S. Ohshima

Bio: S. Ohshima is an academic researcher from Yamagata University. The author has contributed to research in topics: Thin film & Band-pass filter. The author has an hindex of 14, co-authored 107 publications receiving 815 citations.

A simple measurement technique for critical current density by using a permanent magnet

Abstract: We developed a novel nondestructive and contactless method for measurements of critical current density (J/sub c/) in high-temperature superconductor (HTS) thin films by using a permanent magnet (Sm/sub 2/Co/sub 17/). This measurement technique is based on a repulsive force (F/sub r/) and attractive force (F/sub a/) between the magnet and HTS film caused by the shielding currents and pinning forces. The F/sub r/ and F/sub a/ were measured by using a high-resolution load sensor changing the distance (L) between the magnet and HTS film. An effective maximum repulsive force (F/sub m,eff/) could be determined from an extrapolated value of the F/sub r/ vs. L curve for L=0 mm. We investigated the relationship of the J/sub c/ to the F/sub m,eff/ in many HTS (YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// and ErBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//) samples. The standard J/sub c/ of HTS films was measured by using an inductive measurement system produced from THEVA GmbH. We found that the J/sub c/ is almost proportional to the two-thirds power of the F/sub m,eff/ normalized by the film thickness. This result indicates that the simple permanent magnet technique can be easily estimate the J/sub c/ of the HTS films. This system has good reproducibility for the J/sub c/ measurements and is useful to provide a rapid, routine characterization of the superconductivity in the HTS materials.

The correlation of the critical current density and surface resistance of YBa2Cu3O7−δ thin films

Abstract: We have examined the correlation between the critical current density (J c ) and the surface resistance (R s ) of an YBa 2 Cu 3 O 7-δ (YBCO) thin film. YBCO films were prepared in an inductively coupled plasma sputtering system on MgO and BSC/MgO substrates. BaSnO 3 (BSO) was used as a buffer material for the MgO substrates. J c was determined by magnetic measurements, and R s was measured using a sapphire rod resonator method. The correlation of critical current and surface resistance has been reported previously, however, there is few researches in which a systematic correlation of R S and Jc using the same sample has been done. To begin with, we measured R s by the dielectric resonator method, and next, J c was measured using the magnetic field method. As a result, it was proven that there was a strong correlation between R s and J c . We found that the relationship between J c and R s (at 22 GHz) could be expressed by the following equation, R s = 2.0 x 10 7 J c -1, where the unit of R s is the ohm, and the unit of J c is A/m 2 . It was found that the relation was applicable in a wide temperature range under T c . The value of the surface resistance can be estimated, if an accurate critical current is obtained.

Power handling capability improvement of HTS filter with sliced microstrip line resonators

Abstract: We investigated the third-order intermodulation distortion (IMD3) properties of high- temperature superconducting (HTS) filters with sliced microstrip line resonators. Electromagnetic simulations indicated that the current concentration along the outer edges of a sliced microstrip line was lower than that of a conventional microstrip line. We fabricated a 3-pole filter with a sliced microstrip line resonator using HTS YBCO thin films and measured the IMD3. Until the input power increase to 7 W, the filter has proper operation from 50 to 77.3 K. The third-order intercept point value of 64 dBm was observed at 70 K. The ΔIMD3 was less than −55 dBc at 1 W input power when the temperature was 50–77.3 K. The sliced microstrip line indicated low ΔIMD3 to the fundamental two-tone sinusoidal waves, which had a difference of 1 MHz, when the input power was 1 W. The results showed that HTS filters with sliced microstrip line resonators have better power handling capability and can feasibly be used in transmitters.

Design and Performance of Transmit Filters Using HTS Bulk Resonators for IMT-Advanced Applications

Abstract: A transmit band pass filter using circular-disk high-temperature superconductor (HTS) bulk resonators was designed with a 5.0 GHz center frequency and a 100 MHz bandwidth for use in international mobile telecommunication (IMT) advanced applications. The frequency response and electromagnetic field were calculated using a three-dimensional simulator. The mode of the resonator was used because it had the smallest diameter. Using stripline (SL) resonators enhanced both the unloaded quality factor and the coupling between the input/output ports and resonators. The frequency response of the designed SL 3-pole Chebyshev filter meets the requirements of IMT-Advanced applications. Additionally, the maximum surface current of the HTS bulk filter was approximately 86% less than that of a conventional hairpin filter.

Design and fabrication of 5 GHz band pass filter using circle-type HTS bulk resonator

Abstract: We designed and fabricated a transmit band pass filter (BPF) using circle-type high temperature superconductor (HTS) bulk resonators. A Dy–Ba–Cu–O bulk was fabricated using a modified quench and melt growth (QMG) process and cut into specimens of 8.40 and 8.44 mm in diameter and 0.5 mm thick for use as the HTS bulk resonators. A three-pole stripline (SL) BPF was designed based on a Chebyshev function and the frequency response and electromagnetic field of the filter were simulated using a three-dimensional electromagnetic field simulator. From the results of the simulation, the center frequency, bandwidth, insertion loss, and ripple of the designed filter were 4.97 GHz, 100 MHz, 0.03 dB, and 0.048 dB, respectively. In the experimental results on the actual fabricated filter, the filtering response was clearly observed; however, the center frequency of 5.46 GHz was higher than that of the simulation. The simulated maximum surface current in the resonators of the SL filter was approximately 86% smaller than that of a conventional hairpin filter. Furthermore, the measured response of the Dy–Ba–Cu–O bulk filter at an input power of 20 dBm was almost the same as that at 0 dBm. These results mean that an SL filter using a Dy–Ba–Cu–O bulk resonator may be practicable as a high-power transmit BPF.

Gain enhancement methods for printed circuit antennas

Abstract: Resonance conditions for a substrate-superstrate printed antenna geometry which allow for large antenna gain are presented. Asymptotic formulas for gain, beamwidth, and bandwidth are given, and the bandwidth limitation of the method is discussed. The method is extended to produce narrow patterns about the horizon, and directive patterns at two different angles.

Recent advances in pulsed-laser deposition of complex oxides

Abstract: Pulsed-laser deposition (PLD) is one of the most promising techniques for the formation of complex-oxide heterostructures, superlattices, and well controlled interfaces. The first part of this paper presents a review of several useful modifications of the process, including methods inspired by combinatorial approaches. We then discuss detailed growth kinetics results, which illustrate that 'true' layer-by-layer (LBL) growth can only be approached, not fully met, even though many characterization techniques reveal interfaces with unexpected sharpness. Time-resolved surface x-ray diffraction measurements show that crystallization and the majority of interlayer mass transport occur on timescales that are comparable to those of the plume/substrate interaction, providing direct experimental evidence that a growth regime exists in which non-thermal processes dominate PLD. This understanding shows how kinetic growth manipulation can bring PLD closer to ideal LBL than any other growth method available today.

Electrically small, superdirective, and superconducting antennas

Abstract: Preface. 1: ELECTRICALLY SMALL ANTENNAS. 1.1 Introduction. 1.2 Fundamental Limitations. 1.3 Electrically Small Antennas: Canonical Types. 1.4 Clever Physics, but Bad Numbers. 1.5 Pathological Antennas. 1.6 ESA Summary. References. Index. 2: SUPERDIRECTIVE ANTENNAS. 2.1 History and Motivation. 2.2 Maximum Directivity. 2.3 Constrained Superdirectivity. 2.4 Bandwidth, Efficiency, and Tolerances. 2.5 Miscellaneous Superdirectivity. 2.6 Matching Circuit Loss Magnification. 2.7 Non-Foster Matching Circuits. 2.8 SD Antenna Summary. References. Index. 3: SUPERCONDUCTING ANTENNAS. 3.1 Introduction. 3.2 Superconductivity Concepts for Antenna Engineers. 3.3 Dipole, Loop, and Patch Antennas. 3.4 Phasers and Delay Lines. 3.5 SC Antenna Summary. References. Subject Index.

Flux pinning enhancement in YBa2Cu3O7−x films with BaSnO3 nanoparticles

Abstract: Nanoparticles of BaSnO3 were incorporated into YBa2Cu3O7?x (YBCO) films on LaAlO3 substrates for magnetic flux pinning enhancements. More than an order of magnitude improvement in the high field magnetization Jc at 6?T at 77?K was observed as compared to regular YBCO films. The irreversibility field (Hirr) was increased to 8.5?T at 77?K and to 13.4?T at 65?K. The in-field transport current measurements confirmed an order of magnitude improvement in high fields. The angular dependence of the Jc data at 1?T showed that is 1.3 times higher than indicating the presence of c-axis correlated defects. Transmission electron microscopy studies revealed the presence of a large density of uniformly distributed ~10?nm sized BaSnO3 precipitates and strain fields around them. A dual sector pulsed laser deposition target is used to produce the films, thus eliminating reactions between BaSnO3 and YBCO during the target preparation stage, but may allow the BaSnO3 to react locally and create defects that act as pinning centres.