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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.

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. >

Thin‐film, composite mixtures of YBa2Cu3O7−δ (YBCO) and Y2O3 (yttria) have been grown by off‐axis sputtering onto (100) SrTiO3 and (100) and (110) MgO. X‐ray‐diffraction measurements for films grown on (100) substrates show the presence of c‐axis‐oriented YBCO and (h00)‐ordered yttria. The composites deposited on (110) MgO are insulating, and x‐ray analysis shows the presence of Y2Ba1Cu1O5 along with c‐axis YBCO. Scanning electron microscopy analysis shows the films to be very granular. The superconducting transition temperature and resistivity ratios between 300 and 100 K of the composites grown on the (100) substrates have been measured. A dramatic change is seen in these transport properties around 70% (volume) of YBa2Cu3O7−δ, which may be explained by the phase diagram for Y‐Ba‐Cu‐O.

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. >

Three variations of sputtering for growing thin films of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ are examined: a three-metal cosputtering technique requiring a postanneal at high temperatures, an in situ process using a inverted cylindrical magnetron sputter gun, and another in situ process using an planar target in an off-axis geometry. The films are grown primarily on a magnesium oxide substrate because it is readily available, inexpensive, and its low dielectric constant permits convenient microwave circuit design. It is found that the cosputter process with postanneal produces films with depressed T/sub c/'s and J/sub c/'s. The inverted cylindrical magnetron sputtering has yielded the best films, showing complete transitions as high as 87 K and J/sub c/=4*10/sup 6/ A/cm/sup 2/ at 4 K. The off-axis results are not quite as good, with 79 K complete transitions. By lowering the substrates further out of the plasma in the off-axis system, films can be improved by avoiding damage from negative ions.

Y1Ba2Cu3O7 thin films have been grown on MgO by off‐axis magnetron sputtering using mixtures of argon, oxygen, and hydrogen. Reduction in film transition temperature resulting from cumulative target sputtering time (target degradation) is minimized by adding hydrogen to the sputtering gas. Without hydrogen, new targets which had produced films with 87.5 K transition temperatures degraded with deposition time and produced films with transition temperatures of only 82 K. After addition of hydrogen, these targets produced films with transition temperatures of nearly 89 K. Critical‐current densities for the films made at optimum hydrogen flow were greater than 3×106 A/cm2 at 4 K. With the addition of hydrogen, we observed a significant increase in the sputtergun cathode voltage and a dramatic increase in the deposition rate. The films were predominantly c‐axis oriented, and we observed a minimum c‐axis lattice parameter for optimum hydrogen flow. We attribute these improvements in material properties to the c...

Thin‐film composites of co‐sputtered Y1Ba2Cu3O7−x and LaAlO3 have been deposited by off‐axis magnetron sputtering. Scanning electron microscopy (SEM) and x‐ray diffraction studies suggest that LaAlO3 precipitates along the grain boundaries of the Y1Ba2Cu3O7−x grains. The normal state conductivity, transition temperature, and critical current density systematically decreased with increasing LaAlO3 composition, consistent with a decrease in the coupling between Y1Ba2Cu3O7−x grains. Inductive transition widths were only a few degrees wide for all LaAlO3 compositions, indicating homogeneous materials. Initial photoresponse measurements show bolometric behavior near Tc and increased response for greater LaAlO3 compositions.

Tunneling studies have been made using sputtered Cu/sub 2/O as an insulating barrier between an EBCO thin film and a normal metallic layer. Cu/sub 2/O was selected for its low potential for chemical reaction with the superconducting layer and for its photoconductive properties. Having a band gap in the visible (2 eV), Cu/sub 2/O is a suitable candidate for photosensitive tunneling experiments. EBCO thin films were deposited in situ onto MgO substrates using an off-axis sputtering technique. T/sub c/'s of as-deposited films were between 80 and 85 K. Inductively measured J/sub c/'s ranged between 10/sup 6/ and 10/sup 7/ A/cm/sup 2/ at 4 K. A 5-20-nm layer of Cu/sub 2/O was sputtered directly on top of the superconducting film. Room-temperature resistivities of the Cu/sub 2/O layer were typically greater than 10/sup 6/ Omega -cm. A normal metal layer (typically gold) was then deposited onto the Cu/sub 2/O layer. Transport properties of the structure were measured at temperatures ranging above and below the T/sub c/ of the superconducting layer. Measurements of structures fabricated to date showed no tunneling effects. Measurements of the I-V characteristic of a Cu/EBCO bilayer exhibited nonohmic behavior at 85 K and 4 K, indicating the existence of an extraneous interfacial layer. Preliminary results on one such structure indicate possible tunneling effects.

The projected doubling of current levels of atmospheric carbon dioxide concentration ([CO(2)]) during the next century along with increases in other radiatively active gases have led to predictions of increases in global air temperature and shifts in precipitation patterns. Additionally, stratospheric ozone depletion may result in increased ultraviolet-B (UV-B) radiation incident at the Earth's surface in some areas. Since these changes in the Earth's atmosphere may have profound effects on vegetation, the objectives of this paper are to summarize some of the recent research on plant responses to [CO(2)], temperature and UV-B radiation. Elevated [CO(2)] increases photosynthesis and usually results in increased biomass, and seed yield. The magnitude of these increases and the specific photosynthetic response depends on the plant species, and are strongly influenced by other environmental factors including temperature, light level, and the availability of water and nutrients. While elevated [CO(2)] reduces transpiration and increases photosynthetic water-use efficiency, increasing air temperature can result in greater water use, accelerated plant developmental rate, and shortened growth duration. Experiments on UV-B radiation exposure have demonstrated a wide range of photobiological responses among plants with decreases in photosynthesis and plant growth among more sensitive species. Although a few studies have addressed the interactive effects of [CO(2)] and temperature on plants, information on the effects of UV-B radiation at elevated [CO(2)] is scarce. Since [CO(2)], temperature and UV-B radiation may increase concurrently, more research is needed to determine plant responses to the interactive effects of these environmental variables.

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.