Hiroyuki Fuke

2 GHz tunable superconducting band-pass filter using a piezoelectric bender

Abstract We have developed a 2 GHz tunable superconducting band-pass filter using a piezoelectric bending actuator. The filter is a 12-pole microstrip line structure on 50 mm diameter LaAlO 3 substrate and has low loss and sharp skirt characteristics. The center frequency was 1.94 GHz and the attenuation characteristics were 30 dB at 1 and 3 MHz apart from the lower and higher band-edge, respectively. A sapphire (Al 2 O 3 ) plate was stacked on the filter with an air gap and the gap thickness was controlled electrically from the outside of a refrigerator using a piezoelectric bending actuator. The center frequency of the filter could be adjusted within 11 MHz in such a manner that the low loss and sharp skirt characteristics were not degraded.

Enhanced flux pinning in Ag-sheathed (Bi, Pb)2Sr2Ca2Cu3O10+ delta tapes by post-annealing

A significant enhancement of Jc in a magnetic field for Ag-sheathed Bi-Pb-Sr-Ca-Cu-O (BPSCCO) tapes has been achieved by post-annealing at reduced temperature after processing. This post-annealing at reduced temperature introduced extra oxygen into the oxide superconductor. Weak links were eliminated and Jc has been increased in the post-annealed tapes by a factor of 1.5 to 3 at 0 T over the normally processed tapes without post-annealing. The optimized Jc values for the post-annealed tapes ranged from 30000 to 57000 A cm-2 at 77.3 K and 0 T. The irreversibility lines measured by a DC SQUID exhibited an improvement in flux pinning in the post-annealed tapes compared with that in normally processed tapes. The introduced oxygen ions are responsible for the enhanced Jc and may affect two dimensionality in superconductivity for the BPSCCO system.

Tuning properties of 2 GHz superconducting microstrip-line filters

We have fabricated a 2 GHz 12-pole superconducting microstrip line filter and a 2-pole electrically tunable filter using interdigital electrodes and dielectric film. The change of the dielectric constant of a SrTiO/sub 3/ film, stacked on the filter, by applying a bias voltage between the electrodes leads to the tunability. The range of the center frequency modulation was 13 MHz (0.7%) at 60 K, 75 kV/cm. The microwave frequency response was improved by modification of the electrode structure. This tuning method is considered to be promising in that individual resonance elements in multi-pole filters can be tuned independently.

Flux pinning by a‐axis grains in c‐axis‐oriented Y‐Ba‐Cu‐O films

We have investigated the relationship between the microstructures and pinning forces by measuring the magnetic‐field dependence and angular dependence of Jc in several kinds of YBCO thin films having different microstructures. A high‐Jc value was kept even when the magnetic field was applied perpendicular to the film plane in the case of a c‐axis‐oriented film which was studded with a‐axis‐oriented grains. The boundaries between the a‐axis‐oriented grain and the c‐axis‐oriented grain are considered to be effective as pinning centers.

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.

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.

Improvement of In-Plane Alignment of Grains in YBCO Films on Ag Tapes

The in-plane alignment of grains in YBCO superconducting films deposited on poly-crystalline Ag tapes by the ionized-cluster-beam-deposition method was investigated. All of the YBCO films obtained had c-axis orientation. It has been clarified that the in-plane alignment depends on the crystal orientation of the Ag tapes. Namely, the in-plane alignment of YBCO films was improved when Ag (110) was aligned in the tape surface. On the other hand, when the crystal orientation of the Ag tapes was random, the YBCO films did not show any alignment in the tape surface. It is considered that the growth mechanism of YBCO films is explained by the near-coincident site lattice model between YBCO and Ag lattices.

Electrically tunable YBa2Cu3Oy resonators using interdigital electrodes and dielectric film

Abstract We have fabricated electrically tunable YBa2Cu3Oy (YBCO) resonators using interdigital electrodes and a dielectric film, and have investigated their tuning capabilities. The tuning section, which consists of an SrTiO3 (STO) dielectric film and SrRuO3 (SRO) interdigital electrodes, was stacked on a half-wavelength microstrip line resonator. The change of the dielectric constant of the STO film by applying a bias voltage between the electrodes leads to the tunability. The range of frequency modulation and the quality factor were 10 MHz (0.21%) and 2100 at 40 K, 75 kV/cm, respectively. This tuning method has an advantage over conventional methods in that individual filter elements can be tuned independently and a high quality factor is expected.

Growth Mechanism During Firing Process of Single-Coated Thick YBCO Films by TFA-MOD

Single-coating technology for metal organic deposition using trifluoroacetate has various advantages: no inhomogeneous interface in the case of multilayers, no random orientation from the interface, and no deterioration of material through two or more heat treatments. However, single-coated thick film inevitably has many voids. In order to minimize voids, both a calcining process and a firing process are required. This paper focuses on the firing process. During the firing process, balance of <i>V</i>_d and <i>V</i>_g is important. <i>V</i>_d and <i>V</i>_g are the drop rate of quasi-liquid and the growth rate of YBa₂Cu₃O_{7 -}<i>x</i>, respectively. We found that the volume of voids can be reduced in the condition of V_d > V_g, where gas flow is low, temperature high, and humidity low. We prepared gel films whose thickness was about 50 μm and calcined them to realize thick calcined films. The films are fired to realize 5.2-μm-thick superconductor on LaAlO₃ single crystal. Transmission electron microscopy observation revealed oriented layers. The film is still 20% void by volume. However, the film must be 40% void by volume without optimal firing process based on a quasi-liquid network model. The quasi-liquid network model is effective for reducing voids. Optimization of the calcining process will spur further development.