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IEICE Electronics Express

Zero-crossing Shapiro step generated in a niobium in-line Josephson gate

Yoshinao Mizugaki, Haruna Takahashi, Hiroshi Shimada

Published Jan 1, 2014• 2 citations
Publisher

Abstract

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.

DOI
10.1587/elex.11.20140054

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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.
A zero-crossing Shapiro step (ZCSS) in an asymmetric and nonhysteretic two-junction superconducting quantum interference device (SQUID) driven by one sinusoidal rf signal is presented. The operation is based on imbalanced evolution of the junction phases owing to the tilted interference diagrams. Dependence of ZCSS generation upon the frequency and amplitude of a rf signal was numerically obtained. A test circuit was designed and fabricated using a Nb∕AlOx∕Nb junction technology. When we irradiated a rf signal of 10GHz, a ZCSS at the voltage position of the first order was observed. The polarity of the step voltage was switched by a dc control flux to the SQUID loop. The experimental bias-current versus voltage and control-current versus voltage curves were quantitatively reproduced by numerical simulation.
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.
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.
A YBa2Cu3O7-δbicrystal Josephson junction was fabricated by electron beam lithography and Ar ion beam etching. The width of the junction (W) was 12 μm and the Josephson penetration length (λJ) was calculated to be 2.36 μm. As W/λJis 5.1, this bicrystal junction is a long Josephson junction. In critical current - magnetic field characteristics, periodicity is not observed, but the maximum value and the minimum value of the critical current are different at a finite magnetic field. In the I-V curve with irradiating the rf signal, the rounded Shapiro step was observed. By applying the magnetic field, the rounded 1st Shapiro step across the zero current axis was observed.
Zero-crossing Shapiro steps in focused-ion-beam-tailored high-TcT_cTc​ superconducting microstructures
Microwave response of S-shaped Bi2_22​Sr2_22​CaCu2O8+x_{8+x}8+x​ (Bi-2212) micron-scale samples, in which the supercurrent was forced to flow perpendicular to the crystal layers, was investigated. A treatment with a focused ion beam allowed us to reduce the plasma frequency down to fpf_pfp​$\sim$5 GHz at TTT=0.3 K in naturally stacked Josephson junctions in a crystal. We observed Shapiro steps at frequencies as low as $\sim$5 GHz. Well-developed zero-crossing Shapiro steps were observed at frequencies as low as $\sim$10 GHz. They appeared as constant-voltage plateaus with a non-zero voltage occurring at zero bias current. We confirmed that zero-crossing Shapiro steps in the Bi-2212 stacked junctions can be observed when the irradiated frequency is sufficiently larger than fpf_pfp​. The observed high-order fractional steps in the microwave responses indicate that the interlayer-coupled Bi-2212 Josephson junctions have nonsinusoidal current-phase relation. Based on the temperature dependence of the steps we also showed that the finite slope of the steps is due to the enhancement of the phase diffusion effect.