Microwave Device/Metrology/ Physics
REBa2Cu3Oy(REBCO) has a higher critical temperature (Tc), critical current density (Jc), and critical magnetic field (Hc) compared to other superconductors. Therefore, REBCO is expected to be applied to transmission cables, magnets, motors, and microwave devices (filters and NMR pick-up coil). Many research groups have attempted to improve the performance and practical application of these superconducting devices [1-4]. However, research and development of superconducting devices using REBCO is mainly focused on DC or AC devices such as power transmission cables, magnets, and motors.
On the other hand, it is known that the surface resistance of REBCO film is more than three orders of magnitude lower than that of copper in the high frequency above kHz. Therefore, REBCO film is a promising candidate material for high performance microwave devices. Microwave receive filters using REBCO film have excellent performance with low loss and sharp skirt characteristics and are already used in the receiving systems of mobile communication systems [5, 6]. However, practical applications of microwave devices using REBCO films are limited to receive filters. Research and development on superconducting microwave devices is mainly focused on device structure such as shape and size, and is not focused on superconducting materials. Attempts to develop superconducting microwave devices (transmit filter and NMR pick-up coil) have not yet been successful, due to low properties of superconducting films. In this study, we proposed the preparation and properties of trifluoroacetates metal organic deposition (TFA-MOD) derived REBCO films for microwave devices.
A CeO2-buffered R-Al2O3substrate (25*25 mm) was annealed at 1000°C in a tubular furnace with a flow of pure oxygen gas. The (Y,Gd)BCO thin film was deposited on the annealed CeO2-buffered R-Al2O3substrate from a metal organic solution containing Y-, Gd-, and Ba-trifluoroacetates, and Cu-octylic acid salt with a cation ratio of 0.77:0.23:1.5:3.0. The metal organic solution was coated by a spin-coating method at a rotation speed of 6000 rpm. The coated film was calcined in a humid oxygen atmosphere to form an amorphous precursor film by increasing the temperature to 500°C at 5°C/min. For conversion to the (Y,Gd)BCO phase, the amorphous precursor film was heated to 720°C at 20°C/min in a humidified mixed atmosphere of argon and oxygen.
Figure 1 (a) shows a photograph of the 25*25 mm (Y,Gd)BCO film. A black film was formed over the entire substrate surface. For an XRD θ-2θ scan, (Y,Gd)BCO (001)-orientation peaks were evident. Δω and ΔΦ were estimated from the (Y,Gd)BCO (005) peak in the ω scan and the (Y,Gd)BCO (103) peaks in the Φ scan, respectively. The (Y,Gd)BCO film showed high crystallinity (Δω ≈ 0.18° and ΔΦ ≈ 0.85°), which is consistent with our previous report. The Tcfor the (Y,Gd)BCO film only varied from 90.8 to 91.2 K. The average value of self-field Jc (Jcsf) at 77 K for the (Y,Gd)BCO film was 8.9 MA/cm2, which is approximately 3 times higher than that for a co-evaporated YBCO thin film grown on CeO2buffered R-Al2O3 substrate fabricated by Ceraco GmbH (about 3.0 MA/cm2). Figure 1 (b) shows Jcsf / Jcsf max maps at 77 K for the(Y,Gd)BCO film. The fluctuation of Jcsf was within ±~10% of the average. The fluctuation of Jcsf were small, which confirms the uniformity of the film. The surface resistance (Rs) at 21.8 GHz and 77 K for the (Y,Gd)BCO film was 2.3 mΩ, which is lower than that for the co-evaporated YBCO thin film. In the presentation, we will describe the microwave properties of (Y,Gd)BCO film and the filters using TFA-MOD derived (Y,Gd)BCO film.
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This work was supported by JKA and its promotion funds from KEIRIN RACE. A part of this work was supported by JSPS KAKENHI (23K13365), CASIO Science Promotion Foundation, Iketani Science and Technology Foundation, and Fujikura Foundation.
Figure 1. (a) A photograph of the 25*25 mm size (Y,Gd)BCO thin film. (b) The Jcsf at 77 K maps of the (Y,Gd)BCO thin film.
Advanced microwave technologies constitute the foundation of a wide range of modern sciences, including microwave integrated circuits, quantum computing, microwave photonics, spintronics, etc. To facilitate the design of chip-based microwave devices, there is an increasing demand for state-of-the-art microscopic techniques capable of characterizing the near-field microwave distribution and performance. In this work, we integrate Josephson junctions onto a nano-sized quartz tip, forming a highly sensitive microwave mixer on-tip [1]. This allows us to conduct spectroscopic imaging of near-field microwave distributions with high spatial resolution. Leveraging its microwave-sensitive characteristics, our Josephson microscopy achieves a broad detecting bandwidth of up to 300 GHz, as well as remarkable frequency and intensity resolutions. To verify its functionality for applications, our study marks the first implementation of superconducting Josephson probe microscopy for near-field microwave detection on multiple voltage-controlled oscillators [2]. Focusing on spectrum tracking, various phenomena, such as stray spectrums and frequency drifts, were found under non-steady operating states. Parasitic electromagnetic fields, originating from power supply lines and frequency divider circuits, were identified as sources of interference between units. The investigation further determined optimal working states by analyzing features of microwave distributions. Our research not only provides insights into the optimization of circuit design and performance enhancement in microwave oscillators, but also emphasizes the significance of non-destructive near-field microwave microscopy as a pivotal tool in characterizing advance integrated microwave devices.
[1] Ping Zhang, Jingjing Lv, etc., National Science Review (2024) (in press)
[2] Ping Zhang, Yang-Yang Lyu, etc., Nano Letters 24(18), 5453–5459 (2024)
This work was supported by the National Key Research and Development Program of China (2021YFA0718802), the National Natural Science Foundation of China (61727805), and Jiangsu Key Laboratory of Advanced Techniques for Manipulating Electromagnetic Waves.
Figure 1. System setup and application of the Josephson microscope. (a) Microscope assembly. When the probe approaches the chip, Josephson junctions on the apex receive subtle near-field microwaves emitted from the chip surface, causing oscillations of Cooper pairs. The probe is biased to perform coherent detection and characterizations of microwave distribution and spectrum tracking. (b) SEM image of the probe. The image shows the formation of weak-link Josephson junctions on top separated by grooves. (c) Photo of the whole oscillator chip. The red dashed line marks the imaging area in (d). Part of the surrounding pins provides bias voltages required for the normal operation of the chip. (d) Near-field microwave imaging above the oscillator. The probe's voltage signals are spatially tracked during the scanning process, showing a circular microwave distribution. The inset shows the design schematic corresponding to the imaging area. (e) Restored spectra of the oscillator. When the oscillator is in a dysfunctional state, the center frequency shifts and sidelobes appear in its frequency spectrum.
Keywords: near-field probing, microwave microscopy, Josephson junction, frequency mixing
High-temperature superconducting (HTS) tapes are mainly used in cables and motors at relatively low frequencies below 1 kHz, and it has been reported that the AC loss of superconducting tapes in this range increases proportionally to the frequency because the hysteresis loss is dominant. In contrast, the recently developed wireless power transmission system is expected to be applied at 10 kHz or higher when using high-temperature superconducting tapes, and the AC loss in the superconducting tape is generally expected to increase with the square of the frequency due to the eddy current loss in the protective layer as well as the hysteresis loss in the superconducting layer. There are two types of HTS tapes: REBCO tapes and BISCCO tapes. BISCCO tape is buried in a protective layer of silver, which is expected to have large eddy current losses and cannot be used for high frequency applications, while REBCO tape has protective layers such as Hastelloy substrate and silver layer, which cause eddy current losses in this area.
In order to investigate these characteristics of AC loss, this study employed the finite element method (FEM) for the purpose of analyzing the electromagnetic field, with the objective of deriving the AC loss in each layer of the REBCO tape. In the context of wireless power transmission, given the reports of the potential for achieving very high Q-values at low power at high frequencies, this study investigated the AC losses under such conditions. As direct estimation of AC losses at low currents is challenging due to the necessity for a fine mesh, this study initially derived the losses in each layer for the REBCO tape with a critical current of 400 A and energized at 100 to 300 A. The findings demonstrated that the AC losses were found to be affected by each layer. Consequently, it was confirmed that the AC losses in each layer affect each other. Based on these results, equations for fitting up to 0.1A in each layer were proposed. The results demonstrate that eddy current losses in the Hastelloy layer account for the majority of the losses at high frequencies in the low current region.
Herein, we develop a quantum voltage measurement system in a strong magnetic field. One of our goals is to conduct the quantum metrology triangle (QMT) experiment, which operate three electric quantum standards, a Josephson voltage standard (JVS), a quantized Hall resistance standard (QHRS) and a quantum current standard based on the single electron tunneling effect, in one dilution refrigerator [1]. To realize this system, with the strong magnetic field condition for the QHRS, it is necessary to reduce the magnetic field in the vicinity of the JVS device. A magnetic field shielding structure to reduce the magnetic field has been developed and the design of the structure was optimized by the numerical calculation with a finite element simulation, which lead us to the structure composed of a permalloy box covered by two superconducting layers [2]. In this study, we demonstrated an operation of the JVS device in the magnetic field shielding structure at a 4 K stage of the dilution refrigerator. At the bottom of the dilution refrigerator, about 50 cm below the JVS in the magnetic field shielding structure, a superconducting magnet with a cancelation coil was placed and generated a magnetic field. We measured the residual magnetic field inside of the magnetic field shielding structure with a Hall device [3] and evaluated the effect to the JVS operation [4]. The residual magnetic field was less than the geomagnetic field and the operating bias current margin of JVS was maintained, even under a generated magnetic field of 10.2 T. For the next step, the simultaneous operation of both the JVS and the QHRS devices in the refrigerator is planned.
[1] D. Matsumaru, et al., “Development of a Josephson voltage standard module for quantum metrology triangle measurements,” CPEM 2022 Conf. Digest, Nov 2022.
[2] D. Matsumaru, et al., “Simulation study of magnetic shielding effects for operation on a Josephson voltage standard device in a high magnetic field,” ISS2022 Conf. Digest, Nov. 2022.
[3] D. Matsumaru, et al., “Fabrication and evaluation of a superconducting shield for operating a Josephson voltage standard device in a high magnetic field,” ISS2023 Conf. Digest, Nov. 2023.
[4] D. Matsumaru, et al., “Operation of Josephson Voltage Standard with Superconducting Shield in Magnetic-Field Environment,” CPEM 2024 Conf. Digest, Jul. 2024.
This work was supported by JSPS KAKENHI Grant Number JP24K07616.
Keywords: magnetic shielding, magnetic field, Josephson effect, voltage standard
We present a fully 3D model to solve Usadel’s equations in arbitrary geometries using COMSOL Multiphysics. Using this model, we predict the magnitude of the proximity effect or inverse proximity effect in micro scale superconducting devices. Calculating a spatially resolved electronic density of states allows us to determine the thermal and electrical transport properties of arbitrary superconductive electronic devices at mK temperatures. The performance of such devices could be significantly impacted by any unexpected proximitization or inverse proximitization. For example, in the 2D bilayer approximation, thin normal metals in contact with superconductors may no longer conduct electronic heat, but this effect is complicated by real device geometries and the particular properties of the normal and superconducting metals. This work goes beyond bilayer approximations and quasi 2D methods to allow for accurate calculation of transport phenomena in proximitized normal metals and in inversely proximitized superconductors.