Highly sensitive SQUID sensors made of low- and high-Tc superconducting materials have been developed. Utilizing these SQUID sensors, advanced magnetic sensing systems are being developed in many fields, including biomedical, geophysics, material analysis and precise measurement applications. In this article, we review the recent progress in these sensing systems, highlighting several topics developed in Japan.
The development of high temperature superconductor (HTS) resonators, which feature high quality owing to the low surface resistance at high frequency, is important for filters and wireless power transfer (WPT) systems. Filters with HTS resonators have low loss and sharp skirt characteristics, making them well suited for the mobile telecommunication base stations. WPT systems with HTS resonators (coils) exhibit high transfer efficiency, making them well suited for wireless charging of mobile devices, electric vehicles, and so on. In this article, we introduce the recent application of HTS filters and novel HTS wire featuring high quality factor at high frequency, and their application to wireless power transfer.
This paper reports how the superconducting micro-resonators work as high-sensitivity photon detectors, from millimeter-wave signals to high-energy particles. The resonators are called microwave kinetic inductance detectors and are suitable for use in a large format array. The resonators are easily fabricated applying a few photolithographic processes, and the resonator yield is more than 90%. Hundreds of superconducting resonators can be biased with a single readout line, enabling them to be read out together with frequency multiplexing. We introduce the recent progress of scientific projects relying on this technique, and summarize our results for millimeter-wave astronomy and a high-energy particle detection system. The device was made using epitaxial aluminum on a silicon wafer and its noise was as low as the background limit, approximately 6 × 10−18 W/Hz1/2 in the millimeter-wave range. For high-energy particle detection, we investigated two materials: niobium and a hightemperature superconductor, YBa2Cu3O7-δ. The niobium-based device detected the alpha line (5.4 MeV) events. The pulse decay time was approximately 5 μs and energy resolution was approximately 1 MeV.
Superconducting-tunnel-junction (STJ) array X-ray detectors can exhibit excellent performance with respect to energy resolution, detection efficiency, and counting rate in a soft X-ray energy range. The detection area of STJ array detectors for high throughput analysis should be enlarged up to more than 10 mm2 by increasing the pixel number of the arrays. In this work, in order to realize a STJ array with a large number of pixels up to 1000 within a 10-mm-square chip, we successfully fabricated STJ arrays arranging STJ pixels at high density. For the arrays, a wiring layer is embedded in a SiO2 isolation layer underneath the STJ pixels. A STJ array detector with 100 pixels has an operation yield of 93% and a mean energy resolution of 12.5 +/- 0.7 eV in full-width at half-maximum for a C-Kα X-ray. The 12.5 eV is close to the natural line width of C in matrices, which means that the intrinsic energy resolution is a few eV. The detection performance was almost the same as those of conventional 100-pixel STJ array detectors. In addition, we developed an energy-dispersive X-ray spectroscopy analyzer combined with a scanning electron microscope (SEM) and the STJ array X-ray detector to realize nano-scale elemental mapping with high energy-resolving power. The analyzer simultaneously exhibits a high throughput of silicon drift detectors and a high energy resolution of wavelength-dispersive X-ray spectrometers.
EuBa2Cu4O8 (Eu124) films were deposited on SrTiO3 (100) substrates using the molten hydroxide method and the eutectic composition of NaOH-KOH. The starting materials are a mixture of Eu2O3, BaO2 and CuO. The starting materials and SrTiO3 (100) substrates were put into molten NaOH-KOH at 475°C and kept for 12 h. After cooling in the furnace, substrates were extracted from solidified flux and washed using water and ethanol. The epitaxial Eu124 films obtained were heat-treated and decomposed at 550–825°C and P(O2)=2.0×10–1–1.0×10–5 atm in order to obtain the EuBa2Cu3O7−δ (Eu123) phase. Two-dimensionally aligned Eu123 film was formed on the SrTiO3 (001) substrate at 625°C using the decomposition process of Eu124 epitaxial film grown from molten hydroxide at 475°C. The transformation temperature from Eu124 to Eu123 decreased as the oxygen partial pressure during heat treatment decreased. Cu-rich particles and regions with a diameter of about a micron were precipitated on/in the Eu123 matrix of the decomposed film, and observed by SEM/EDS images. The films decomposed in P(O2)=10−4 atm at 625°C showed Tc=93 K by ρ-T measurement.