出版者・発行元：IEEE Transactions on Applied Superconductivity  

This study shows that we can obtain an enhanced mutual coupling by using a superconducting quantum interference device (SQUID) containing a π-phase shift Josephson junction (π-junction). The π-junction can increase the current flowing in the loop inductance of the SQUID, thereby enhancing the magnetic flux generated in the inductance. Therefore, the effective self-inductance of the SQUID can increase, resulting in an increase in the mutual coupling. We use a 0-π SQUID formed by a conventional Josephson junction (0-junction) and a π-junction. Our numerical analysis for a simplified structure revealed that the ratio of effective to original self-inductance increased to 50. To confirm this, we measured the modulation patterns of critical current in dc-SQUIDs in which part of the loop inductance was replaced with a series of 0-π SQUIDs. The observed modulation pattern revealed that the effective self-inductance became more than 10-fold larger than the original self-inductance under the conditions that the applied current is a few μA while the critical current of the 0-junction in 0-π SQUIDs is 35 μA.

DOI： 10.1109/TASC.2023.3249657

Web of Science

Scopus

出版者・発行元：IEEE Transactions on Applied Superconductivity  

We report optimization of half-flux-quantum (HFQ) circuits composed of SQUIDs containing conventional Josephson junctions and π-shift magnetic Josephson junctions. The SQUID can act as a Josephson junction with an extremely small critical current and be easily switched by a weak driving force with the assistance of the circulating current that is induced due to the π-shift and quantization conditions. It leads to a drastic reduction of both static and dynamic power consumption. However, the parameter margins of the HFQ circuits obtained so far is narrow compared with conventional single-flux-quantum circuits. Since HFQ circuits use many circuit elements and require the two junctions in the SQUID to switch alternately and symmetrically, circuit parameter optimization is much more difficult. In this study, we developed an optimization tool for HFQ circuits, including operation judgment and margin calculation. We improved the critical margin of the HFQ Josephson transmission line to 39%. We also optimized the HFQ splitter, confluence buffer, and D flip-flop. We discuss design guidelines for HFQ circuits based on our systematic exploration of the optimized circuit parameter sets. We suggest that the optimized loop inductances are around 0.5 and 1.5 of Φ0/2 for data transmission and storage, respectively.

DOI： 10.1109/TASC.2023.3258374

Web of Science

Scopus

出版者・発行元：Applied Physics Letters  

We adopt superconductor/ferromagnet/insulator/superconductor (SFIS) Josephson junctions (JJs) as both switching JJs and intrinsic πphase shifters in superconductor digital circuits. The critical current density (Jc) and characteristic voltage (Vc) of the SFIS junctions are about 22 A/cm2 and 22 μV, respectively. The intrinsic πphase shift is confirmed by measuring the suppressed nominal critical current Icn and half-period-shifted modulation pattern of a π-π-πsuperconducting quantum interference device (SQUID) that contains three π-JJs in a superconducting loop. A single-flux-quantum (SFQ) circuit composed of a DC/SFQ, Josephson transmission line (JTL), and SFQ/DC converter based on SFIS JJs alone is demonstrated at 4.2 K. The energy dissipation of the SFQ/DC converter decreases by 80% because some JJs are self-biased by the πphase shifter. The intrinsic circulating currents induced by the πphase shifters lead to a narrow bias margin (±5%) and even error function, which can be solved by parameters optimization or circuit initialization in the future. The half-modulation period (φ0/2) of a half-flux-quantum (HFQ) SQUID (a partial HFQ JTL) exhibits propagation of HFQ between π-π-πSQUIDs, indicating that more complex HFQ circuits can be developed with π-JJs alone in the future.

DOI： 10.1063/5.0144604

Web of Science

Scopus

出版者・発行元：Superconductor Science and Technology  

We directly observed π-leaps of superconducting phase differences in π-junction-based superconducting quantum interference devices (SQUIDs). The SQUIDs studied here are formed by introducing a π-junction to a conventional-junction (0-junction)-based direct current (DC)-SQUID, which is referred to as the 0-0-π SQUID. Either clockwise or counter-clockwise-circulating currents flow spontaneously in the 0-0-π SQUID because of a π-phase shift of the π-junction. In other words, the 0-0-π SQUID has a bistable state corresponding to the directions of circulating currents. π-leaps are generated by transiting between the two states of the bistable state. π-leaps are an ultra-fast phenomenon and are difficult to observe as they are. We prepared a half-flux quantum (HFQ)-SQUID that comprised two 0-0-π SQUIDs. π-leaps are reflected in a static characteristic, that is, a modulation pattern of the critical current in the HFQ-SQUID. We formed π-junctions with the PdNi layer on 0-junction-based circuits supplied by the National Institute of Advanced Industrial Science and Technology. The modulation pattern of the HFQ-SQUID had a period corresponding to π-leaps as expected, although some microstructures were observed. We demonstrated that the microstructures originated from the asymmetry inside each 0-0-π SQUID by analyzing the relationship between the phase change of 0-0-π SQUIDs and the modulation patterns.

DOI： 10.1088/1361-6668/acb38d

Web of Science

Scopus

記述言語：英語   出版者・発行元：Scientific Reports  

We studied niobium nitride (NbN)-based π-junctions with a diluted ferromagnetic Pd89Ni11 interlayer (NbN/PdNi/NbN junctions). In the NbN/PdNi/NbN junctions with various PdNi thicknesses, we observed a non-monotonic dependence of the critical currents on PdNi thickness, indicating the effects of the exchange interaction on the superconducting order parameter. From theoretical fitting of the experimental data, we found that the NbN/PdNi/NbN junctions showed a significantly smaller degree of spin-flip scattering in the PdNi interlayer than in the CuNi interlayer of NbN/CuNi/NbN junctions reported previously. The weak spin-flip scattering leads to a longer decay length of the Josephson critical current, so the critical currents were observed over a wide range of PdNi thicknesses (10–40 nm). We also fabricated superconducting quantum interference devices (SQUIDs) including the NbN/PdNi/NbN junction, using a PdNi thickness in which the π-state was expected. A half-flux-quantum shift, as evidence of the π-state, was observed in the magnetic field-dependent critical currents of the SQUIDs. This result represents an important step towards the practical application of NbN-based π-Josephson junctions.

DOI： 10.1038/s41598-022-10967-6

Web of Science

Scopus

PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1109/tasc.2021.3060351

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1109/tasc.2021.3072846

担当区分：筆頭著者,　最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1088/0256-307x/36/4/047401

担当区分：筆頭著者,　最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1103/physrevb.99.100506

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1109/tasc.2018.2803108

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1063/1.5030348

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国名：アメリカ合衆国  

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国名：日本国  

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会議種別：口頭発表（一般）  

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