Anthony R. E. Garcia

Epitaxial CeO2 films as buffer layers for high-temperature superconducting thin films

We have prepared epitaxial (100)CeO2 thin films on LaAlO3, sapphire, and yttria‐stabilized zirconia using pulsed laser deposition. It is demonstrated in this letter that the CeO2 films are chemically and structurally compatible to the high‐temperature superconductor YBa2Cu3O7−δ (YBCO). Epitaxial YBCO films on CeO2/LaAlO3 had a zero resistance temperature and critical current density in a zero field of 90 K and 5.9×106 A/cm2 at 75 K, respectively. Furthermore, epitaxial multilayers of CeO2/YBCO were prepared. This work demonstrated that CeO2 is an excellent buffer layer material for the high‐temperature superconductors.

Effect of deposition rate on properties of YBa2Cu3O7−δ superconducting thin films

Superconducting thin films of YBa2Cu3O7−δ on (100) SrTiO3 are prepared in situ by a pulsed laser deposition technique at deposition rates from 1 to 145 A/s. Crystallinity of the films is examined by Rutherford backscattering in the channeling mode using a 2.2 MeV He+ ion beam. The backscattering minimum yield (χmin) increases with the deposition rate. A χmin of 3% is observed in the films deposited at the lowest deposition rate. Even at a deposition rate of 145 A/s, the films show good crystallinity with χmin of 15%, indicating epitaxial growth. The x‐ray diffraction measurements show that the films have strong c‐axis orientation normal to the substrates. The films have metallic resistance versus temperature behavior with zero resistance temperatures of 90 K. The results indicate that the pulsed laser deposition technique could be used to deposit large‐area films efficiently with adequate substrate movement.

Large critical current densities in YBa2Cu3O7−x thin films made at high deposition rates

Critical current densities (Jc) in YBa2Cu3O7−x films made at deposition rates from 0.1 to 14.5 nm/s (∼50 μm/h) were measured using a direct transport method. As the deposition rate was increased by two orders of magnitude, the films exhibited no marked degradation in current carrying capability with Jc of ∼4×106 A/cm2 at 77 K and zero field. Jc for all the films showed similar behavior under a magnetic field up to 8 T, although extra structural defects were found in the films deposited at the higher rates. The results from this experiment indicate the feasibility for coating wires, tapes, and other macroscopic shapes with high Tc superconductors using proper vapor deposition techniques.

A versatile substrate heater for use in highly oxidizing atmospheres

We report the design, construction and performance of a versatile substrate heater that meets the severe design constraints of heating substrates to high temperatures (≤900 °C) in relatively high pressures (hundreds of Torr) of oxidizing gases. The heater has been used to heat substrates by both thermal conduction via a conductive high temperature cement and by direct radiation. In particular, the production of high‐quality YBa2Cu3O7−δ thin films grown on radiatively heated LaAlO3 substrates is demonstrated using the pulsed laser deposition (PLD) technique.

Studies of excimer laser annealing of in situ deposited thin films of YBa2Cu3O7−δ

Abstract Superconducting films of YBa 2 Cu 3 O 7−δ have been prepared on yttrium stabilized cubic zirconia in situ by laser (308 nm) ablation. Films have been deposited at 650°C with and without the presence of a second (annealing) excimer radiation pulse (308 nm) incident on the substrate. These films are compared to high-quality films prepared at 790 ° C. In annealing experiments, time delays between the synchronized excimer lasers were varied from 0 to 1000 μs to explore the effect of the annealing radiation throughout the mass arrival time range at the substrate. Little improvement was observed in the overall quality of the deposited films at any time delay. Model calculations indicate that a possible reason for the lack of improvement originates in a temporal width mismatch of the temperature increase of the substrate surface and the mass arrival of the ablated material.

Pulsed laser deposition of oxide superconducting thin films

Pulsed laser deposition (PLD) is emerging as a popular thin film deposition technique for complex materials, especially metal oxides such as high temperature superconductors. Advantages over other thin film techniques are ease of stoichiometric deposition, high deposition rates, and processing pressure independence. Furthermore, multilayer deposition can be accomplished by simply switching different targets into the laser beam. Two common problems associated with PLD are particle deposition and target modification. In this paper we summarize some of the advantages, difficulties, and future directions of PLD.

Growth of high-Tc superconducting thin films for microwave applications

High Tc superconducting thin films of YBa2Cu3O7-(delta ) (YBCO) were deposited on sapphire substrates with buffer layers of yttria-stabilized zirconia and cerium oxide by using pulsed laser deposition (PLD) technique. The epitaxial relationship between the substrate and buffer layers as well as the buffer layer and the superconducting film was established. Furthermore, using the same thin film technique, YBCO superconducting thin films were deposited on both sides of substrates of up to 5 cm in diameter. The superconducting properties of the films on both the sapphire with buffer layers and large-area substrates were comparable to the ones of the best YBCO films.

LASER PROCESSING OF HIGH TEMPERATURE SUPERCONDUCTING THIN FILMS

ABSTRACT High quality thin films of high temperature superconductors (HTS) have been fabricated using pulsed or cw lasers as physical vapor deposition sources. There is current interest in scaling the pulsed laser deposition (PLD) technique to coat 3-in. wafers with HTS. Process control and diagnostics relevant to achieving the required scaling and reproducibility will be reviewed.

Were ancient seals secure

Forgeries of ancient seals have been found in modern times, but there has been little previous analysis of how much security ancient seals might have offered. In this paper, we demonstrate four different vulnerabilities of clay seal impressions using attack methods and materials that were available thousands of years ago. The success of these attacks suggests that ancient stamp and cylinder seals may have been highly vulnerable to spoofing.

Plume dynamics of excimer-laser-vaporized YBa2Cu3O7-delta material

Pulsed laser deposition of thin films is a technology that has been explored in some detail. Because of the difficulty of monitoring in real time either the ablation process itself, or thin film growth, many studies have relied on diagnostics of either the ablated plume (emission, absorption, fluorescence) or the resulting films (Tc, Jc, X-ray, RBS) to infer information about the overall process. This indirect approach has provided some vital information for improving the production of high-temperature superconductors. In this study the plume dynamics during the in-situ pulsed laser deposition of YBa2Cu3O7-(delta ) thin films are investigated. The 248 and 308 nm lines of an excimer laser were used to generate a plume from a bulk YBa2Cu3O7-(delta ) target. Variations in the plume distribution as a function of processing gas, pressure, fluence, energy, and spot size were monitored by resulting film distribution and composition and time resolved emission imaging. Results indicate that the plume distribution can be controlled to some extent by the incident laser beam size; in addition, broadening increases with increasing oxygen pressure.