Brady J. Gibbons

Microstructure and dielectric properties of Ba1−xSrxTiO3 films grown on LaAlO3 substrates

We report a systematic study of the microstructure and dielectric properties of barium strontium titanate, Ba1−xSrxTiO3, films grown by laser ablation on LaAlO3 substrates, where x=0.1–0.9 at an interval of 0.1. X-ray diffraction analysis shows that when x 0.4, compared with the peak temperatures of the bulk Ba1−xSrxTiO3. At room temperature, the dielectric constant and tunability are relatively high when x⩽0.4 but start to decrease rapidly as x increases.

Reel-to-reel preparation of ion-beam assisted deposition (IBAD)-MgO based coated conductors

We report on our efforts in developing and scaling-up the systems for IBAD-MgO based coated conductor fabrication. The overall fabrication process involves a number of different processes including: electropolishing of the substrates; barrier-layer, seed-layer, and IBAD-MgO deposition by e-beam evaporation; and pulsed laser deposition of buffer and YBCO layers. All processes are realized in reel-to-reel processing systems. Latest results have shown that the IBAD-MgO approach yields coated conductor performance comparable to the best results achieved elsewhere to date.

Thermal conductivity of nano-grained SrTiO3 thin films

We measure the thermal conductivities of nano-grained strontium titanate (ng-SrTiO3) films deposited on sapphire substrates via time-domain thermoreflectance. The 170 nm thick oxide films of varying grain-size were prepared from a chemical solution deposition process. We find that the thermal conductivity of ng-SrTiO3 decreases with decreasing average grain size and attribute this to increased phonon scattering at grain boundaries. Our data are well described by a model that accounts for the spectral nature of anharmonic Umklapp scattering along with grain boundary scattering and scattering due to the film thickness.

Accelerated coated conductor program at Los Alamos National Laboratory

In order to accelerate research and development of coated conductors (CC), a new facility has been developed at Los Alamos National Laboratory (LANL) with labs dedicated to scaled-up fabrication and characterization of CC. These laboratories include facilities for metal tape preparation, ion-beam assisted deposition (IBAD) of template layers, superconductor deposition by laser ablation (PLD), as well as materials characterization including low-temperature transport measurements. The work builds on the prior successes of the LANL CC program, such as the benchmark results of critical currents over 300 A on 1-cm-wide short samples deposited via IBAD and PLD. The new labs have recently been completed and include the ability for reel-to-reel tape processing and characterization. Fabrication of long lengths of tape (>10 m) allow for high-throughput experimentation by linear combinatorial design. This facility is also made to enable collaboration with industrial and other partners.

Photovoltaic response and dielectric properties of epitaxial anatase-TiO2 films grown on conductive La0.5Sr0.5CoO3 electrodes

We have grown epitaxial anatase-TiO2 (001) films on La0.5Sr0.5CoO3 (001) bottom electrodes using pulsed-laser deposition. The small lattice mismatch (0.5%) between the anatase-TiO2 and the La0.5Sr0.5CoO3 makes it possible to grow anatase-TiO2 films with excellent crystallinity on conductive metal oxides. The photovoltaic properties of the epitaxial anatase-TiO2 on the La0.5Sr0.5CoO3 were characterized using a Kelvin probe. The optical band-gap energy was found to be 3.05 eV. The dielectric properties of the epitaxial anatase-TiO2 films were characterized using a capacitor structure of Au/anatase-TiO2/La0.5Sr0.5CoO3 on a LaAlO3 substrate. The dielectric dispersion exhibited a power-law dependence, and the dielectric constant measured at room temperature and 1 MHz was 38.

Continuous fabrication of IBAD-MgO based coated conductors

Reel-to-reel tape processing capable of fabricating tens of meters of coated conductor has been set up at Los Alamos National Laboratory. These reel-to-reel facilities include electropolishing, IBAD-MgO texturing, buffer layer deposition, REBaCuO pulsed laser deposition and position dependent critical current and ac-loss measurement. The systems enable development of long-length coated conductors as well as high-throughput experimentation. IBAD-MgO is an extremely fast process, demonstrated at 100 meters/hour, but requires smooth surfaces over long lengths, which is achieved in our laboratory by electropolishing of metal alloy tapes. Barrier and buffer layers are optimized to minimize interdiffusion from the substrate into the superconductor and to maximize crystalline alignment. YBCO is deposited in a multi-zone heater that allows for an engineered heating power profile as the YBCO film is deposited. Currently the best critical current result on a 1 meter length for a 3 /spl mu/m thick YBCO layer is 350 A (self field, 75.5 K).

Dielectric response and structural properties of TiO2-doped Ba0.6Sr0.4TiO3 films

Dielectric Ba0.6Sr0.4TiO3 films doped with different weight percentages of TiO2 were deposited by pulsed-laser deposition. The dielectric constant, dielectric loss, and the dielectric tunability of the films were found to be a strong function of the weight ratio of TiO2/Ba0.6Sr0.4TiO3. Compared to the pure Ba0.6Sr0.4TiO3, the TiO2-doped Ba0.6Sr0.4TiO3 exhibits lower dielectric loss while maintaining a significantly adjustable dielectric constant and desirable capacitance tunability. The change of the dielectric properties of TiO2-doped Ba0.6Sr0.4TiO3 films is closely related to the change in microstructure of the films as revealed by transmission electron microscopy.

Effect of crystallinity on the transport properties of Nd0.67Sr0.33MnO3 thin films

Highly crystalline Nd0.67Sr0.33MnO3 (NSMO) thin films were grown on (001) LaAlO3 (LAO) substrates using pulsed-laser deposition. By studying the microstructural properties of NSMO using x-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution TEM, we have found that high degree crystallinity leads to improved transport properties such as smaller resistivities at both peak and room temperatures. A peak temperature of 187 K has been observed from a resistivity-temperature measurement, and around 200 K from a magnetization measurement.

Effect of the misalignment between the applied and internal magnetic fields on the critical currents of “tilted coated conductors”

We present angular-dependent measurements of the critical current density (Jc) of a YBa2Cu3O7 film grown on tilted buffered Ion-Beam-Assisted-Deposition MgO metal tapes. We compare samples with the current parallel and perpendicular to the tape’s direction. In samples with the current parallel to the tape, we observe an asymmetric angular dependence of Jc and a shift of the ab-planes maxima position with the magnetic field strength. The shift is a result of the misalignment between the applied and internal magnetic fields. The misalignment effect takes place at fields lower than 3T at T=75.5K. For samples with the current perpendicular to the tape, we find an overall reduced value of Jc, the angular behavior is symmetrical, and no shift in the maxima position as a function of the magnetic field is observed. These results indicate that the effect of misalignment between the applied and internal magnetic fields must be taken into account when studying the angular dependence of Jc.

Influence of tilted geometries on critical current in superconducting thin films

We present measurements of the angular dependent critical current (J/sub c/) of DyBa/sub 2/Cu/sub 3/O/sub 7/ on single crystal SrTiO/sub 3/ and YBa/sub 2/Cu/sub 3/O/sub 7/ on tilted Ion Beam Assisted Deposition MgO substrate. These experiments demonstrate that J/sub c/ angular dependences of tilted defects are a useful tool to detect misalignments between the internal and applied magnetic fields. The results show that, in order to understand vortex pinning in thin anisotropic samples below certain value of magnetic field, this misalignment must be taken into account.