High-T_c superconducting flux flow transistors were fabricated with co-evaporated thin films of YBaCuO. The vortex flow channels (2 µm in width) and the device patterns were formed by Ar ion milling. The three-terminal characteristics, vortex flow characteristics, transresistance, and current gain of the device were measured. The AC input-output characteristics of the device with an Au load resistor were also measured. The measured flow voltage, transresistance and current gain are discussed in relation to these AC input-output measurements.

A YBaCuO-Nonsuperconductive YBaCuO-YBaCuO coplanar Josephson junction has been fabricated, using Nonsuperconductive YBaCuO thin film deposited on an MgO(100) substrate with intentional and very local damage which was created by Focused Ion Beam. The YBaCuO grown on the damaged section of the substrate turned out to be non-superconductor, due to implanted Ga ions and the change in the crystal quality, facilitating formation of an S-N-S junction. We found the important fact that the critical current density decreased exponentially with inverse of the junction length which was changed from 0.2 to 1 µm, and that Ga ion was detected in the thin films of the junctions, and that the thin films of the junctions were formed by a mixture of an amorphous, a polycrystal and a crystal, which is confirmed by Transmission Electron Diffraction. And the damaged substrate gave rise to Ga segregation and the mixed crystal, which played an very important role to form the normal metallic YBCO thin film of the Josephson junction. All these facts are related with the S-N-S junctions.

The capacitance and the resistance of the Au/PrBa₂Cu₃O_y (PBCO)/YBa₂Cu₃O_x (YBCO) structure were measured at liquid helium temperature. A film of YBCO was deposited by rf magnetron sputtering at 700, and its thickness was 250 nm. A film of PBCO was deposited by rf magnetron sputtering at 690, and its thickness was less than 375 nm. The inverse capacitance and the resistance of the structure increased with PBCO thickness when PBCO thickness was more than 70 nm. However, the inverse capacitance was near zero, and the resistance was much less than that of PBCO itself when PBCO thickness was less than 70 nm. These results show the possibility that the electric property of PBCO within 70 nm from the PBCO/YBCO interface is different from that of PBCO itself, that is, there is a low-resistance region in PBCO near the YBCO/PBCO interface.

A Superconductor-Insulator-Superconductor (SIS) mixer using two junctions connected in parallel through a stripline inductance has been studied. The essential point of the two-junctions device is that the capacitance of the junctions was tuned out by the inductance to obtain a broadband operation without mechanical tuning elements. It has been shown by theoretical analysis that the performance of this type of device is excellent and nearly quantum-limited performance of the mixer can be obtained. It has been demonstrated that the double sideband (DSB) noise temperature of a receiver employing this type of device was less than 40 K over the bandwidth of 90-120 GHz and that the lowest receiver noise temperature of 18 K, which is only 3.2 times as large as the quantum limited photon noise was obtained around 118 GHz. Junctions used in the two-junctions device have significantly larger area, i.e. larger capacitance, and smaller normal resistance than conventional ones. In order to obtain a good impedance match between the source and the junctions, an impedance transformer made of a superconductiong stripline was integrated with the junctions. This type of two-junctions device can easily be scaled to submillimeter frequency without using submicron-sized SIS junctions.

We have investigated the Josephson microwave self-radiation and the linewidth from different types of YBa₂Cu₃O_y(YBCO) grain boundary junctions: natural grain boundary junctions, step-edge junctions and bicrystal junctions. The Josephson self-rediation was directly observed using a total power radiometer receiver with receiving frequencies f_REC=1.7-72 GHz. All junctions exhibited microwave self-radiation peaks with intensity of order of 10^-12-10^-14 W. For step-edge and bicrystal junction, they appeared at a voltage related to the Josephson frequency-voltage relation, V=n(h/2e)f, while for natural grain boundary junctions, the above relation did not hold, suggesting a Josephson medium property. For all types of junctions the observed Josephson linewidth deviated from the theoretical RSJ values due to the extra noise source in the grain boundary junction. The Josephson linewidth decreased with increasing the receiving frequency for all type of junctions. The reduction of Josephson linewidth at higher frequencies indicates that the critical current fluctuations due to a critical current spread at small bias voltages and a crystalline disorientation at the junction boundary generate an additional noise in grain boundary junctions.

A calculation method by the improved three-fluid model is shown to describe phenomenologically temperature and frequency dependence of surface resistance R_s for high-T_c superconductors. It is verified that this model is useful to describe temperature dependence of R_s for such high-T_c superconducting films as Y-Ba-Cu-O (YBCO), Eu-Ba-Cu-O, and Tl-Ba-Ca-Cu-O films. For the frequency dependence of R_s of a YBCO bulk, furthermore, the measured results which have not depended on f² in the frequency range 10-25 GHz, can be described successfully by this model. Finally, a figure of merit is proposed to evaluate material quality for high-T_c superconductors from the values of electron densities and momentum relaxation time determined by the present model.

Recenty, the Josephson effect-based voltage standard has been realized by using the Josephson junction array which is constructed by integrating many Josephson junctions. In this article, the 1-V Josephson-junction-array voltage standard used in routine calibration work and further development of the 10-V Josephson junction array at the Electrotechnical Laboratory (ETL) are introduced.

This paper describes a SQUID magnetometer and the measurement of small signals. It also describes the current state of SQUID technology developed in the SSL project.

This study shows that using the direct offset integration technique (DOIT) and additional positive feedback (APF) in a high-T_c dc superconducting quantum interference device (SQUID) improves the effective flux-to-voltage transfer function and reduces the flux noise of a magnetometer, thus improving the magnetic field noise. The effective flux-to-voltage transfer function and the flux noise with APF were measured at different values of the positive feedback parameter β_a, which depends on the resistance of the APF circuit. These quantities were also compared between conditions with and without APF. This investigation showed that a β_a condition the most suitable for minimizing the flux noise of a magnetometer with APF exists and that it is β_a=0.77. The effective flux-to-voltage transfer function with APF is about three times what it is without APF (93 µV/Φ₀ vs. 32 µV/Φ₀). The magnetic field noise of a magnetometer with APF is improved by a factor of about 3 (242 fT/Hz vs. 738 fT/Hz).

The relationship between the flux-trapping phenomenon and the device-structure of a SQUID has been studied using three types of SQUIDs; a SQUID with a guard-ring, a SQUID with a moat, and a SQUID without these structures. The change in the voltage-flux characteristics of the SQUIDs due to the flux-trapping are measured. For the measurements, an acceleration of the flux-trapping is realized by applying a magnetic field during cooling of the SQUIDs. From the measured results, the SQUID with the guard-ring and that with the moat can reject tha external magnetic field more effectively than the SQUID without these structures. The reason of the difference in the rejection of the external magnetic field is thought to be the existence of superconducting closed loops. However, the flux-trapping of the SQUID with the guard-ring and that with the moat occur more easily than the flux-trapping of the SQUID without these structures for the cooling under the finite magnetic field. Therefore, the moat structure and the guard-ring structure need a higher-grade magnetic shielding for a practical use.

We present a superconducting neural network which functions as an RS flip-flop. We employ a coupled-SQUID as a neuron, which is a combination of a single-junction SQUID and a double-junction SQUID. A resistor is used as a fixed synapse. The network consists of two neurons and two synapses. The operation of the network is simulated under the junction current density of 100 kA/cm². The result shows that the network is operated as an RS flip-flop with clock speed capability up to 50 GHz.

Cryocooler cooled superconducting magnets using Bismuth based high-T_c current leads have been successfully demonstrated. The magnets mainly consisted of a superconducting coil, current leads and a radiation shield which are cooled by a two stage Gifford-McMahon cryocooler without using liquid helium. Our first liquid helium-free 4.6 T (Nb, Ti)₃Sn superconducting magnet with a room temperature bore of 38 mm operated at 11 K has recorded a continuous operation at 3.7 T for 1,200 hours and total cooling time over 10,000 hours without trouble. As a next step, we constructed a (Nb, Ti)₃Sn liquid helium-free superconducting magnet with a wider room temperature bore of 60 mm. The coil temperature reached 8.3 K in 37 hours after starting the cryocooler. The magnet generated 5.0 T at the center of the 60 mm room temperature bore at an operating current of 140 A. An operation at a field of 5 T was confirmed to be stable even if the cryocooler has been stopped for 4 minutes. These results show that the liquid helium-free superconducting magnets can provide an excellent performance for a new application of the superconducting magnet.

MRI is a widely used diagnostic imaging modality because it has excellent diagnostic capabilities, is safe to use and generates images not affected by bone artifacts. Images are obtained by utilizing the phenomenon of Nuclear Magnetic Resonance (NMR) by which protons located in a static magnetic field absorb radiofrequency (RF) pulses with a specific frequency and release a part of the energy as a NMR signal. Potentially MRI has the ability to provide functional and metabolic information (such as flow, temperature, diffusion, neuron activity) in addition to morphological information. This paper describes the imaging principles and provides a general outline of some applications: flow imaging, metabolite imaging and temperature imaging.

The superconducting magnet on a maglev vehicle vibrate and heats up inside under the influence of various disturbances in running. We have investigated the characteristics of heating in the superconducting magnet vibrating under the electro-magnetic disturbance from the ground coils. This magnetic disturbance has a frequency component ranging widely from 0 Hz to several hundred Hz which is proportional to the speed of the maglev vehicle. It was revealed that an extreme increase of heat load on the inner vessel of the energized magnet occurred at a particular frequency and it surpassed the capacity of the refrigerator installed in the tank of the superconducting magnet. As a result of the investigation, we could identify broadly three factors of heating, and now we have good prospects of largely suppressing the heating by reducing the disturbance through the folded arrangement of the ground coils and a structural improvement of the magnet.

A Monte Carlo method for the passage of electrons based on a single scattering model is developed. A code based on this method is operable on personal computers, and has been applied to analyze electron behavior in a layered system consisting of Ti (an accelerator window), air, cellulose triacetate (CTA) and backing material irradiated by 300 keV electrons. The energy spectra and the angular distributions of electrons on the CTA surface as well as depth distributions of energy deposition in the CTA for various backing materials have been obtained. Some of these results are compared with experiments, and show fairly good agreement.

This paper describes an innovative heat-pipe cooling technology for asynchronous transfer mode (ATM) switching multichip modules (MCMs) operating with a throughput of 40 Gb/s. Although high-speed ATM link-wires are connected at the top surface of the MCMs, there is no room to cool the MCM by forced air convection, because power and the system clock signal are supplied by connectors on the rear and periphery of the MCM. We therefore chose to attach a cold-plate to the back of each MCM. The condenser part of the heat pipe, which is mounted behind the power supply printed circuit board, is cooled by low-velocity forced air. Total power dissipation is about 30 watts per MCM. With a 2 m/s foreced airflow, the sub-switching-element module (four MCMs) operates at a throughput of 80 Gb/s with a maximum junction temperature of less than 85. Measured thermal resistance between the switch LSI junction and air is about 6/W. This heat-pipe cooling system has a small system footprint, compact hardware, and good cooling capacity.

Reducing switching noise is a key point in increasing signal transmission capability. This noise is related to the pin assignment of connectors and the inner layer structure of the printed circuit board (PCB). This paper presents and evaluates experimental results on the relationships between pin assignment, the number of the signal outputs, and switching noise. It shows that calculated and experimental results agree well if we assume that the distribution of return current, causing switching noise in a connector, does not uniformly decrease with increases in the number of ground pins. We also assume that the effective number of ground pins is related to the number of signal pins even if there are more ground pins than there are signal pins.