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.
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.
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. Outgrowths are easily confused with the particulate deposition problem associated with laser deposition. In these experiments, the particulate problem was essentially eliminated by using freshly polished targets for each run.
Photodissociation of mass-selected populations of trapped ions is used as a tool to determine the spatial distributions of the ion clouds under a variety of trapping conditions. These ion tomography experiments are performed in both the axial and radial dimensions, and the results show that the ion cloud expands significantly in the radial dimension as the number of trapped ions is increased. This expansion correlates with an increasing error in mass assignment due to delayed ion ejection. Furthermore, both effects appear to be related to the occurrence of compound-dependent (rather than mass/charge ratio-dependent) effects on ion ejection. The molecular ions of nitrobenzene and n-butylbenzene, and the benzoyl cation, examined under fixed conditions using the same number of ions, each displays different mass shifts which correlate with differences in the magnitudes of their radial distributions. These results demonstrate that the spatial distribution of a collection of ions depends on their physicochemical properties. Furthermore, alterations in the geometry of the trap are shown to be a means of controlling the compound-dependent positional distributions as well as the corresponding mass shifts. Ion tomography measurements of the size of the ion clouds are made for all three types of ions as a function of the number of trapped ions. They show that the compound-dependent mass shifts can be eliminated by symmetrically increasing the spacing of the end cap electrodes, a procedure which deliberately increases the positive octapolar field component. The implications of these results for exact mass measurements using ion traps are considered.
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