Tatsunori Hashimoto

Josephson characteristics in a‐axis oriented YBa2Cu3O7−δ/PrBa2Cu3O7−δ’/YBa2Cu3O7−δ junctions

A sandwich‐type all‐oxide Josephson junction consisting of an a‐axis oriented YBa2Cu3O7−δ/PrBa2Cu3O7−δ’/YBa2Cu3O7−δ structure has been realized and its Josephson characteristics experimentally investigated. Almost all the fabricated junctions showed the Josephson characteristics, including a resistively shunted junction (RSJ)‐like current‐voltage characteristic and a Fraunhofer‐like magnetic‐field dependence of the critical current. The obtained maximum values of the critical‐current density jc and the IcRn value were 110 A/cm2 and 80 μV, respectively, for a 50‐nm‐thick PBCO interlayer at 3 K. Besides, the temperature dependencies of the critical current and the normal resistance were measured. Ic was proportional to exp(− aT1/2), where a is a constant.

a-axis oriented YBa2Cu3Oy thin films grown on novel buffer layers

Abstract Preferable growth conditions for preparing a -axis oriented YBa 2 Cu 3 O y (YBCO) and PrBa 2 Cu 3 O y (PBCO) thin films on SrTiO 3 substrates were investigated. The structural analysis of thin films deposited at various substrate temperatures and growth rates revealed that cubic structure films with high crystallinity were grown at relatively low substrate temperatures in the YBaCuO system and at a high deposition rate in the PrBaCuO system, respectively. Although the a axis orientation growth of YBCO was found to be enhanced on the cubic-YBCO film with excellent lattice matching with YBCO, a large density of segregated particles was observed both in the cubic-YBCO films and the YBCO films grown on them. The surface morphology of a -axis oriented YBCO thin films was significantly improved by utilizing a cubic-YBCO/cubic-PBCO bilayer buffer. The surface roughness of the 250 nm thick a -axis oriented YBCO thin film grown on the bilayer buffer was confirmed to be less than 2.5 nm and the T c onset reached 90 K.

HTS hairpin microstrip filter on r-cut sapphire substrate

We have developed a 17-pole high-temperature superconductor thin-film hairpin microstrip filter on r-cut sapphire substrate. We determined the optimum orientation of hairpin resonators on r-cut sapphire substrates, considering the anisotropy of dielectric permittivity. With optimum orientation, the desired frequency response was realized. On the other hand, when the orientation was not the optimum, the frequency response was violently disturbed. The cause of the disturbance of the frequency response was investigated.

7 GHz-Band Quasi-Elliptic Function Narrow-Band Superconducting Filter on Sapphire Substrate

We have developed a 7 GHz band superconducting 8-pole quasi-elliptic function filter on sapphire substrate. To realize a high Qu microstrip line resonator at 7 GHz band, the structure was used for a resonator so as to suppress the radiation, which consists of two adjacent radiation points with opposite phase. Also, the radiation decreased in proportion to odd times of half-wavelength. Hairpin microstrip line resonator with 1.5 times wavelengths realized sharp-cut filter characteristic by both high Qu and transmission zeros. Measured center frequency of the 8-pole quasi-elliptic function filter agreed with the designed value. Given these features, a superconducting filter with a sharp skirt characteristic for next-generation RF applications is expected to be realized by applying these filters

Bandwidth tunable high-Tc superconducting filter

Abstract We have developed a high- T c superconducting band-pass filter, whose center frequency and bandwidth can be tuned independently, with steep attenuation at both lower and higher band edges. This tunable filter is composed of two different types of band-pass filters, one with steep attenuation at the lower band edge and the other with steep attenuation at the higher band edge. Two filters are connected in series and each filter is covered with additional dielectric plates, respectively. The tuning characteristics of the entire filter can be controlled by adjusting the distances between the dielectric plates and the substrate. The entire filter was fabricated monolithically on a 30-mm-wide and 60-mm-long LaAlO 3 substrate. A center frequency of 1.93 GHz, a 3 dB bandwidth of 20 MHz, and an attenuation of 30 dB, 1.0 MHz apart from both band edges, were obtained for the entire filter. Then, a sapphire plate was set above each of two filters, and a center frequency tuning of 10 MHz and a bandwidth tuning of 5 MHz were obtained by adjusting the distances between the substrate and the sapphire plates.

High-Tc superconducting tunable filter with steep attenuation

Abstract We developed a compact RF receiver front-end subsystem using a high- T c superconducting sharp-skirt band-pass filter with a center frequency tuning function. The subsystem had two pairs of the tunable filter and an LNA on board. A 23-pole hairpin-type 2-GHz microstrip-line filter was fabricated with YBa 2 Cu 3 O y thin films deposited on a 35-mm-wide and 70-mm-long sapphire substrate. Attenuation characteristics were more than 40 dB at 1 MHz, apart from both the lower and the higher pass-band edges. For center frequency tuning, a 1-mm-thick dielectric sapphire plate was stacked on the filter, and the filtering characteristics were tuned by moving the plate using a piezoelectric bending actuator. The range of the center frequency modulation was more than 10 MHz with no significant degradation of the low-loss and sharp-skirt characteristics.

Josephson Effect in YBa2Cu3O7/Au-Ag/Pb Junctions

A comprehensive study was carried out on the Josephson effect in YBCO/Au-Ag/Pb SNS junctions prepared on YBCO thin films with various crystal orientations. Most of the fabricated junctions showed a dc Josephson current with a clear Fraunhofer pattern dependence on the applied magnetic field, which ensured the uniformity of the fabricated junction structure. Experimental results were quantitatively compared with the conventional junction theory based on the Usadel equation. The junction characteristics were confirmed to be reasonably explained by a junction model in which a thin insulating or highly resistive layer was assumed to exist at the YBCO-Au interface.

Ultra-narrowband HTS filter with 2.5-wavelength hairpin resonators in 7 GHz band

We have developed an ultra-narrowband high-temperature superconducting (HTS) filter with a fractional bandwidth of 0.14% in the 7 GHz band. To realize a filter with which a narrow bandwidth and low insertion loss are compatible, resonators with high unloaded Q (Qu) value are required. Hairpin microstrip resonators with 2.5 times wavelength were adopted to suppress the radiation loss and realize a high Qu value. With both the hairpin resonators and transmission zeros supplied by an 8-pole quasi-elliptic function filter construction, designed filter properties were realized. Next-generation RF applications are expected to be realized by applying these filters.

Superconducting latching/SFQ hybrid RAM

We have developed a 256-bit superconducting latching/SFQ hybrid (SLASH) RAM block as the first step in developing a 16-Kbit SLASH RAM, which enables high-frequency clock operation up to 10 GHz. The SLASH RAM is composed of ac-powered latching devices and dc-powered SFQ devices. The 256-bit SLASH RAM block is composed of 16/spl times/16 matrix array of vortex transitional memory cells, SFQ-NOR decoders, latching drivers, latching sense circuits, and address buffers. The 256-bit SLASH RAM block chips were fabricated and tested. We confirmed that the 256-bit SLASH RAM block functioned successfully.

Superconducting second-order sigma-delta modulators utilizing multi-flux-quantum generators

We have been developing a superconducting second-order sigma-delta analog-to-digital converter with a large bandwidth and a large dynamic range for software radio applications. In a superconducting second-order sigma-delta modulator utilizing a passive integrator and single-flux-quantum (SFQ) feedback, gain to compensate the large attenuation in the passive integrator of the modulator is required to obtain a large dynamic range. In this paper, a second-order modulator utilizing multi-flux-quantum (MFQ) generators creating the needed gain is reported and the design and experimental test results of the elementary circuits including a comparator, a first-order modulator, and the MFQ generator are described. The digital outputs of the comparator and the first-order modulator were experimentally evaluated, where the input dependence of the comparator switching probability and the noise-shaping properties of the modulator were clearly observed. The low speed operation of the MFQ generator creating three SFQ pulses was experimentally confirmed. The other circuits were also tested at low frequencies.