High frequency detector and circuit applications often require device fabrication on medium-to-low dielectric constant substrates ((epsilon) < 12). Silicon-on-sapphire (SOS) substrates have acceptably low dielectric constants and provide other important advantages, including the possibility of monolithic integration of silicon and superconducting circuitry. Our initial results with YBa₂Cu₃O_7-x(YBCO) edge- geometry superconductor/normal-metal/superconductor (SNS) weak links fabricated on r-plane SOS substrates using cubic zirconia (YSZ) buffer layers revealed problems with grain boundary nucleation in the YBCO counterelectrode. These results motivated development of a new multilayer buffer system consisting of an epitaxial YSZ film grown on an SOS substrate, overlaid by a thin YBCO 'seed' layer, and an epitaxial SrTiO₃ (STO) layer. STO - YBCO bilayers grown over the YBCO seed layer show a remarkable improvement in epitaxial quality and in YBCO electrical properties relative to similar bilayers grown directly on the YSZ buffer. In addition, SNS weak links fabricated on SOS substrates using the multilayer buffer system exhibit dramatically improved electrical characteristics compared to devices produced on YSZ buffer layers. These are the first epitaxial edge-geometry SNS weak links produced on SOS substrates.

We report our studies on the fabrication of YBa₂Cu₃O_x (YBCO) thin-film structures, patterned with a laser-writing technique. We demonstrate that this patterning method can be successfully implemented in fabricating a variety of electronic and optoelectronic high-T_c devices and circuits. Laser patterning implements a focused beam from a continuous-wave Ar- ion laser to selectively heat-up an epitaxial YBCO film in a controlled (oxygen or oxygen-free) atmosphere. Depending on the film's initial oxygen content and the ambient atmosphere, laser heating allows oxygen to diffuse in or out of the annealed region and form oxygen-rich (YBa₂Cu₃O₇; superconducting) patterns next to oxygen-depleted (YBa₂Cu₃O₆; insulating at low temperature) ones. The width of the YBa₂Cu₃O₇- YBa₂Cu₃O₆ interface is less than 1 micrometers . The laser-writing procedure is noninvasive, does not require a patterning mask, and results in completely planar, monolithic structures, free of surface contamination or edge degradation. Our oxygen-rich lines (typically 4 to 60 micrometers wide), patterned on the high-quality, intentionally deoxygenated YBCO films, exhibit zero resistivity at 90 K and critical current density of approximately 3 MA/cm² at 77 K. Their superconducting properties remain unchanged even after eight months of shelf storage. On the other hand, oxygen-poor regions are semiconducting and characterized at low temperatures by a 3D, variable-length hopping transport. Below 100 K, they exhibit low microwave losses, their dc resistance is above 10 M(Omega) /square, and dielectric permittivity is below 20 at about 10 GHz. A number of devices and circuits patterned by laser writing, such as a microbridge, coplanar transmission line, open- ended coplanar microwave resonator, photoconductive switch, and YBCO field-effect transistor, are presented. All structures are intrinsic to the YBCO material, and they combine in a new and unique way superconducting and dielectric properties of the YBa₂Cu₃O₇ and YBa₂Cu₃O₆ phases.

We report on a systematic study of the transport characteristics of YBa₂Cu₃O₇ (YBCO) step-edge Josephson junctions as a function of the step angle. The microstructure of a YBCO film depends very critically on the step angle (alpha) in a SrTiO₃ or LaAlO₃ substrate. Briefly, on shallow steps (0 < (alpha) < 44 degree(s)) the film grows epitaxially across the step. On steep steps (46 degree(s) < (alpha) < 85 degree(s)) two grain boundaries occur on the step. In this paper it is shown that the I-V curves of the step-edge junction reflect the microstructure of the YBCO film on the step. The I-V curves on shallow steps are of flux-flow type. On 45 degree(s) steps there are several Josephson junctions with different critical currents, while on steep steps two critical currents are found in the I-V curve. Summarizing all data, we conclude that the grain boundaries formed on steep steps are responsible for the Josephson behaviour of the junctions.

We have investigated the Josephson radiation from different types of YBa₂Cu₃O₇ (YBCO) thin film junctions: step-edge (SEJ), biepitaxial, and superconductor-normal conductor- superconductor (SNS) with N=Au and PrBa₂Cu₃O₇. The radiation was detected using a nonresonant radiometer system with a receiving frequency of 11-12 GHz. The current-voltage characteristics were measured simultaneously with the radiation spectra in the temperature range from 4.2 to 90 K. All junctions exhibited a large emission peak at a voltage which was related to the frequency through the second Josephson relation. Typically, for high temperatures, and, therefore, small critical currents, the experimental data of the radiation linewidth agreed well with the theoretical predictions of the RSJ model. At lower temperatures the experimental linewidths deviated from the theoretical values due to additional noise sources in the junctions. Some of the SEJs showed a nonmonotonic dependence of the linewidth on temperature. Such SEJ data will be discussed in terms of a model which treats the SEJ as an interferometer consisting of a parallel array of Josephson junctions.

A superconducting quantum interference grating (SQUIG), consisting of many Josephson junctions in parallel, exhibits quantum interference analogous to the optical interference pattern produced by a diffraction grating, just as the dc SQUID is the magnetic analog of the double slit in optics. The field- dependent critical current of a SQUIG with sufficiently low inductance exhibits a series of sharp peaks, similar to the position-dependent intensity of a multiple-slit interference pattern. Such a device has the potential for improved sensitivity as a magnetic sensor because of its enhanced flux-to-voltage transfer coefficient. The behavior is quite different, however, when the inductance is large. In this limit, the field-dependent critical current becomes extremely hysteretic and exhibits self- organized phase coherence, in which a peak in the critical current (indicating coherence of the junction phases) is observed after each reversal of the field sweep. We have observed this phenomenon in large inductance arrays with up to ten YBa₂Cu₃O₇ (YBCO) bi-crystal grain boundary junctions in parallel. We have modeled the behavior using a Frenkel-Kontorova model generalized to include global interactions due to mutual inductance. According to our model calculations, self-organized phase coherence occurs when the system relaxes from its critical state, causing the phases to simultaneously relax to minima in the periodic potential representing the Josephson coupling energy, and thus to become coherent modulo 2(pi) .

YBaCuO-PrBaCuO-YBaCuO a-axis oriented trilayer structures was realized by in-situ ICM (Inverted Cylindrical Magnetron) sputtering method, including a template step. Cross type junctions of micrometer size were patterned with a trilayer SNOP process. Their Josephson and tunneling characteristics have been investigated for various thicknesses of the PrBaCuO barrier (10- 100 nm). The critical current was found proportional to exp(-(alpha) T^1/2) (SNS type in the dirty limit) where (alpha) was a constant. Strong and reproducible non-linearities were observed on the I-V curves especially above 10 mV and analyzed in terms of tunneling process. Tunnel barrier and interface models are proposed to explain the low critical current-normal resistance products (0.4 mV at 4.2(Kappa) ) as well as the I-V curves.

Several electric field effect devices have been built with YBaCuO/PrBaCuO layers and multilayers. The layers are deposited by an inverted cylindrical magnetron sputtering process (ICM) on MgO substrates. This process has been proven to give very good performances, even for thinner and ultra thin films. The films used for the field effect transistor devices consist of one or two bilayers of YBaCuO and PrBaCuO with a nominal thickness of each material of about 4 nm. Small channel geometries of 50 microns x 60 microns are etched by argon ion milling and the channel is covered by a silicon dioxide dielectric layer (50-100 nm). The gate contacts, as well as the drain and source contacts, are made by sputtered gold pads. These devices are tested in static and dynamic measurements: the silicon dioxide layer inhibits symmetric I-V characteristics of the dielectric layer with high breakdown field strength up to 4 MV/cm. The gate capacity of a few pF allows the test of the device in a kHz frequency range. Tests with sine wave gate voltages show low distortion of the output signal with a voltage gain of about 0.01. Smaller dielectric layers could raise the gain close to one. The results of these field effect experiences are compared with optically induced effects. The optical measurements are performed with a helium-neon laser (633 nm) with a power density of 600 W/cm². The laser pulses with frequencies up to 1 MHz gives responses somewhat similar to the observed field effect responses. Combining field effect and optical irradiation, it is found that the laser pulses seem to create charge carriers screening the field effect. Based on this principle a new photodetector will be proposed.

At the Smithsonian Astrophysical Observatory, we have been developing expertise in the construction of high-frequency heterodyne receivers using SIS junctions as their first mixing elements. This research has been carried out in support of the Submillimeter Array (SMA), a radio interferometer soon to be built on Mauna Kea that will require multiple reciever systems at a number of bands between 177 GHz and 920 GHz. In this paper, we describe research on the fabrication of all-refractory SIS junctions. To be useful as high-frequency mixers, SIS junctions must have small area and high critical current density. At the lower SMA frequencies, Nb/Al-Oxide/Nb ahs proven to be an ideal material system, and optical photolithography has sufficient resolution for junction definition. At the higher frequencies, NbN/MgO/NbN may enjoy an advantage due to its larger energy gap, and electron-beam lithography may be necessary for defining junctions much smaller than one micron in size. We report good initial results for Nb/Al-Oxide/Nb junctions, fabricated using a self-aligned optical technique, with area $AP 0.5 micrometers ² and critical current density $AP 10,000 Amp/cm².

We report on a study of S-N and N-S current switching in high quality YBaCuO films deposited onto ZrO₂ and NdGaO₃ substrates. The films 60-120 nm thick prepared by laser ablation were structured into single strips and were provided with gold contacts. We monitored the time dependence of the resistance upon application of the voltage step on the film. Experiment performed within certain ranges of voltage amplitudes and temperatures showed the occurrence of the fast stage both in S-N (shorter than 300 ps) and N-S transition. We discuss the mechanism of switching taking into account the hot electron phenomena in YBaCuO. The contributions of various thermal processes in the subsequent stage of the resistance dynamic are also discussed. The basic limiting characteristics (average dissipated power, minimum work done for switching, maximum repetition rate) of a picosecond switch which is proposed to be developed are estimated.

The discovery of high temperature superconductors has revived the interest in rf SQUIDs, which, in the case of conventional superconductors had been surpassed in performance by the dc SQUID. Several advantages are offered by the rf SQUID, like the requirement for only a single weak link and a low 1/f noise. With high bias frequencies (> 100 MHz) it is possible to obtain flux noise values comparable to dc SQUIDs. At present, HTS rf SQUIDs offer a field sensitivity of less than 100 fT/(root)Hz ( 1 Hz). This is already sufficient for a number of serious applications. This paper reviews recent developments towards practical rf SQUIDs made of high-T(subscript $c/ superconductors.

A description is presented of the fabrication and properties of high Tc DC SQUIDs and flux transformers fabricated by dry processing of pulsed laser ablated YBa₂Cu₃O_7-x (YBCO) thin films. SQUIDs have been fabricated with either bicrystal substrate or step edge junctions. For all devices fabricated thus far, measurements indicate a similar character to the flux noise spectra with a significant 1/f noise component below 100-500 Hz. The transfer function and energy sensitivities as a function of SQUID inductances in the range 60-200 pH have been measured for bicrystal DC SQUIDs and compared with estimates. Various techniques have been employed to improve the magnetic field sensitivity of the uncoupled DC SQUID toward more practical levels including the use of large area washers, single layer magnetometers and 3-level flux transformers fabricated from trilayers of YBCO/SrTiO₃YBCO. The properties of open input coils are presented as well as the performance of closed loop transformers coupled via flip chip geometry to the SQUID washer. A white magnetic noise level of approximately equals 130 fT rms/(root) Hz above 200 Hz has been reached with a flux transformer with a 15 turn input coil and pick-up loop area of 13 mm².

The effect of microwave and dc magnetic fields on the unloaded quality factor and the resonant frequency of two differently coupled hysteretic 3 GHz microwave SQUIDs was investigated. The SQUID loops are integrated into and solely inductively coupled to the superconducting resonator, respectively. The dissipative and the reactive response are analyzed in terms of a phenomenological model by Silver and Zimmerman. The temperature dependencies of the threshold field for microwave induced quantum transitions and of the coupling strength between SQUID ring and resonator are discussed. Conditions for optimized operation are deduced for both samples. The results indicate that it is possible to reduce the coupling coefficient, k², by a suitable loop design while maintaining, at a reduced temperature, a sufficiently large dc field induced variation of the microwave losses (transfer function).

We have developed scanning probe magnetic microscopes which use dc Superconducting QUantum Interference Devices (SQUIDs) to obtain images of the magnetic field above a sample surface. In our instruments, the SQUID is held fixed and the sample is scanned in a raster pattern by means of a computer-controlled cryogenic positioning mechanism. We record the output of the SQUID as a function of sample position and use this to construct gray-scale or false color images of the magnetic field above the sample. We presently have two microscopes which can scan in 3D; one microscope uses a high transition temperature ((Tau) _c) YBa₂Cu₃O₇ SQUID while the other microscope uses a low-(Tau) _c Nb-PbIn SQUID. Our high-(Tau) _c microscope operates with the sample and SQUID in liquid nitrogen and typically achieves a spatial resolution of about 20-80 micrometers and a magnetic field resolution of about 20-200 pT for a 1 second average. Our low-(Tau) _c microsope operates with the sample and SQUID in a liquid-helium cooled vacuum space and is designed to allow the imaging of samples at different temperatures. We report on the microscopic magnetic imaging of currents, ferromagnetic inks, superconducting films and normal metal samples.

A 4-channel YBa₂Cu₃O₇-y (YBCO) thin film dc-SQUID magnetometer was fabricated. Biomagnetic measurements were performed by using the 4 channel SQUID array at 77$KAPPA in a magnetically shielded room. We have successfully measured 4 channel magnetocardiac signals clearly in real time mode. The best magnetic field resolution of the four SQUIDs was 370fT/Hz^1/2 at 10Hz and 200fT/Hz^1/2 in the white noise region.

Relaxation Oscillation SQUIDs (ROSs) based on 4x4 micrometers ² Nb/AlO_x Josephson tunnel junctions have been fabricated and characterized. A ROS consists of a hysteretic dc SQUID shunted by an inductor L and a resistor R in series to induce the relaxation oscillations. The values of L range from 20 nH up to about 300 nH, whereas the time constant L/R are between 8 and 45 ns. Frequency-flux characteristics have been recorded with the help of a spectrum analyzer, directly connected to the ROS. The relaxation frequencies range from 5 to 180 MHz. The experimental characteristics can be explained very well with a simple model describing the oscillation cycle. The effect of the self-induced magnetic field due to a magnetic coupling between the dc SQUID and the shunt circuit has been studied in detail. The sensitivity of ROSs and DROSs (Double Relaxation Oscillation SQUIDs) improves with increasing relaxation frequency. In (D)ROSs based on unshunted, hysteretic tunnel junction dc SQUIDs, the maximum relaxation frequency and the sensitivity are limited by LC resonances due to the SQUID capacitance and the shunt inductance. It is shown that the relaxation frequency can be increased up to frequencies of the order of 1 GHz if an extra resistor is integrated to damp these resonances. The optimum value of the damping resistor can be obtained from the ROS parameters. For stable operation of a (D)ROS, the shunt resistance should not be too large. The optimum value of this resistance can be calculated from the effective McCumber parameter and the bias current. Theoretically, the sensitivity of a ROS with a SQUID capacitance of 1 pF, a SQUID inductance of 20 pH and a relaxation frequency of 1 GHz equals about 5h, where h is Planck's constant. In a DROS with voltage readout based on similar SQUIDs, the theoretical sensitivity at a relaxation frequency of 1 GHz is 17h, with an estimated flux-to-voltage transfer coefficient of 5 mV/(phi) ₀.

In a Relaxation Oscillation SQUID (ROS), the magnetic fields (Self-Magnetic Fields, i.e. SMF) generated by the current in the shunt circuit coupled to the SQUID loop. It has been proven that SMF caused asymmetry and a shift of the magnetic flux-frequency ((Phi) -f) characteristics and the frequency fluctuation. The calculated results of the (Phi) -f curve taking account of the SMF are in good agreement with the experiment results. The asymmetric $PHI-f curve provides a larger df/d(Phi) , which leads to the good flux resolution. While the frequency fluctuation, which is observed as the broad spectrum, increases in proportion to the transfer coefficient (df/d(Phi) ). The SMF depends on the mutual inductance between the shunt inductance and the SQUID loop.

Pulsed eddy-current detection and characterization of wall- thinning in aircraft lap-splices due to corrosion is studied theoretically. The relevant lap-splices consist of two one mm thick sheets of aluminum bonded together by bolts and separated a small distance by a insulating sealant. Corrosion changes the thickness of both plates and the size of the gap between them. The problem is to determine nondestructively the thickness of both the 'top' and 'bottom' plates as well as the intervening gap. We calculate the time-domain current-voltage response function of a small cylindrically symmetric coil of wire that is placed next to a lap-joint and excited by a step-function current. The result for air-core coils is obtained as a simple quadrature, while coils that contain ferrite-cores are modeled with a finite element code. The characteristic features of the transient response are shown to depend sensitively on the thickness of the top plate, the gap and the bottom plate.

The effects of radio frequency radiation on the dc SQUID are examined. Simulations show how the shape of the SQUID transfer characteristic is distorted by radio frequency interference (RFI). How this affects three commonly used SQUID modulation methods is discussed, and the results explain why we experimentally observe the bias current reversing readout method to be the least susceptible to RFI. The commonly seen increase in the low frequency flux noise power spectrum of dc SQUIDs in unshielded environments is also explained.

Applications of Superconducting Quantum Interference Devices (SQUIDs) usually require high sensitivity at relatively low frequencies, often down to 1 Hz or lower. Excess noise, typically with a spectral density scaling inversely as the frequency, can substantially reduce the sensitivity of SQUIDs at low frequencies. We have studied 1/f noise in niobium rf-SQUIDs in some detail. To reduce any noise contribution of the readout electronics, a cryogenic preamplifier was used. When one measures the signal voltage of the SQUID directly, a pronounced 1/f noise was observed in all samples, and was nearly independent of the bias frequency used. The crossover between 1/f and white noise moved towards higher frequencies as the bias frequency was increased, because of a lower white noise contribution. The 1/f noise scaled approximately as the inductance of the SQUID. When operated in a flux locked loop, however, no 1/f noise could be observed above 0.5 Hz. Operating a rf SQUID in a flux locked loop can thus substantially reduce 1/f noise in rf SQUIDs.

We have designed and tested an optically driven inductance modulator, for use as part of a 1/f noise reduction scheme for SQUID magnetometers. The modulated element is a Nb coil photolithographically patterned on a 6x6 mm silicon chip. The coil is covered by three cocentric superconducting washers, patterned from a layer of PbInAu deposited above the Nb. Leads from the washers extend to a meander region directly below an optical fiber, which is coupled to a diode laser. With the laser off, the superconducting washer loops diamanetically shield the coil, and the coil has a low effective inductance. With the laser on, the meander is driven normal and the superconducting washer loops are open, greatly reducing the shielding. The coil then has a high effective inductance. The inductance is measured using a SQUID connected to the coil. Using this method, we have modulated the coil inductance from 2 (mu) H to nearly 20 (mu) H. By using four of these inductance modulators, we plan to fabricate a low- noise chopping network at the input of a SQUID, which can be used to reduce the 1/f noise in SQUID measurements.

We investigated effective flux-to-voltage transfer function and flux noise characteristic corresponding to positive feedback parameter (beta) a on Ketchen-type dc-SQUID with additional positive feedback (APF) by comparing the experimental results with calculated results. The measurement and calculated results for the effective flux-to-voltage transfer function and white flux noise characteristic are in good agreement. We quantitatively obtained the white flux noise characteristic of dc-SQUID (dc-Superconducting Quantum Interference Device) with APF. As (beta) a approaches unity, the flux noise caused by preamplifier voltage noise is decreased and the flux noise caused by the intrinsic flux noise of dc-SQUID with APF is increased. We find that an optimum value of dc-SQUID with APF exists, determined by the trade-off between the flux noise caused by preamplifier voltage-noise and the intrinsic flux noise of dc- SQUID with APF.

We have investigated the interaction between high critical temperature (high-T_c) superconductor-normal metal- superconductor step-edge junctions coupled through a non- superconducting feedback loop. We have characterized the strength of the interaction as a function of frequency and temperature for both a circuit without a groundplane and an all high-T_c multilayer circuit incorporating a superconducting ground plane. We observed relative locking strengths (the ratio of the measured locking current I_L to the junctions average critical current I_c) as large as I_LI_cequals9% and peak locking frequencies as high as 1.06 THz. The maximum temperature at which locking occurred was 35 K. An analysis of the temperature dependence of the locking current accounting for thermal fluctuations in the context of Johnson noise from resistive elements in the circuit agrees well with our experimental observations.

TRW is developing High Temperature Superconducting (HTS) electronics to dramatically improve the performance of space- based communications systems. These systems demand high speed analog-to-digital converters, high speed digital multiplexers, and quadriphase modulation of rf carrier signals. TRW has demonstrated an HTS high-speed multiplexer and a digital phase modulator that operates at microwave frequencies. The latter is the fundamental building block of a Quadriphase Modulator Exciter (QME). TRW has also developed an HTS cryogenic packaging subsystem which uses a commercially available palm-sized cryocooler. This package represents the first demonstration of an HTS self-supporting system. Package verification tests revealed that the package is capable of supporting high speed I/O and demonstrates reliable connectivity through multiple interfaces. The successful operation of a digital phase modulator has been demonstrated with this package.

Superconducting devices operate at speeds where coaxial copper cables can be a limiting factor. Coaxial cables can limit frequency response and impose significant thermal loading on a cyrogenic system. The high bandwidth of optical fibers and their low thermal conductivity make them good candidates for providing data into and out of superconducting circuits. In this paper, we present the results of our experience in operating photodetectors and laser diodes together with superconducting circuits in the same low temperature (4.2 K) environment. Using these photodetectors, we demonstrate the input of optical signals to an analog superconducting circuit at 6 GHz. Output from a superconducting circuit operating at 500 MHz is fed into a laser diode, and optically coupled to room temperature electronics. By combining these two techniques, we demonstrate a fully operational superconducting shift register with both input and output signals supplied by optical fiber.

A low Tc Pb alloy Superconductor-Insulator-Superconductor (SIS) tunnel junction heterodyne receiver has been constructed for astronomical use and tested over the frequency range of 400 to 540 GHz. Various alloy structures have been investigated in order to allow the production of small area SIS junctions with stable electrical characteristics and resistance to stress on cooling from 300 K to 4.2 K. Improvements in photolithography and thin film deposition techniques have been made that allow the fabrication of reliable sub-micron area junctions using suspended photoresist stencil and E-beam evaporation techniques. A single sub-micron area junction is mounted in a reduced height two tuner waveguide structure, which provides an optimum impedance match between the junction and the received signal. Performance measurements made with the receiver installed on the James Clerk Maxwell Telescope, Hawaii, show a total system double sideband noise equivalent temperature of 160 K at 460 GHz and 220 K at 490 GHz, measured in a 1 GHz instantaneous IF bandwidth centered at 4 GHz. The receiver demonstrates that Pb alloy tunnel junctions provide excellent sensitivity at submillimetre wavelengths and are sufficiently stable and reliable to allow use at a remote observing site.

This paper presents the progress we have made on our design, fabrication, and testing of a fully parallel superconducting analog-to-digital converter (ADC) with multi-GHz clock frequencies and input bandwidth. To our best knowledge, this converter is the first flash-type analog-to-digital converter ever reported in Josephson technology that fully integrates a quantizer and a thermometer-to-binary encoder to achieve binary outputs. In this design, the quantizer consists of 2(superscript N-1 comparators, each of which is realized using a hysteretic one- junction sampling SQUID driving a two-junction readout SQUID. A new logic family has been designed based on the same comparator building block and has been used to implement the thermometer-to- binary encoder. Taking advantage of the fact that the encoder's input is thermometer-coded, we have designed three-input and four-input quasi-XOR gates with only three NAND gates and therefore reduced significantly the total gate count. Functionalities of all the sub-circuits have been verified experimentally at clock frequencies up to 3 GHz, which is limited by our currently available testing equipment.