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Japanese Journal of Applied Physics

RF-Induced Steps in the V-I characteristics of a Superconducting Closed Loop Formed form 2N-Josephson Junctions

Kenji Takanaka, Tadayuki Kobayashi, Tsutomu Yamashita

Published Sep 1, 1984• 2 citations
Publisher

Abstract

This paper reports a theoretical study of a superconducting closed loop consisting of a parallel connection of two linear series arrays of N-Jesephson junctions, exposed to a microwave field of frequency F. Constant-voltage steps in the current-voltage characteristics are found to appear at V dc=2m N F Φ_0, where m is an integer and Φ_0 is the flux quantum, if the loop inductance is small.

DOI
10.1143/jjap.23.1216

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This paper reports a theoretical study of a superconducting closed loop consisting of a parallel connection of two linear series arrays of N-Jesephson junctions, exposed to a microwave field of frequency F. Constant-voltage steps in the current-voltage characteristics are found to appear at V dc=2m N F Φ_0, where m is an integer and Φ_0 is the flux quantum, if the loop inductance is small.
Abstract When the impedance of a Josephson junction is too low, the elements are connected in series. We connected two arrays with N series elements of Josephson junctions each (denoted 2N‐JJ loop) and tried to find conditions for it to operate coherently by investigating voltage‐current curves. the computer simulation showed that coherent operation is easier with this circuit than with a series connection of the elements when the elements have different junction parameters. Pb‐alloy tunnel‐type junctions were produced which have N = 2 and 3, and we found that voltage steps generated by application of rf current with frequency F have a voltige interval corresponding to NF. This agrees with the simulation results and indicates coherent operation.
Both resonant and microwave-induced steps in the V-I characteristics of four-Josephson-junction loop with a small junction-parameter spread have been investigated. An rf current was superimposed onto a dc current and the radio frequency was tuned to the resonance frequency of the loop. In the high-Q region, the rf-power dependence of the step amplitude was similar to that of a single junction. However, we obtained V-I characteristics which indicated that the behavior of the junction in the loop was coherent in the low-Q region.
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.
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.
The DC voltage output from a hysteretic Josephson junction which is locked to an AC frequency source differs from the ideal Josephson relation if the junction drives a current about a closed superconducting circuit. The difference in voltage Delta V from two hysteretic Josephson junctions driven in series opposition is proportional to the difference in their driving frequencies Delta omega if the junctions are each biased to the nth voltage step. It is shown here, however, that Delta V is systematically smaller than the voltage difference Delta V 0 predicted by the ideal Josephson relation Delta V 0 =(nh(cross)/2e) Delta omega . If the loop inductance approaches zero, the smallest detectable voltage difference Delta V 0 between two junctions is limited by the intrinsic Josephson inductance. For arrays of more than one junction, however, Delta V 0 remains proportional to the loop inductance.
Microwave detector response ( eta ) for three types of superconducting structures-single edge-type Josephson junctions, arrays of series-connected junctions, and arrays of RF-series and DC parallel-connected junctions-were investigated in the frequency range 35-120 GHz. Experimental dependence of eta versus biasing voltage is mainly determined by the interaction of junctions in the array. The selective response, which means a narrow linewidth of Josephson radiation, is observed for the third type of superconducting structure in a small magnetic field changing the phase shift between closely-spaced junctions. These arrays have the advantage in coupling with the external microwave system.