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Acta Physica Sinica

Comparison of electric coupling with magnetic coupling between the Josephson tunnel junction and applied microwaves

Dai Yuan-Dong, Cui Guang-Ji, Meng Xiaofan, Bin Jian

Published Apr 1, 1986• 0 citations
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Abstract

The electric coupling and the magnetic coupling between the Josephson tunnel junction and applied microwaves have been studied separately. The theoretical analysis (based on the forced vibration model) and experimental results indicate that for a half-wavelength junction only even microwave-induced steps can be obtained under magnetic coupling, but for the electric coupling, odd steps appear simultaneously, and that for an integral wavelength junction only even steps can be observed under the two couplings. The efficiency of the electric coupling, according to the theory and the experimental results, is found to be better than that of the magnetic coupling for the Nb-NbO/sub x/-Pb junction.

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IEEE Transactions on Magnetics•Mar 1, 1991•1 citation
The electric coupling and the magnetic coupling between the Josephson tunnel junction and applied microwaves have been studied separately. The theoretical analysis (based on the forced vibration model) and experimental results indicate that for a half-wavelength junction only even microwave-induced steps can be obtained under magnetic coupling, but for the electric coupling, odd steps appear simultaneously, and that for an integral wavelength junction only even steps can be observed under the two couplings. The efficiency of the electric coupling, according to the theory and the experimental results, is found to be better than that of the magnetic coupling for the Nb-NbO/sub x/-Pb junction.
We have studied the power dependence of the microwave-induced dc voltages across "unbiased" Pb---Pb oxide---Pb Josephson tunnel junctions and found a systematic dependence. We have also found that the Josephson effect in the form of induced quantum voltages was observable in a large dc magnetic field as high as 0.5 kG, about one-half of the Hc2{H}_{c2}Hc2​ of the superconducting Pb film.
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.
The microwave response of the dc quasiparticle tunneling current in Josephson tunnel junctions, where the Josephson current is suppressed by an external magnetic field, has been studied quantitatively in order to clarify its characteristics as a probe for the measurement of the junction capacitance. Extensive experiments for both small and long junctions are carried out for distinguishing between microwave behaviors of lumped and distributed constant junctions. It is shown that the observed voltage dependence of the dc quasiparticle tunneling current modified by an applied rf field is in good agreement with a theoretical result which takes into account the influence of the microwave circuit connected to the junction. The comparison between theory and experiment gives the magnitude of the internal rf field in the junction. Together with the applied rf field, this internal rf field leads to the junction rf impedance which is dominated by the junction capacitance in our experimental condition. In the case of lumped junctions, this experimental rf impedance is in reasonable agreement with the theoretical one with the junction capacitance estimated from the Fiske step of the distributed junction fabricated on the same substrate; the obtained ratio of the experimental impedance to the theoretical one is approximately 0.6–1.7. In the case of distributed junctions, however, experimental values of their characteristic impedances are approximately 0.2–0.3 of theoretical values calculated by assuming the one-dimensional junction model and taking account of the standing-wave effect in the junction.
The amplitudes of the current steps in the I-V characteristics of mutually coupled two-dimensional distributed Josephson tunnel junctions driven by microwaves are investigated. For this purpose a numerical computation algorithm that is based on a planar resonator model for the individual Josephson tunnel junctions is used to calculate the DC current density distribution. In addition to the fundamental microwave frequency, harmonic contents of the tunneling current are also considered. The lateral dimensions of the individual junctions are small compared to the microwave wavelength and the Josephson penetration depth, giving an almost constant current density distribution. Therefore, the coupled junctions can give much greater step amplitudes than a single junction with an equal tunneling area, because of their nonuniform current density distribution. The calculation show a strong dependence of the Shapiro step amplitudes with respect to the coupling of the tunnel junctions. These studies imply that the choice of proper coupling conditions is important to achieve maximum step amplitudes. These results are helpful in designing tunnel junctions with optimized step amplitudes.