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Superconductor Science and Technology

Rf-induced dc voltages across unbiased terminals and current–voltage characteristics of microwave-driven high-TcSQUIDs

Jie Li, L Chen, Ping Chen, C. K. Ong

Published Jan 10, 2002• 0 citations
Publisher

Abstract

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.

DOI
10.1088/0953-2048/15/2/308

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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.
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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.
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Experimental I-V curves of microwave-driven Josephson tunnel junctions with resistive shunts are reported. The results are in very good agreement with numerical calculations using the resistively shunted junction model. In the low-frequency regime there are three distinct regions in the I-V curves and two different types of Shapiro steps. It is demonstrated that the overall shape of the I-V curves can be explained by using an adiabatic interpretation of the junction response. The two different types of Shapiro steps are related to a compensation effect between dc and rf bias currents.
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.
We have fabricated an artificial BaPb1- x Bi x O3 grain boundary tunnel junction on a SrTiO3 bicrystal substrate. Current-voltage characteristics exhibit supercurrent and quasi-particle tunneling which is commonly observed for superconductor-insulator-superconductor (SIS) type junctions. A Shapiro step is observed for a junction with a large area under microwave radiation. On the other hand, microwave-induced dc voltages at zero bias current are observed for a narrow junction. This dependence on the junction width indicates that generation of the microwave-induced voltages is related to normal resistance or capacitance of a junction.