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We examined a portable cryostat utilizing liquid nitrogen with temperature control function for operation of high-temperature superconducting quantum interference devices (HTS-SQUIDs) to have a higher slew rate. To operate a non-destructive evaluation system with HTS-SQUID in city areas with serious environmental electromagnetic noise, we often need a higher slew rate. A glass Dewar is installed in an outer case made of aluminum, which is sealed hermetically. The temperature of liquid nitrogen is controlled by using a vacuum pump connected to a feedback circuit, which monitors the temperature of liquid nitrogen. The temperature of liquid nitrogen could be controlled in the range between 73 and 77.2 K with a 0.1 K step. For example, the temperature was kept at 75 K with precision of ±0.05 K for 9 hs and SQUID was operated stably for this period. The ΔV and the slew rate of a test SQUID were increased from 22 μV and 3.2 mT/s at 77.2 K to 37 μV and 5.9 mT/s at 73 K, respectively. The white noise was also improved from 48 fT/√Hz at 77.2 K to 32 fT/√Hz at 74 K. We believe that this technology is useful to extend application of HTS-SQUIDs in the fields.

To perform an early stage detection of the deterioration of steel parts used in infrastructures, a nondestructive testing (NDT) method is required, which is capable of evaluating hidden parts such as a steel deck covered with an asphalt pavement. To achieve this, a magnetic field detector should be sufficiently large to detect magnetic fields over a wide range to account for high lift-off. In addition, the detector must be capable of functioning at high frequencies for a high-speed inspection of surface cracks, and at low frequencies to compensate for the skin effect during the inspection of inner or rear side cracks. In this paper, we developed a magnetic field detector for eddy current testing in a wide frequency range by combining a high-temperature superconducting (HTS) coil made using DI-BSCCO tapes with a tunnel magnetoresistive (TMR) sensor. The TMR sensor was attached on the inside of the loop of the HTS coil to maximize the shielding characteristic. We applied the developed system to NDT, which can detect a signal from an artificial crack with a lift-off of 75 mm, which is greater than the thickness of the asphalt pavement. In addition, inner cracks that are 2.7 mm beneath the surface can be detected with a 10 Hz applied magnetic field.

We have developed a three-channel high-temperature superconductor (HTS) superconducting quantum interference device (SQUID) eddy current testing (ECT) system for detection of a fatigue crack which occurs in a steel deck plate used in expressway bridges. To avoid physical interference between adjacent channels, a dipole coil with compact design is employed. A field test using the three channel ECT system was performed on an expressway bridge having an orthotropic steel deck structure under traffic lane restrictions. Although a signal corresponding to existence of a fatigue crack was not detected due to a low occurrence rate of fatigue crack, we could demonstrate a stable and long-time operation in an urban area during the daytime and acquisition of correct data corresponding to some structural features of the expressway bridge.

We have developed an high-temperature superconductor (HTS)-superconducting quantum interference device (SQUID) eddy current testing (ECT) system using an HTS-SQUID module and external pickup coil made of Bi-2223 tape to investigate the feasibility of nondestructive evaluation of a steel plate paved with asphalt. Steel plates with simulated cracks were measured by using the system with a liftoff of 80 mm, which corresponds to a typical thickness of the asphaltic pavement. A signal from a backside crack in the steel plate can be detected by reducing the frequency of the excitation field to less than 40 Hz. In addition, it was found that a fluctuation of the background signal caused by an inhomogeneous magnetization decreases with decreasing the frequency while the signal from the crack does not depend on the frequency, resulting in clearer detection of the signal from the crack at lower frequencies. Since the signal from the eddy current decreases with decreasing the frequency, the observed signal from the crack probably originated from the flux leakage of the excitation field while the fluctuation of the background signal originated from the eddy current. The successful detection of the backside crack in the steel plate is expected to lead to diagnosis of an early-stage fatigue damage of social infrastructures.

We have tried to develop an induction logging system for oil fields utilizing a high-temperature superconducting quantum interference device (HTS-SQUID). Enhanced oil recovery (EOR) technology utilizing carbon dioxide (CO2 ) is a promising technique for efficient production of petroleum. To control the EOR, it is important to improve the accuracy of monitoring technique of CO2 injected into the oil reservoir. The HTS-SQUID has a potential to monitor CO2 behavior in crude oil reservoir because of its high sensitivity in a wide frequency range from dc to over 1 MHz. As the first step of the developments, we prepared a vessel for SQUID system by using carbon fiber reinforced plastic with pressure tightness of 70 MPa and thermal durability of 200 °C. By using the SQUID system placed at a depth of about 300 m in a steel-cased borehole filled with water, magnetic signals from a transmitter coil in another borehole were successfully measured.

Magnetic sensing techniques have been recently developed to detect biomarkers using magnetic nanoparticles (MNPs) in liquid-phase assays. In the case of alternating current (ac) magnetic susceptibility when detecting a low number of MNPs, the diamagnetic signal of water becomes a major problem at a low signal-to-noise ratio. Therefore, we developed a high-temperature superconducting-quantum-interference-device-based ac susceptibility measurement system that can detect third-harmonic signals from MNPs. On the basis of the nonlinear characteristic of MNPs with large ac magnetic excitation (1.06 kHz), the third-harmonic signal detection (3.18 kHz) of MNPs leads to the elimination of the diamagnetic signal of water. The system consists of excitation and gradiometer pickup coils and a multifunctional circuit device. To improve the signal-to-noise ratio of the third-harmonic measurement, the system has two cancelation functions regarding the fundamental magnetic field. The MNPs are magnetized by using the fundamental magnetic field using the excitation coil. Third-harmonic signals from the MNPs were then detected during the up-and-down movement of the MNPs. We evaluated the detection sensitivity of our system using MNPs. We confirmed that the limit of detection consistency of MNPs was 10 ng by using the third-harmonic measurement method.

We developed a hybrid cooling system consisting of a liquid-nitrogen Dewar and a cryocooler with the aim of cooling HTS-SQUIDs fast at operation sites, extending cooling period time, and avoiding noise increase caused by the cryocooler. Liquid nitrogen is evaporated mainly by thermal inflow into the Dewar. Thus, we tried reducing the thermal inflow into a glass Dewar by cooling its inner surface using a small Stirling cryocooler with a cooling capacity of 16 W at 77 K, and examined the cooling period and the operation procedure. We successfully kept 0.6 liter of liquid nitrogen for one week with 0.2 liter reduction. It was also indicated that long time and low noise operation of HTS-SQUIDs would be possible in the hybrid cooling system by temporarily stopping the cryocooler during measurements.

Direct detection of magnetic fields generated by the ion currents in electrolytic cells was performed and the ion currents were mapped during a cyclic voltammetry experiment. A high-Tc superconducting quantum interference device (SQUID) gradiometer with ramp-edge Josephson junctions was used as the detector and thus the first derivative of the magnetic field along the electrolytic cell surface was obtained. The electric potential vs. the reference electrode was measured between −0.3 V and 0.8 V and the hysteresis signals caused by the redox reactions of the electrolyte were observed. The vector component of the ion current during cyclic voltammetry was mapped along the electrolytic cell. As a demonstration of visualization of ion transportation, the magnetic signal was also measured at various positions above the electrolytic cell surface.

We have investigated the fabrication of a high-temperature superconducting (HTS) gradiometer with long baseline for geophysical applications. The proof-of-concept gradiometer using a 1-turn pickup coil made of a GdBa 2 Cu 3 O y coated conductor (GCC) and 5.5-turn input coil integrated on a SQUID was fabricated in our previous work. In this study, we have optimized the device structure to improve the frequency response, gradient field sensitivity and gradiometer balance. The fabricated flux transformer consists of a 6-turn planar gradiometric pickup coil and a 26-turn input coil made of an HTS thin film. A low-melting-point alloy was used to connect polished Ag surfaces of the CGG pickup coil and Au pads of the input coil. An HTS SQUID was formed on another substrate and stacked on the input coil. A mechanical balancing structure using three pieces of GCC as a superconducting shield was also implemented. The fabricated gradiometer showed a gradient field noise of 0.8 fT/cm Hz 1/2 in the white noise regions, a gradiometer balance of 1/142, and a cutoff frequency of 9 Hz corresponding to a 2 mΩ contact resistance between the pickup coil and the input coil.

We have fabricated a large high- T c superconducting (HTS) SQUID gradiometer with a baseline length of 130 mm for geophysical application. It consists of an HTS-SQUID gradiometer chip with an integrated input coil and a large pickup coil made of an HTS coated conductor (CC). In this work, fabrication of the chip containing ramp-edge Josephson junctions and multilayer structures is reported. An MgO(1 0 0) substrate with a size of 15 × 15 mm 2 was used. The half area of the substrate was occupied by gold pads for connection with an HTS-CC. A directly-coupled thin-film gradiometer with a baseline length of 7 mm was prepared in the remained half area. SmBCO and La-doped ErBCO having T c ’s around 90 K were used for base- and counter-electrodes, respectively. A 5.5-turn input coil made of La-doped ErBCO was formed on the thin-film pickup coil made of SmBCO. The fabricated gradiometer chip was operated by supplying electrical current through the input coil, but modulation corresponding to only three flux quanta was observed in the V – Φ curve. We found that the critical current density of the thin-film pickup coil was insufficient. We speculated that the rather wide pickup coil was not fully oxygenated. After longer annealing treatment, the number of modulation peaks in the V – Φ curve was significantly increased.

For accurate and stable monitoring of the oil/gas and fluid distributions in the reservoir during the EOR (Enhanced Oil Recovery) operation even if the well distance exceeds a thousand meter, JOGMEC (Japan Oil, Gas and Metals National Corporation) is developing a crosswell electromagnetic survey system with applying a super-sensitive magnetic sensor "SQUID" (Superconducting Quantum Interference Device). In this study, we have developed the component technologies of borehole SQUID magnetometer and powerful transmitter coil. The SQUID sensor performs in the temperature condition of 77 K, where the liquid nitrogen is available for cooling. To deploy the SQUID sensor in the deep borehole safely, we designed and trial fabricated pressure- and heat-resistant vessel which contains the glass Dewar of liquid nitrogen, release tubes of vaporized nitrogen gas, electrical circuits, and other devices to control the SQUID sensors. The developed transmitter coil has magnetic moment of 27,500 Am2, which will satisfy the intensity for our purpose. We demonstrated the stable operations of the prototype models of borehole SQUID magnetometer and transmitter systems with using the research wells of the JOGMEC field, of which maximum depth is 300 m and cased with conductive tube. The SQUID magnetometer has the ability to be deployed in the environment of 3,000 ~ 4,000 m deep boreholes by the laboratory experiments. At the next stage, we will demonstrate the systems at the actual borehole, whose typical depth is greater than 1,000 m. On the other hand, we have been developing the algorithms to interpret and correct the effects of conductive casing tubes, and three-dimensional modeling of underground structure. For the demonstration of their performance, we will also conduct the crosswell measurement with using the test wells.

Recent developments of electronic devices containing Josephson junctions (JJ) with high-Tc superconductors (HTS) are reported. In particular, the fabrication process and the properties of superconducting quantum interference devices (SQUIDs) with a multilayer structure and ramp-edge-type JJs are described. The JJs were fabricated by re-crystallization of an artificially deposited Cu-poor precursory layer. The formation mechanism of the junction barrier is discussed. We have fabricated various types of gradiometers and magnetometers. They have been actually utilized for several application systems, such as a non-destructive evaluation (NDE) system for deep-lying defects in a metallic plate and a reel-to-reel testing system for striated HTS-coated conductors.

We developed a non-destructive evaluation (NDE) technique combining the high-speed scanning laser observation system and the SQUID gradiometer array system for multi-filamentary coated conductors (CCs). The high-speed scanning laser observation system can detect anomalies in height of the surface and reflectivity, and marks the information of defects on the backside of the substrate. GdBCO CCs striated into five lines were evaluated by using the NDE technique, and three kinds of delamination defects and the electrical shorts between the filaments were evaluated. The SQUID system and the laser system have different strong points and weak points for evaluation of striated CCs. All three types of delamination defects could not be identified unless both the SQUID and the laser systems were used. The combination of these evaluation systems is useful to develop reliable CCs and especially for AC power applications.

We have studied the fabrication process for high- T c superconducting quantum interference devices (HTS-SQUIDs) containing ramp-edge-type Josephson junctions and multilayer superconducting wiring. Integrated HTS-SQUID magnetometers with a 20-turn input coil were fabricated. SmBCO and La-doped ErBCO were used for base- and counter-electrodes having T c ’s around 90 K, respectively. MgO (1 0 0) substrates with a size of 15 × 15 mm 2 were used. Two types of magnetometers with different washer sizes and different coil widths were fabricated: Type-I had a 2-μm-wide coil on a 200 × 200-μm 2 washer, and Type-II had a 4-μm-wide coil on a 300 × 300-μm 2 washer. Both the magnetometers exhibited rather low white noise level around 25 fT/Hz 1/2 (@77 K, 1 kHz). Peak-to-peak modulation voltage around 16–20 mV was obtained for both the magnetometers. When the external magnetic field increased, the modulation curve was abruptly distorted. Modulation corresponding to 51 flux quanta appeared in the V – Φ curve of a Type-I magnetometer. On the other hand, modulation up to 121 flux quanta could be seen for a Type-II magnetometer. It was likely that the critical current of the superconducting loop consisting of a pickup and an input coil restricted the number of the observed flux quanta. It seems that the wider coil in the Type-II allowed a larger superconducting current in the loop. We succeeded in improving dynamic range of the V – Φ characteristics.

We examined the ability to detect deep-lying defects in multilayer stacks of aluminum plates using an eddy-current testing technique with HTS planar-type SQUID gradiometers. In addition to a previous gradiometer with a baseline of 1 mm, a new gradiometer with a baseline of 8.5 mm and a larger effective area fabricated by HTS multilayer and ramp-edge junction technologies was used. The frequency and depth dependences of the SQUID signal were compared between the two gradiometers. By employing the longer-baseline gradiometer and an excitation frequency of 35 Hz, a slit-like defect located at about 38 mm in depth could be clearly observed with a signal-to-noise ratio larger than 10.

We have fabricated a two-axis high-Tc planar gradiometer. Two gradiometers with planar gradiometric pickup coils lie diagonally on a square substrate. Therefore, they can measure field gradients, dBz/dx and dBz/dy at the same site. The two-axis gradiometer was fabricated on a 15 × 15 mm MgO (100) substrate using an HTS multilayer process including crossovers, superconductive contacts and ramp-edge junctions. The width of the pickup coil and the baseline length are 6 mm and 8.5 mm, respectively. Voltage modulations in V-Φ characteristics more than 20 μV at 77 K were obtained for both the gradiometers. Parasitic effective areas measured by applying a uniform magnetic field were about 0.0001 mm-2. The gradiometer balance was estimated to be about 10-2. The parasitic effective area was not due to the imbalance of the gradiometric pickup coil, but due to a sensitivity of SQUID itself. Flux noise level as low as 10 μΦ0/Hz1/2 at frequencies down to 3 Hz and a cross-talk ratio less than 1/230 were obtained.

We have studied the fabrication process of HTS-SQUIDs with ramp-edge Josephson junctions (JJs). SmBa2Cu3Oy (SmBCO) and Er0.95La0.1Ba1.95Cu3Oy (L1ErBCO) having Tc's around 90 K were used for base- and counter-electrodes, respectively. SrSnO3 (SSO) was used for insulating layers. A black-colored Pr1.4Ba1.6Cu2.6Ga0.4Oy (P4G4) layer was first grown on MgO substrate via a thin BaZrO3 buffer layer. It was expected to work as a temperature homogenizer over the whole substrate area during deposition of the upper layers. We fabricated planar gradiometers having pickup coils with sizes of 0.5 × 1.0, 1.0 × 1.0 and 3.0 × 3.0 mm2. Their baselines are 1.0, 1.0 and 3.0 mm, respectively. The gradiometers exhibited flux noise at 77 K as low as 3.3 ~ 4.5 μΦ0/Hz1/2 at 1 kHz. Directly and inductively coupled magnetometers having a 13.5 mm square pickup coil were also fabricated. The latter has an integrated 20-turn input coil. They showed field noise levels of 55 and 10 fT/Hz1/2 at 1 kHz, respectively.

We examined the ability of detecting defects in multilayer aluminum plates by using an HTS SQUID gradiometer in an unshielded environment. The planar HTS SQUID gradiometer with 1 mm × 1 mm pickup loops and 1 mm baseline was fabricated by using HTS multilayer and ramp-edge junction technologies, and cooled by thermal conduction from a LN 2 cryostat. We tried non-destructive evaluation (NDE) of a structure consisting of 5–10 layers of 2 mm thick aluminum plates with a through hole, which has a shape of slit, 30 mm in length and 0.5 mm in width only in the bottom layer, by changing the frequency of eddy current induced by a double-D type coil. It was found that the observed frequency and depth dependences of the peak gradiometer signal were well fitted by theoretical curves taking account of decay of eddy current and defect-induced magnetic field depending on the distance between the defect and the gradiometer. By employing 200 Hz frequency, the slit in the 10-layer structure could be clearly detected, indicating the ability of detecting defects located in depth more than twenty times larger than the gradiometer baseline.

We have studied the fabrication process for high- T c superconducting quantum interference devices (HTS-SQUIDs) containing ramp-edge-type Josephson junctions and multilayer superconducting wiring. 5-Channel gradiometer arrays for non-destructive evaluation of striated coated conductors were fabricated. Our previous process contained some problems which often brought about property spreads of the gradiometers and/or degradation in superconducting electrodes. We attempted to improve homogeneity of substrate temperature during barrier preparation. The thickness of a black-colored Pr 1.4 Ba 1.6 Cu 2.6 Ga 0.4 O y embedded at the lowest in the multilayer was increased. The substrate on which bare ramp-surfaces were arranged was heated from the rear side by thermal radiation. A sapphire plate as a temperature homogenizer was inserted between the substrate and the heater. At the final procedure of the process, careful oxygenation containing surface cleaning by dry etching prior to heat-treatment was carried out. By employing them, five HTS-SQUIDs exhibiting the peak-to-peak modulation voltage over 50 μV were successfully fabricated within 6 × 6-mm 2 area in a chip. Their noise levels at 1 kHz and 77 K were as low as 4 μΦ 0 /Hz 1/2 .

We developed a non-destructive evaluation (NDE) system using an HTS SQUID array in order to examine rare-earth (RE)-123 HTS coated conductors striated into multi-filamentary lines. The 5-channel HTS SQUID gradiometer array was composed of ramp-edge junctions with LaErBaCuO and SmBaCuO electrode layers, and fabricated by using an HTS multi layer fabrication technique. The planar gradiometers with 1 × 1 mm 2 pickup loops and a baseline of 1 mm detected the vertical element of magnetic field gradient induced around defects by an eddy current. The gradiometer array cooled by thermal conduction from a liquid nitrogen bath was placed above the coated conductor on the main stage with a lift-off of about 1.5 mm. A coated conductor was fed from a reel to reel, and cooled blow its T c by stages connected to Gifford–Mcmahon (GM) coolers. By employing a 3 kHz induction current generating the maximum field of 0.14 mT, we could identify a distribution of defects in a long-length non-striated conductor. Furthermore, we could detect and distinguish three kinds of defects, existence of a spotty normal-state region, electrical short between striated filaments, and delamination of the superconducting layer from the Hastelloy tape for each filamentary superconducting line at a high speed up to 30 m/h.

We developed a non-destructive testing (NDT) system utilizing multi-channel high-temperature superconductor (HTS) SQUID gradiometers in order to examine rare earth (RE)-123 coated conductors striated into multi-filamentary lines. The 5-channel HTS SQUID gradiometer array was composed of ramp-edge junctions with LaErBaCuO and SmBaCuO electrode layers, and fabricated by using an HTS multilayer fabrication technique. The parallel gradiometers detected the vertical element of magnetic field gradient induced around defects by an eddy current. It was demonstrated that we can detect macroscopic defects in each filamentary superconducting line at high speed up to 30 m/h.

We have fabricated high-temperature superconductor (HTS) dc superconducting quantum interference devices (SQUIDs) with a multilayer structure and ramp-edge Josephson junctions. For stable operation at 77 K, SmBa2Cu3Oy (SmBCO) and La0.1-Er0.95Ba1.95Cu3Oy (La-ErBCO) films with a T c above 90 K were employed as materials for the base-electrode and counter-electrode layers, respectively. Junctions were fabricated by employing a Cu-poor La-ErBCO layer as a precursor of the barrier. The fabricated parallel gradiometers with a baseline of 1-3 mm exhibited the field-induced modulation voltage of 20-50 muV at 77 K, and a clear modulation up to 86.8 K. Noise measurements at 77 K revealed a white flux noise of 4.5-10 muPhi0/Hz1/2 at 1 kHz. The magnetometer with an integrated 20-turn flux transformer and the effective area of about 1.8 mm2 exhibited a field noise of 25 fT/Hz1/2 or less at 1 kHz.

We proposed a novel multilayer structure having a new combination of oxides for integrated high- T c SQUID with ramp-edge Josephson junctions. La 0.1 Er 0.95 Ba 1.95 Cu 3 O y (L1ErBCO) and SmBa 2 Cu 3 O y (SmBCO) were used as counter- and base-electrodes, respectively. An SrSnO 3 (SSO) layer was deposited as an insulating layer. Prior to deposition of SmBCO, Pr 1.4 Ba 1.6 Cu 2.6 Ga 0.4 O y (P4G4) and SSO were deposited on MgO (100) substrate in order to improve film quality of L1ErBCO/SSO/SmBCO layers. The black-colored P4G4 layer was expected to work as a temperature homogenizer over a whole substrate area during deposition of the upper layers. All the layers except L1ErBCO were deposited by an off-axis magnetron sputtering. An L1ErBCO layer was deposited by a pulsed laser deposition method. A thin Cu-poor L1ErBCO layer was initially deposited to form an adequate barrier on the ramp-edge of SmBCO. Gradiometer arrays having multilayered feedback coils and pickup loops were made in a chip and their proper operation at 77 K was confirmed. The present multilayer structure is promising for application to SQUIDs with more complicated designs.

This paper describes a compact cooling system using a single-stage stirling-type cryocooler for a practical HTS sampler. The system was designed to cool down an HTS sampler module below 50 K, enabling a bandwidth of the chip more than 100 GHz. The system measures 150 mm in width, 140 mm in height and 310 mm in depth, and weighs 5 kg. Semi-rigid coaxial cables made of brass with a silver coated inner conductor were adopted for a signal to be measured and a trigger pulse. The loss for the signal line was less than 1.5 dB at 50 GHz with relatively small thermal inflow. Thermal inflows from low frequency lines, IF signal lines for control/output of the sampler and dc bias lines, were minimized by choosing proper wires. A new sampler module with reduced weight was placed on the cold stage, which was surrounded by double magnetic shields. The module was successfully cooled down to less than 50 K with cooling time of 1 h in the system. We have also succeeded in observing sinusoidal waveforms with the HTS sampler cooled by the compact cooling system.

We have succeeded in observing a pico-second current pulse accompanying a single flux quantum (SFQ) using a high- T c superconducting (HTS) sampler. The sampler circuit used in this study, an on-chip sampler, has two pulsers connected to each other at the comparator in one chip. The waveform of an SFQ current pulse from one pulser is measured by using another SFQ pulse from the other pulser as a sampling pulse. We observed a waveform exhibiting two peaks with 5.8 and 1.6 ps widths. Based on these values, we estimated the sampling-pulse width as 2.8 ps, suggesting the potential bandwidth for the HTS sampler of 125 GHz. This value coincides well with the bandwidth expected from the critical current-normal resistance ( I c R n ) product of the Josephson junctions of 1 mV.

We improved the fabrication technology of high-temperature superconducting (HTS) single-flux-quantum (SFQ) circuits by suppression of oxygen loss. Among a lot of difficulties in the HTS circuit fabrication process, loss of oxygen is one of the most significant problems. To understand the oxygen going in and out at each step of the fabrication process, we utilized a room-temperature probing system and a low-temperature probing system. We found that some of the steps after the fabrication of junctions, such as deposition of an insulating layer, ashing of photo-resist, and RF sputtering of an Au wiring layer, cause loss of oxygen from the junctions. We constructed the peripheral fabrication process by combining the annealing process in oxygen and the process with oxygen loss, and obtained controlled I c , sufficiently large I c R n , and T c of the junction.

We are developing waveform measurement systems using a high- superconducting (HTS) sampler. As one of the methods for introducing high-frequency signals into the sampler chip, we are testing optical input systems. A uni-traveling-carrier photodiode (UTC-PD), which has potential bandwidth of over 300 GHz, is used in the systems for converting optical signals to electrical ones in front of the sampler circuit. Our preliminary experiments revealed high enough sensitivity even at temperatures as low as below 40 K for the UTC-PD that is to be cooled with the sampler chip. In the present study, we test two types of modules. One packages the UTC-PD separately from the sampler chip, and the other integrates both of them. In the former type, the optical and sampler modules are connected using V-connectors. The modules are cooled with a cryocooler to the operating temperature of the sampler circuit, which is around 40 K. Signals to be measured are input into the UTC-PD with a 1.55-mum carrier wave. We have succeeded in observing sinusoidal waveforms of up to 20 GHz using both types of modules.

We have developed a magnetic prospecting system with HTS SQUID gradiometers for exploration of metal resources. The SQUID gradiometer consists of a flux transformer chip made of a YBCO thin film and a SQUID gradiometer chip which is stacked on the transformer chip. The SQUID gradiometer chip was fabricated by using an HTS multilayer and ramp-edge junction technology at ISTEC. Their effective volume and the balance estimated by using a Helmholtz coil were approximately 3 x 10-9 m3 and 1/500, resulting in the gradiometric field noise of 7 pT/m/Hz1/2 at 10 Hz. Two assembled SQUID gradiometers which measure dBz/dx and dBz/dy field gradients were cooled with liquid nitrogen in a cryostat of the magnetic prospecting system. The cryostat was suspended from the frame of the system and its attitude was self-controlled by gravity. The magnetic prospecting system with the HTS SQUID gradiometers, flux-gate sensors, a GPS module, and a gyro sensor was also tested in a field near an old mine.

Photosensitive polyimide was selected as a protection layer for the YBa2Cu3O7−x circuits and stability of ramp-edge-junction characteristics against atmospheric moisture was investigated. Both coating and patterning of the protection layer could simultaneously be formed on the ramp-edge-junctions by using the polyimide film. Characteristic change of the ramp-edge-junctions that were coated with a polyimide film were less than 10% after thermal cycling more than ten times between room temperature and liquid He temperature. On the other hand, characteristic change was much more than 20% for the bare junction as narrow as 2 μm.

We have investigated the flux state in high- and low-Tc SFQ circuits having superconducting ground planes with moats by a scanning SQUID microscope (SSM) system. Observation of YBa2Cu3Oy (YBCO) sampler circuits having superconducting ground planes with 5 µm-wide moats confirmed that magnetic flux was excluded from the moat-surrounded area including ramp-edge Josephson junctions. The threshold value for complete flux exclusion was about 0.8 µT slightly depending on chips, which can be achieved readily with dual μ-metal magnetic shielding. Moreover, we have observed the flux state in Nb multilayer structures having a superconducting ground plane with 4 µm wide moats, which was designed and fabricated using the CONNECT cell library. It was confirmed that magnetic flux was trapped selectively into moats with appropriate configuration, which were fabricated within each 40 µm × 40 µm unit cell.

We constructed a laser-plume monitoring system to stabilize fabrication process of high-temperature superconducting (HTS) circuits with films and junctions prepared by a laser ablation method. The system utilizing a CCD camera with wide band sensitivity accumulates intensity of ultraviolet ray for many plumes as an averaged matrix data of 256 × 256. Using the matrix data, we successfully estimated the magnitude or length of plumes objectively with a measurement error of ±0.15 mm. We found that target temperature, laser gas condition and target consumption have strong influences on the magnitude of plumes even if the laser power monitor shows the same output power. We found the conditions to obtain a stable magnitude of plumes and successfully controlled the magnitude of plumes even in unstable conditions of laser ablation. The junctions fabricated by using the stable plumes had resistively shunted junction (RSJ) type current–voltage ( I – V ) characteristics with a percentage of success of 93%.

This paper describes an overview of our plan and recent progress on the HTS sampler development with the aim of demonstrating its potential performance of a bandwidth over 100 GHz using a compact cryocooler. Several changes from the NEC’s former work have been made in the device fabrication process, the circuit design and the system design. We employed the HTS circuit process with a lower ground-plane structure, and improved the circuit design and the related packaging technology, which enabled a higher bandwidth. We have focused on observing 50 GHz waveforms by the sampler as the mid-point goal, and then succeeded in observing a 45 GHz sinusoidal waveform in magnetically coupled current measurement and a 50 GHz waveform in voltage measurement with a high-frequency module. An on-chip sampler observing short-duration sampling pulse was also designed and the observation was successfully demonstrated.

We have investigated the flux state in YBa 2 Cu 3 O y (YBCO) sampler circuits having superconducting ground planes with 5 or 7-μm-wide moats by a scanning SQUID microscope (SSM) system. It was found that magnetic flux was trapped into continuous moats with the length larger than 20 μm, and magnetic flux was excluded from the moat-surrounded area which includes ramp-edge Josephson junctions and ground plane contacts. We ensured that flux trapping into ground plane contacts is suppressed by reducing its size to typically 5 × 5 μm 2 . The threshold value for complete flux exclusion is about 0.8 μT slightly depending on chips, which can be readily achieved with dual μ-metal magnetic shielding. In addition, we have observed the magnetic flux due to electric noise which comes from outside the SSM chamber.

We propose a new ion etching procedure to level the surfaces of lower layers in multilayer structures for HTS SFQ circuits. In order to make a flat lower layer, we have utilized a variation in the etching rate with the ion incident angle in ion etching process. The etching process under the acceleration voltage of 280 V and the ion incident angle as low as 15° for 5 min improved the average surface roughness (Ra) of the SrSnO3 insulating layer from 2.8 nm to 0.8 nm. This procedure was applied just before the La–YBCO base layer growth on the SrSnO3/La–YBCO groundplane bilayer, which resulted in the base layer with Ra much less than 2 nm.

We succeeded in observing high-frequency current waveforms up to 45 GHz using a high-temperature superconducting (HTS) sampler. In this experiment, we used a sampler circuit with a superconducting pickup coil, which magnetically detects current signals flowing through a micro-strip line on a printed board placed outside the cryochamber. This type of measurement enables non-contact current-waveform observation that seems useful for analyses of EMI, defects in LSI, etc. Computer simulation reveals that one of our latest versions of HTS sampler circuits having Josephson transmission lines with optimized biases as buffers has a potential of sampling high-frequency signals with a bandwidth above 100 GHz. To realize the circuit parameters required in the simulations, we developed an HTS circuit fabrication process employing a lower ground plane structure with SrSnO 3 insulating layers. We consider that improvement of the circuit fabrication process and optimization of the pickup coil lead to much higher signal frequency observable by the sampler.

We report on a latching-type driver operated at 30 K for the output interface of single-flux-quantum (SFQ) circuits. Ramp-edge-type high-temperature superconductor (HTS) junctions showed only small inherent hysteresis in current–voltage characteristics at a temperature of 30 K. To have hysteresis large enough for latching operation, we chose La-doped YbBaCuO as a counter electrode of an interface modified junction, and used amorphous LaSrAlTaO to form an additional barrier to the interface modified barrier. The junction shunted with optimized capacitance showed sufficiently large hysteresis of 33%, which is as large as that of a non-shunted junction at 4.2 K. We employed 20 kA cm−2 as the critical current density (Jc) of the junction to fabricate dc/SFQ, Josephson transmission lines (JTLs) and the latching driver on the same chip. It is smaller than the optimum current density that gives the smallest punchthrough probability of the junction, which was calculated to be 10−12 for a 30 Gbps ac bias by using the junction parameters at 30 K. However, the probability for Jc of 20 kA cm−2 is still smaller than that of low-temperature semiconductor junctions because of the larger Jc of the HTS junctions. The fabricated latching driver successfully converted the SFQ pulse, which passed through several JTLs from the dc/SFQ circuit, to a 1 mV level signal at 30 K.

Latching-type driver circuits integrated with high-temperature superconductivity (HTS) junctions and two types of resistor were fabricated. These circuits could successfully perform latching operation by means of a single-flux-quantum (SFQ) signal input. Ramp-edge-type HTS junctions were fabricated by interface engineering and showed a hysteretic current-voltage characteristics under a temperature of 50 K. The critical current (I/sub c/) of these 5-micron-wide junctions was about 0.2 mA and the I/sub c/R/sub n/ product was about 1.7 mV at 4.2 K. The circuits included two types of resistors, which were made of RF-magnetron-sputtered indium-tin-oxide (ITO) and Au films. A driver with a parallel 2-junction-stack could convert an SFQ pulse, which was transformed through several Josephson transmission lines (JTL's) from a DC/SFQ circuit, to a 2.3-mV level signal.

A ramp-edge junction was fabricated with 200-nm-thick YBa 2 Cu 3 O 7− x (YBCO) electrodes and an intermediate insulating layer of 300-nm-thick LaSrAlTaO (LSAT). Extraneous electrode areas are placed on top of their ramps as a result of the fabrication process. The junction exhibited an RSJ-like I–V curve with a swelling or steps depending on the shape of the extraneous electrode areas. The junctions on a 4-mm□ chip were polished and the extraneous electrode areas were removed. We obtained quasi-planar junctions with planarized surfaces and base structures composed of YBCO and LSAT. The swelling and steps were explained as results of the resonance of parasitic inductance and capacitance as they disappeared after polishing. The I c R n product did not decrease, which suggests that the junction was not damaged by polishing. We think that polishing will be important in the fabrication of high- T c superconducting circuits with ramp-edge junctions.

We developed a latching-type driver using capacitively shunted high-temperature superconductivity (HTS) junctions for Single-Flux-Quantum (SFQ)-semiconductor output interfaces and fabricated it using ramp-edge-type HTS junctions. Assuming a junction IcRn product of 2 mV, a circuit simulation shows that the driver can produce an output of about 8 mV from an SFQ input pulse with a sufficiently short rise time for an interface clock operation of several gigahertz. HTS junctions were fabricated using the interface engineering method, and capacitors were made from an Indium oxide insulator (the dielectric constant was about 23 at 20 K) sandwiched by YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) electrodes. The hysteresis of the I-V characteristics of the junctions increased by increasing the area of capacitance. The latching operation of the driver was observed with an output voltage of up to 3 mV.

As a first step toward fabricating transistors to control supercurrent, we fabricated two- and three-terminal devices that have coplanar Nb electrodes coupled with a SrTiO/sub 3/ substrate. In the two-terminal devices, the Nb electrodes were coupled with an Nb-doped SrTiO/sub 3/ substrate with a carrier concentration of 2/spl times/10/sup 19/ cm/sup -3/. Electrode spacing was about 50 nm. The devices showed Josephson effect at 4.2 K. Three-terminal devices were made on a thinly doped surface of a non-doped 0.4-mm-thick SrTiO/sub 3/ substrate with gate electrodes on the non-doped area of the substrate. Resistance of the three-terminal devices was controlled by applying voltages to the gate electrode at 4.2 K. >

We measured the current-voltage characteristics of YBa/sub 2/Cu/sub 3/O/sub 7-x//oxide/n-SrTiO/sub 3/ diodes using NdGaO/sub 3/, LaAlO/sub 3/, CeO/sub 2/, and MgO as the oxide. MgO films had the highest current density. We then fabricated dielectric-base transistors with a YBa/sub 2/Cu/sub 3/O/sub 7-x/(YBCO) emitter/collector on a SrTiO/sub 3/ dielectric base with an MgO barrier. The transistors had both voltage and current gains exceeding unity at 4.2 K. The emitter current density was about 4/spl times/10/sup 3/ A/cm/sup 2/ at a collector-emitter voltage of 10 V and base-emitter voltage 10 V; this is 2 to 3 orders of magnitude larger than that of transistors with NdGaO/sub 3/ emitter-base barrier. We obtained a transconductance of around 0.4 mS at a collector-emitter voltage of 10 V for a device with a 6-/spl mu/m-diameter emitter. >