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Japanese Journal of Applied Physics

Zero-Crossing Shapiro Step in Asymmetric Two-Junction Superconducting Quantum Interference Device

Yoshinao Mizugaki, Kazuyuki Katoh, Masataka Moriya, Kouichi Usami, Tadayuki Kobayashi

Published Jun 16, 2006• 3 citations
Publisher

Abstract

We demonstrate a zero-crossing Shapiro step in an asymmetric two-junction superconducting quantum interference device (SQUID) driven by one sinusoidal RF signal. The operation is based on the imbalanced evolution of the junction phases due to the tilted interference diagrams. A test circuit was fabricated using Nb/AlO x /Nb junction technology. When we applied an RF signal of 10 GHz, we observed a Shapiro step crossing the zero-current axis at the voltage position of the first order. The polarity of the step voltage was switched by a DC control flux to the SQUID loop. The experimental current–voltage curves were quantitatively reproduced by simulation.

DOI
10.1143/jjap.45.l621

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We demonstrate a zero-crossing Shapiro step in a three-junction SQUID (3J-SQUID) coupled with two phase-shifted RF signals. The operation is based on periodic flux quantum transitions in the 3J-SQUID under the zero- and negatively-biasing conditions. A test circuit is fabricated using a Nb/AlO/sub x//Nb junction technology. When we feed RF signals up to 10.5 GHz, we observe a Shapiro step crossing the zero-current axis at the voltage position of the 1st order. The experimental current-voltage characteristics are quantitatively reproduced by simulation. Zero-crossing conditions for the DC offset flux and the RF power are obtained experimentally, which is also confirmed by simulation. A programmable Josephson voltage standard is discussed as one of the possible applications.
On an asymmetric two-junction superconducting quantum interference device (2J-SQUID) driven by one sinusoidal rf signal, the observation of zero-crossing Shapiro steps have been reported. In this paper, the loop inductance, L and the ratio of critical currents of Josephson junctions of the 2J-SQUID were optimized by a simulation. Using three voltage states those this asymmetric 2J-SQUID has, the bipolar quantum voltage generator that has three asymmetric 2J-SQUIDs and two control lines, was designed based on obtained parameters. Within the bias current -voltage curves with seven different combinations of the control currents, seven quantized voltages were obtained at the zero bias current.
We demonstrate a zero-crossing Shapiro step generated in a magnetically-coupled Josephson gate, where a long Josephson junction and a control line are placed in-line. The operation is based on the imbalanced evolution of the junction phases owing to the different amplitudes of the positive and negative critical current. A test circuit was fabricated using a Nb/AlOx/Nb junction technology. When we applied a rf signal of 15.2GHz, we observed a Shapiro step that crossed the zero-current axis at the voltage position of the 1st order. The experimental results were quantitatively reproduced by simulation.
Shapiro steps in an RF-field-driven dc-SQUID (RFDS), where an RF signal is coupled to the SQUID loop magnetically, are numerically investigated. We calculate the current–voltage characteristics of RFDSs under no dc offset flux, which show the enhancement of the even-order Shapiro steps. First, the maximum second (the lowest even-order) step height is evaluated as a function of the normalized frequency Ω. In the case of the inductance parameter βL ≥ 1.0, the maximum second step height is maintained at a certain value in the low frequency region. In the case of βL < 1.0, on the other hand, the maximum step height decreases in the low frequency region of Ω < 0.10. The dependence of the current positions of the step upon the RF signal amplitude shows that the voltage transition at the non-superconductive area between the superconducting interference lobes limits the upper current position of the step.
We demonstrate the operation of a single-flux-quantum (SFQ) pump that is electromagnetically dual to the single-electron pump. The circuit is composed of a three-junction superconducting quantum interference device with three control lines. By applying two phase-shifted rf signals magnetically, the SFQ-pump transfers one SFQ per one cycle of the rf signals even at zero- and negatively-biased condition. The circuit was fabricated using a Nb/AlOx/Nb Josephson junction technology. When we fed rf signals of 4.5 GHz, we observed a Shapiro step crossing the zero-current axis. Periodic dependence of voltage at the zero-biasing state upon dc offset flux was also confirmed.