Laura H. Allen

Temperature and field dependence of the critical current densities of Y-Ba-Cu-O films

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.

Paraconductivity in YBa2Cu3O7 and YBa2Cu3O7 thin film composites with cerium oxide and gold

The paraconductivity for thin films of YBa2Cu3O7 (YBCO) and composites of YBCO with CeO2 and gold was investigated and analyzed using the Aslamazov and Larkin theory. Plain YBCO thin films sputter deposited on MgO, SrTiO3, and yttrium stabilized zirconium typically showed no dimensional crossover, were described by three dimensional (3D) fluctuation conductivity, and had a critical slope just above the critical region in agreement with the prediction of theory. With few exceptions, the CeO2 and gold composite films showed a dimensional crossover and a reduced critical exponent. The change in dimensionality was typically from 3D to two dimensional (2D) fluctuation conductivity. These composite films all had reduced values for the critical exponent which we attribute to an enlarged or extended critical region. This is in agreement with the critical current response of these composites to small magnetic fields. Additionally, the composites showed a two step transition in the paraconductivity and a minimum be...

Thin film composites of Au and YBa2Cu3O7−δ

Using a novel bilayer deposition process, we have grown composite films of Au with YBa2Cu3O7−δ on various substrates. Our composites have well‐segregated regions of Au and YBa2Cu3O7−δ, and we have not observed structural degradation of the grains or grain boundaries in the films. Transition temperatures and critical current density measurements have behaviors consistent with regions of weakened grain boundary coupling in the films. Photoresponse measurements are also reported. We believe that Au is diffusing into the grain boundaries and weakening the coupling between YBa2Cu3O7−δ grains. By varying the amount of Au in the films, the coupling can be adjusted. These Au/YBa2Cu3O7−δ composite films are promising materials for device applications relying on vortex motion.

Observation of a transverse voltage in the mixed state of YBCO thin films

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 earths 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.<<ETX>>

YBa2Cu3O7−y/Y2BaCuO5 composites: Growth and characterization

We have prepared thin film composites of Y1Ba2Cu3O7−y (‘‘123’’) and Y2Ba1Cu1O5 (‘‘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, and (00L) ordered Y2O3, but no indication of 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 Tc and Jc, 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.

Y1Ba2Cu3O7-x and LaAlO3 composite thin films by off-axis magnetron sputtering

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.

The properties of Y1Ba2Cu3O7−δ thin films with silver doping prepared by spray pyrolysis

Superconducting, silver‐doped films of Y1Ba2Cu3O7−δ have been deposited by spray pyrolysis of aqueous nitrate solutions onto MgO substrates. The superconducting transport properties, microstructure, and microwave losses have been characterized for various amounts of AgNO3 added to the spraying solution. These films had resistive transition temperatures between 79 and 85 K with widths from 3 to 7 K. The room‐temperature resistivity was a strong function of the silver doping, dropping by a factor of 50 for the heavily doped films. Critical current densities at 4 K were typically several 103 A/cm2 with little correlation to the silver doping. Lattice constants also were not significantly affected by the silver doping level, however, lightly doped films were denser, had the strongest c axis preferred orientation, and a smoother surface. Rf surface resistance was measured at 18 GHz, and for the best films dropped a factor of 10 below copper by 40 K. The London penetration depth was estimated to be approximatel...

Role of hydrogen in the growth of Y1Ba2Cu3O7 on MgO substrates by off‐axis magnetron sputtering

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...

Morphology and transport of YBa2Cu3O7−x sputtered in argon, oxygen, and hydrogen: Dependence on deposition temperature

YBa2Cu3O7−x films have been deposited on MgO by reactive, off‐axis magnetron sputtering in an argon, oxygen, and hydrogen gas mixture. The material and electrical properties of the films were studied for deposition temperatures from 600 to 760 °C. The films, all approximately 300 nm thick, were predominantly a‐axis oriented when deposited at or below 620 °C but were c‐axis oriented when deposited at temperatures above 640 °C. The surfaces of films deposited between 640 and 710 °C were partially covered with a‐axis grains. Surface roughness measurements indicated the smoothest films occurred for deposition temperatures below 680 °C. Resistance ratios as great as 3.1 were observed for some films. Transition temperatures exceeded 89 K and resistivities at 100 K were less than 150 μΩ cm for the best films. Low‐temperature critical current densities exceeded 107 A/cm2 for films deposited from 640 to 720 °C. The temperature dependence of the critical current density near the transition temperature had a power l...

Critical current characteristics of composite thin films of Au and YBa2Cu3O7

Abstract 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.