A. Saito

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

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.

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

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.

Relationship between surface resistance and critical current density of HTS thin films in a DC magnetic field

We investigated the relationship between the surface resistance (R/sub s/) and critical current density (J/sub c/) of DyBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (DyBCO) thin films in a dc magnetic field. The R/sub s/ of DyBCO films at 21.8 GHz was measured using a dielectric resonator method and the J/sub c/ was determined by magnetic measurement and Beans model. A dc magnetic field of up to 5 T was applied along the c-axis of the films. The relationship between the R/sub s/ and J/sub c/ of DyBCO thin films under a zero magnetic field was explained by the following equation, R/sub s/=/spl alpha//spl times/10/sup 7//J/sub c/, where /spl alpha/ is a constant of 1.2, and the units of the R/sub s/ and J/sub c/ are ohm and A/m/sup 2/, respectively. In addition, the R/sub s/ in dc magnetic fields up to 5 T were inversely proportional to the J/sub c/. However, the parameter /spl alpha/, which was fitted from the R/sub s/-J/sub c/ characteristics in each magnetic field, increased slightly with an increase the magnitude of the applied field. These results indicate that the relationship between the R/sub s/ and J/sub c/ in a zero magnetic field may be a general property of HTS thin films and that the /spl alpha/ in DyBCO thin films depends on the magnitude of the applied magnetic field.

Design for transmit bandpass filter and trimming method using superconducting bulk ring resonator and dielectric rods

We designed transmit bandpass filters (BPFs) by using superconducting bulk ring resonators and dielectric trimming rods at a 5.0 GHz center frequency with a 100 MHz bandwidth. The filter with ring resonators was smaller than the filter with disk resonators, where the size was defined as the distance between the input/output ports. We found that the size contributes significantly to the miniaturization of multi-pole filters. A trimming mechanism was also evaluated. The frequency shift region of a single ring resonator reached more than 500 MHz after we moved a dielectric rod whose dielectric constant was 39 into the ring resonator. A 3-pole strip-line BPF was also designed using a Chebyshev function by using three pairs of ring resonators and dielectric trimming rods with the same dimensions and by adjusting the positions of the trimming rods. We obtained the optimal frequency response with a low loss and passband ripple for this BPF.

Intrinsic Surface Resistance of YBCO Thin Films Under DC Magnetic Field

We investigated the dc magnetic field and temperature dependences of the microwave intrinsic surface resistance (<i>R</i>_s^int) of YBa₂Cu₃O_y (YBCO) superconducting thin films with various thicknesses. We used YBCO films from 100, 200, and 300 nm thick YBCO films deposited on CeO₂/Al₂O₃ substrates. The <i>R</i>_s^int is means the surface resistance of YBCO film without the dielectric loss of the substrates. The <i>R</i>_s^int was obtained from measured surface resistance (<i>R</i>_s) by using a phenomenological equation. The <i>R</i>_s was measured using the dielectric resonator method at 21.8 GHz. A dc magnetic field of up to 5.0 T was applied parallel to the c-axis of the superconducting thin films during the <i>R</i>_s measurements. The <i>R</i>_s^int of the YBCO thin films increased when the dc magnetic field was applied. These relations could be explained with the two-fluid model for high-frequency and high-magnetic field limits. The <i>R</i>_s^int ratio (defined as <i>R</i>_s^int (5T)/<i>R</i>_s^int (0T)) slightly decreased with the film thickness. These results indicate that the <i>R</i>_s^int of HTS films depends on the film thickness in the dc magnetic field and the <i>R</i>_s^int in a dc magnetic field may be related to the pinning center in the HTS films.

Comparison of power handling capabilities of sliced and conventional microstrip line filters

We examined a high power filter using a sliced microstrip line. We simulated the current distribution in the microstrip line by using electromagnetic field analysis software. The current flowing along the outer edge of the sliced microstrip line was less concentrated than in the conventional microstrip line. The electric test power of the sliced microstrip line filter was larger than that of the conventional filter. The electric proof power of the filter can be estimated from the current flowing along the outer edge of the microstrip line.

Design and Fabrication of 5 GHz Miniaturized Bandpass Filter Using Superconducting Microstrip Quasi-Spiral Resonators

We propose a miniaturized novel microstrip quasi-spiral resonator (QSR) system composed of rewound-type spiral resonator and c-type spiral resonator. We have also designed a 5 GHz superconducting 4-pole bandpass filter (BPF) using QSR for international mobile telecommunication (IMT)-advanced base station receiver applications. The filter has been fabricated using a single-sided MgB2 superconducting thin film deposited by thermal co-evaporation technique. The measured performance of the BPF has been found to show close agreement with the simulated one.

A submillimetre-wave SIS mixer using NbN/MgO/NbN trilayers grown epitaxially on an MgO substrate

We have designed, fabricated and tested a quasi-optical superconductor–insulator–superconductor (SIS) mixer employing distributed NbN/MgO/NbN tunnel junctions and NbN/MgO/NbN microstriplines at submillimetre-wave frequencies. These trilayers were fabricated by dc- and rf-magnetron sputtering on an MgO substrate at ambient temperature so that the NbN and MgO films were grown epitaxially. Our SIS mixer consists of an MgO hyperhemispherical lens with an antireflection cap and a self-complementary log-periodic antenna made of a single-crystal NbN film, on which the distributed SIS junctions and the two-section impedance transformers were mirror-symmetrically placed at the feed point of the antenna. As designed, the junctions are 0.6 μm wide and 15.5 μm long, which is sufficient to absorb the incoming signal along this lossy transmission line, assuming a current density of 10 kA cm−2. The mixer showed good I–V characteristics, with subgap-to-normal resistance ratios of about 13, although weak-link breaks were observed above the gap voltage. The minimum double side band receiver noise temperature was 334 K at 678 GHz, including the input noise of about 200 K as estimated by the standard technique. The noise temperature gradually rose to 672 K at 820 GHz. This behaviour may be caused by RF losses from the weak-link parts, due to the steps between the distributed junctions and the microstriplines being less than 1 μm wide.

Electromagnetic Evaluation of HTS RF Coils for Nuclear Magnetic Resonance

We propose a Helmholtz-type probe coil that uses a high-temperature superconductor (HTS) for use as 700-MHz nuclear magnetic resonance (NMR) RF coils with a high unloaded quality factor (Qu). We analyzed the microwave resonant properties of a one-turn coil set face-to-face. The coils were made of a YBa2Cu3O7-δ (YBCO) film deposited on an r-plane sapphire substrate. The film thickness was 300 nm. The resonant frequency was found to split into in-phase (lower frequency) and out-phase (higher frequency) modes. We focused on the self-resonant frequency of the one-turn coil. We simulated the horizontal magnetic field caused by the RF coil. Although the coil could generate a sufficiently large transverse magnetic field in the in-phase mode, such a large field could not be generated in the out-phase mode. Therefore, we decided to use the in-phase mode. We also measured and simulated Qu to confirm that it was large enough for use in the 700-MHz NMR system.

Characteristics of MOD Bi-2212 Thin Films on r-Cut Sapphire With

We have studied Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) thin films fabrication by a metal organic decomposition method. When the Bi-2212 thin films were prepared on c-cut sapphire substrates, the maximum critical temperature (T_C) was 25 K by 820°C air annealing in an electric furnace for 30 min. When the Bi-2212 thin films were prepared on r-cut sapphire substrates with CeO₂ buffer layer, the Bi-2212 thin films were grown epitaxially on the CeO₂ buffer layer. The maximum TC obtained in the Bi-2212/CeO₂/r-cut sapphire was 62.5 K by 840°C air annealing in an electric furnace for 30 min. To improve the surface morphology, face-to-face annealing process was introduced for the Bi-2212/CeO₂/r-cut sapphire. By face-to-face annealing, TC was increased up to 65 K, and the critical current density of 1.03 × 10⁵ A/cm² was obtained at 5 K.