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Thin film composites of YBa 2 Cu 3 O 7 (YBCO) with gold were deposited by inverted cylindrical magnetron sputtering on SrTiO 3 (STO), MgO, and yttrium stabilized zirconium (YSZ) substrates using a bilayer deposition process. The material and electrical properties of crystal structure, surface morphology, resistivity, transition temperature ( T c ), resistance ratio, and critical current ( I c ) as a function of temperature and small magnetic fields were investigated. Films deposited on STO showed little degradation in superconducting properties up to 500 A of gold with 1 kA of YBCO; for YSZ and MgO, the films showed a rapid fall-off in T c with gold thickness. For YSZ, 3–10 A of gold caused the films to become insulating; however, for gold greater than 25 A, the films were superconducting but T c rapidly fell to zero by 500 A of gold. Composites on MgO showed the most rapid fall-off in T c going to zero by 200 A of gold. The temperature dependence of I c was predominately quadratic. However, some films showed a crossover between quadratic and linear temperature dependence in I c . The I c of all the composites showed a response to small magnetic fields with the films on STO being the least sensitive and those on MgO the most. There was a trade-off between T c and field response with the film having the greatest response having the lowest T c . These composite films show potential for vortex flow device development.

We have measured the current-voltage (I-V) characteristics of thin film composites made from YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) and Ag. When deposited upon a substrate, this material appears to form arrays of superconducting-normal-superconducting (SNS) junctions. To make these measurements, the films were patterned into microbridges and immersed directly into the cryogen to provide a constant temperature environment and to avoid problems of local heating. The data were fitted by several phenomenological models in an effort to understand the dissipation mechanism appropriate to these materials.

We have investigated thin film composites of YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) with Ag for fluxonic device applications. YBCO/Ag composite films are produced by first depositing a layer of Ag onto a substrate and then heating the film to the YBCO deposition temperature of 680/spl deg/C or higher. YBCO is deposited by off-axis sputtering onto the Ag-coated substrate. The resulting YBCO/Ag film is a composite of YBCO with well-defined Ag regions several microns in size. Scanning electron micrograph images of the films' surfaces show a background of smooth YBCO grains dotted with Ag clusters. For a wide range of increasing Ag composition, the transition temperatures of the composite films on SrTiO/sub 3/ remain high, while the critical current densities have been reduced as much as 65 times. On MgO substrates, critical current density has been reduced by more than four orders of magnitude. Also on MgO, significant voltage response is seen in external magnetic fields of less than 1 mT. These measurements suggest that the films may be arrays of superconductor-normal-superconductor (SNS) junctions formed by weakly coupled YBCO grains with Ag in the grain boundaries. The field responsivity and low critical current densities of these composites make them potentially useful for fabrication of fluxonic devices.

This research involves the study of thin films for use in three-terminal, non-linear two-terminal, and photodetector superconducting devices. The morphological and electrical properties of composite films fabricated from YBCO and noble metals have been investigated for use in these devices. Films of Ag and Au, when heated to YBCO deposition temperatures, coalesce into small, approximately micron-sized islands. We take advantage of this property to form a composite film with distinct YBCO and noble metal regions by depositing a layer of YBCO on the array of noble metal islands. With certain compositions the films exhibit reduction of critical current density by a factor of 65 over YBCO and show no significant degradation of critical temperature. Also, the current-voltage characteristics of the composites are sensitive to small, applied magnetic fields. These properties make the YBCO/noble metal composites potentially well suited for the fabrication of devices whose transport is dependent upon vortex motion. Photoresponse measurements taken on a 1 mm × 1 mm YBCO/Au bridge irradiated by red laser light showed a possible non-bolometric response below the composite superconducting transition temperature.

The occurrence of a transverse voltage peak has been observed in YBCO thin films. This phenomenon occurs under conditions of temperature and current in the earth's magnetic field such that the films are in the mixed state. The magnitude of the peak was found to scale with both temperature and current, and exhibited a Sin(/spl theta/) orientational dependence measured between the film-substrate and the position of the measurement electrodes on the film surface. These observations are similar to observations made in conventional superconducting thin films such as Nb, NbN, NbCN, and PbBi. >

We are using a novel bilayer deposition process to grow composite films of Au with YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//. The composites have well-separated regions of Au and YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//, and we see no evidence for structural or chemical degradation of the grains and grain boundaries. For composite films on MgO substrates, the transport properties indicate the presence of weakened superconductivity in the films. Composites on SrTiO/sub 3/ and LaAlO/sub 3/ do not show this as strongly. The MgO composites behave like arrays of Josephson junctions and show promise as materials for flux flow devices. >

Single-crystalline cubic SiC Layers have been grown on Si(111) substrates by molecular beam epitaxy (MBE) using graphite and silicon solid sources at relatively low substrate temperatures (800°C). The growth process employs initial carbonization of the (111) Si surface followed by direct deposition of both Si and C. Reflection high energy electron diffraction (RHEED), X-ray diffraction, cross-sectional and plan-view transmission electron microscopy (TEM), Auger electron spectroscopy (AES), and optical Nomarski microscopy were used to characterize the films. Atomic ratio of Si to C during the growth was found to be critical in terms of the crystalline quality as well as surface morphology of the films. To the extent of the instrumental accuracy, AES shows the SiC films to be stoichiometric. X-ray diffraction and TEM measurements confirm the crystalline nature of the SiC films.

Multilayer films of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ with composites of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ and LaAlO/sub 3/ have been grown on

YBa/sub 2/Cu/sub 3/O/sub 7/ thin films have been deposited on MgO substrates by off-axis magnetron sputtering in argon, oxygen, and an additional gas. Additions of nitrogen, hydrogen, methane, air, and water vapor in amounts of up to 25% were used in this investigation. Each gas stabilized the target voltage and growth rate for the duration of the deposition. Growth rates improved by over 50% with as little as 3% hydrogen. When used with a target which had degraded after more than 150 h of use, the added gas improved T/sub c/ and J/sub c/ beyond levels attained from the new target. T/sub c/ increased from 82 K without hydrogen to 89 K with hydrogen and J/sub c/ (12 K) from 3*10/sup 4/ to >7*10/sup 6/ A/cm/sup 2/. Nitrogen yielded less dramatic effects. These improvements are attributed to the catalytic effect of these gases in maintaining elevated levels of atomic oxygen during film growth. >

We have prepared thin film samples composed of Y 1 Ba 2 Cu 3 O 7−y («123») and Y 2 Ba 1 Cu 1 O 5 («211») by off-axis sputtering from separate targets of the « 123» and «211» material. X-ray diffraction on the films shows the presence of c-axis oriented «123», but no indication of ordered «211» lines. The c-axis lattice constant does not show any change compared to our pure «123» films. As the volume percentage of «211» material increases, we see a reduction in T c and J c , and an increase in the resistivity. The temperature and field dependence of the critical current are different for our composites compared to our pure «123» films

In our studies of the behavior of materials forming thin film composites with Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO), we have characterized thin film composite mixtures of YBCO and Y/sub 2/O/sub 3/ (yttria) grown onto (100) MgO and (100) SrTiO/sub 3/ substrates by off-axis sputtering. We have examined the transport properties of the films, in particular the critical current density, J/sub c/. The temperature dependence of J/sub c/ for composites on SrTiO/sub 3/ is identical to that of our pure YBCO films, while for composites on MgO the J/sub c/ is similar to measurements on our earlier composites of YBCO and Y/sub 2/Ba/sub 1/Cu/sub 1/O/sub 5/ ("211"). The field dependence of J/sub c/ for the YBCO/yttria composites is different from our pure YBCO films, and from our earlier composites, but the magnitude of the flux pinning is similar. >

We have investigated the properties of off-axis co-sputtered films of YBCO and CeO/sub 2/ having CeO/sub 2/ concentrations up to 43%. Morphology investigations suggest that films with more than 29% CeO/sub 2/ may consist of a new material phase, T/sub c/ and resistance ratio decreased with increasing CeO/sub 2/ concentration. Degradation of T/sub c/, after photoresist processing the films was observed and reversed by a 20 minute oxygen plasma etch. The temperature dependence of the critical current near T/sub c/ showed two power law dependence regions with a crossover near 0.99 T/sub c/ for films having low concentrations of CeO/sub 2/. I/sub C/ 's were decreased by small applied magnetic fields, and for some samples the response was observed to increase at lower temperature. For a sample having 26% CeO/sub 2/, a 40% reduction in critical current was observed for a 2 Gauss applied field. This response indicates that this may be a candidate material for the development of Josephson vortex flow devices.

We have grown a series of composite films of Au with YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// on various substrates using a bilayer deposition technique to study their suitability for flux flow applications. The T/sub c/'s and J/sub c/'s were measured, and results compared with plain YBCO films for the different substrates. The patterning process resulted in some damage to the composites, but an oxygen plasma treatment was effective in restoring T/sub c/'s and resistance ratios, J/sub c/ measurements suggest that Au/YBCO composites behave like SNS-coupled arrays of superconducting grains. Their increased response to small magnetic fields is consistent with Josephson vortices dominating the transport. These findings indicate that Au/YBCO composites are a promising material for flux flow applications.

A technique is described which enables contactless measurements at microwave frequencies of the superconducting transition. The approach employs an electrically small microwave loop antenna to sense the change in the reflected microwave signal as flux is expelled, due to the Meissner effect, from the superconductor. Advantages of this technique include the ability to measure small areas of a superconducting thin film after photolithographic patterning into a device geometry. This approach is very sensitive in the frequency range from 0.05 GHz to 5 GHz and for some YBCO films a dependence of the transition temperature width on frequency has been observed. Such frequency-dependent signatures may provide valuable information regarding improvements in film deposition and device processing.

We have grown thin film composites of and YBa2Cu3O7−x and LaAlO3 by co-sputtering. A film with composition of 10% LaAIO3 by volume had a transition temperature of 58K and critical current density of 1.2 × 106 A/cm2 at 4K. We measured the temperature dependence of the critical current and obtained a good fit with Ginzburg-Landau theory for thin films. A perpendicular magnetic field was applied, and the critical current and pinning force measured. At 70 kOe, the critical current was suppressed only a factor of 4 from its zero-field value. The measured pinning force was 2 × 109 dyn/cm3 at 70 kOe and is comparable to that of YBa2Cu3O7−x films.

The performance of SQUID (superconducting quantum interference device)-based electronics may be degraded from that found in laboratory operation. Investigations on superconducting tubes, wires, and sheets have been conducted to identify contributions to such noise. Results have been obtained for bulk and thin-film samples utilizing both the conventional low-temperature materials and the new high temperature oxide materials. Experiments have been conducted to quantify flux redistribution and flux motion in superconducting samples subjected to temperature changes, temperature gradients, and magnetic field gradients. These investigations have been conducted for magnetic fields typical of many SQUID applications, with field intensities much smaller than the critical values H/sub c1/. Penetration-depth, flux-pinning, and flux-motion effects have been observed. The various types of experiments conducted along with specific results are described. >

The authors have studied the microwave properties of a single crystal of ErBa/sub 2/Cu/sub 3/O/sub 7-x/. The crystal was placed in the bottom of a rectangular TE 103 copper cavity whose resonant frequency was 9.2 GHz. The Q of the system, cavity and sample, as well as the frequency shift, was monitored as the system was cooled. Below about 50 K the measurements became limited by the copper cavity. Close to the superconducting transition, about 90 K, the real surface impedance is at least one order of magnitude higher than that calculated from Mattis and Bardeen's (1958) formula. The authors have also studied using both resonant and nonresonant methods the microwave properties of an oriented film of YBa/sub 2/Cu/sub 3/O/sub 7-x/ grown on a MgO substrate. For both types of measurements they have studied the effects of small magnetic fields, less than 100 G, on the microwave losses at 9.2 and 10 GHz in a microwave cavity and by a nonresonant technique. The results are discussed in terms of flux entry and pinning at grain boundaries. >

We have prepared thin films of Y1Ba2Cu3O7−x using a coevaporation technique and measured their critical current densities as a function of both temperature and field using a transport technique. For a stoichiometric composition we find that Jc at 4.2 K in zero field is 1.04×106 A/cm2 and does not drop below 105 A/cm2 until T>60 K. Fitting the data near Tc shows that Jc follows a power law of (1−t), with t being the reduced temperature. In an applied field perpendicular to the film’s surface, Jc also drops slowly, and up to 90 kOe Jc >105 A/cm2 for T=4.2 K. Measurements for an off‐stoichiometric film, Y1Ba3 Cu2O7−x, give lower Jc values that fall off faster in temperature than for stoichiometric compositions.

Thin films of the YBaCuO superconductor have been deposited by evaporation on SrTiO/sub 3/ substrates. After a post-anneal, the films were patterned into a four-probe geometry, and critical-current density measurements were made using a transport technique. Compositions both on and off the stoichiometric Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ were studied, and measurements were made as a function of temperature and magnetic field. At 4.2 K and in ambient field, a stoichiometric film has a J/sub c/ greater than 10/sup 6/ A/cm/sup 2/ which falls off slowly out to approximately=55 K and then follows a power law dependence (1-t)/sup 3.3/, where t is the reduced temperature. The magnetic-field dependence of J/sub c/ at 4.2 K is weak, and for applied fields of 90 kOe, J/sub c/ has dropped only a factor of 10 from the ambient field. Off-stoichiometric films have lower J/sub c/'s which fall off faster and at lower temperatures. >