N. S. Nogar

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.

Effects of beam parameters on excimer laser deposition of YBa2Cu3O7−δ

Broad angular distributions have been observed for XeCl laser ablation plumes used in the deposition of YBa2Cu3O7−δ thin films. Distributions (inferred from film thickness) and film stoichiometry were measured as a function of laser fluence, beam shape, and oxygen pressure. Parallel to the long axis of the laser spot, plumes exhibited a cos3.5(θ) spread and composition varied with angle; in the perpendicular direction, more diffuse [cos1.5(θ)] plume distributions were associated with stoichiometric deposition. The observed phenomena are consistent with formation of a Knudsen layer near the target surface.

Origin of surface roughness for c -axis oriented Y-Ba-Cu-O superconducting films

One problem with the growth of high quality c‐axis oriented YBa2Cu3O7−x films is the tendency of the film surface to become rough. We studied the film growth mechanism as a function of deposition rate using pulsed laser deposition. These films form by the classic nucleation and growth process; the thickness at which the nucleated islands coalesce increased with decreasing deposition rate. The film has pinholes prior to coalescence and nucleates outgrowths during coalescence. The outgrowths enlarge rapidly because they contain materials and crystallographic directions with growth rates faster than that of the c‐axis film. A smooth surface is obtained if the substrate temperature and deposition rate are chosen such that coalescence is just completed at the final film thickness. We observed the outgrowths nucleating at coalescence and propose that certain defects, related to the c‐axis growth habit, may be the fundamental cause of outgrowth formation. Outgrowths have not been observed in a‐axis films. Outgro...

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.

Mass spectroscopic identification of wavelength dependent UV laser photoablation fragments from polymethylmethacrylate

Samples of polymethylmethacrylate have been irradiated with laser pulses of 266 nm wavelength along with the wavelengths generated via hydrogen‐shifted stimulated Raman scattering. A quadrupole mass spectrometer monitors in real time the photoablation products produced during the irradiation. At wavelengths of 266 nm and above, the products are dominated by monomer, CO2, and CO. At wavelengths below 266 nm, a dramatic change of ablation products is observed, with methyl formate appearing as a major photochemical product.

Epitaxial yttria‐stabilized zirconia on (11̄02)sapphire for YBa2Cu3O7−δ thin films

Epitaxial yttria-stabilized zirconia (YSZ) films were deposited on (1{bar 1}02) sapphire by pulsed laser deposition. The films are formed in a cubic phase with the {ital a} axis normal to the substrate surface. Ion beam (2.8 MeV He{sup ++}) channeling measurements show that the YSZ films are highly crystalline with a channeling minimum yield of 8%. The epitaxial relationship between the film and substrate is further confirmed by a cross-section transmission electron microscopy study. Epitaxial YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} thin films deposited on YSZ/sapphire have {ital T}{sub {ital c}} and {ital J}{sub {ital c}} of up to 89 K and 1{times}10{sup 6} A/cm{sup 2} at 77 K, respectively.

Laser photodissociation probe for ion tomography studies in a quadrupole ion-trap mass spectrometer

Abstract Ion tomography studies based on spatially resolved photodissociation of benzoyl cation (C 6 H 5 CO + ) in an ion-trap mass spectrometer are presented. Changes in radially averaged parent ion distributions are observed as a function of applied RF potential during photodissociation and under conditions of resonant excitation with a supplementary ac voltage. This work also demonstrates stabilization of ion trajectories by helium buffer gas when the trap is used in the mass-selective instability mode of operation.

Ablation of high-temperature superconductor studied by resonance ionization mass spectrometry (RIMS)

Laser ablation of bulk high‐temperature superconductor (HTS) material provides a useful means of producing high‐quality HTS thin films. Mass spectrometric probes of the ablation plume provide a microscopic understanding of the ablation event and plume development as well as providing a process monitor for the thin‐film production. Resonance ionization mass spectrometry (RIMS) detection of the ablated neutral species supplies valuable physical information about the ablation event necessary for developing models of the process. Time‐of‐flight/RIMS detection of Cu, Y, and BaO ablated from YBa2Cu3O7−x indicates the ablation involves post‐desorption gas phase collisions, which influence the ablation chemistry and dynamics (e.g., angular and velocity distributions). Approximately equal velocities are observed for all neutral species at constant ablation laser fluence. In addition, BaO exhibits a rotational temperature far less than the directed translational temperature.

Use of two-photon excitation in resonance ionization mass spectrometry

We report the detection of a series of elements via 2 + 1 (photons to resonance + photons to ionize) resonance ionization mass spectrometry. The efficiency of this process is demonstrated, as the two-photon step can often be saturated. Examples are given where experimental observations of ion yields are compared to theoretical calculations based on a simple formalism used for two-photon transition rates, coupled with a rate-equations calculation of the total ionization rate. Advantages of 2 + 1 ionization compared with more widely used 1 + 1 processes are discussed.