetrical parallel arrays of YBa2Cu3O7−δ Josephson junctions (JJ) irradiated with microwave (MW) radiation in the presence of an applied magnetic field B. The preferential direction of motion of the Josephson vortices is due to the asymmetry-induced ratchet effect and has a dramatic impact: for a particular positive dc bias current I when the flux-flow is robust multiple pronounced Shapiro-steps are observed consistent with a coherent operation of the array. This suggests an efficient emission/detection of MW in related applications. In contrast, when we reverse the direction of I, the flux-flow is reduced and the Shapiro steps are strongly suppressed due to a highly incoherent operation that suggests an inefficient emission/detection of MW. Remarkably, by changing B slightly, the situation is reversed: Shapiro steps are now suppressed for a positive I while well pronounced for a reverse current −I. Our results suggest that a preferential vortex-flow has a very significant impact on the coherent MW operation of superconducting devices consisting of either multiple JJs or an asymmetrically biased single long JJ. This is particularly relevant in the case of flux-flow oscillators for sub-terahertz integrated-receivers, flux-driven Josephson (travelling-wave) parametric amplifiers, or on-chip superconducting MW generators, which usually operate at bias currents in the Shapiro step region.