Wataru Ito

Influence of Crystal Strain on Superconductivity of a-Axis Oriented YBa2Cu3Ox Films

Ion channeling measurements were executed to evaluate the crystal strain in a-axis oriented YBa2Cu3Ox (YBCO) films with different film thicknesses. The average Ba-Ba distance in the film plane as determined from the channeling angle of the Ba atomic row in the (and ) direction was found to be close to the lattice constant of the SrTiO3 substrate regardless of the film thickness. A similar result was also obtained for oxygen atoms. These results indicate that the films grow while matching their lattice constant to that of the substrate. The simultaneous strain relief induced by the increase in film thickness was suggested from the channeling yield in the obtained dip curve. Furthermore, the relaxed grain was confirmed near the surface of the 3500 A film from the Ba axial channeling. The influence of the crystal strain on the superconductivity of the a-axis oriented YBCO films is explained on the basis of variation in the site of apical oxygen.

TEM Studies on Microstructure of a-Axis Oriented YBCO Films

The microstructure of a-axis oriented YBCO films made by newly developed dc-100MHz hybrid plasma sputtering was investigated using the transmission electron microscopy. The films employed in this study were grown on (110) NdGaO3 and (100) SrTiO3 substrates and were confirmed to have an excellent crystallinity by X-ray diffraction and Rutherford backscattering measurements. The interface between the film and the substrate was found to be very sharp for both films. From the plan view of the TEM image, both films were found to consist of two kinds of grains having the c-axis aligning along two perpendicular directions in the plane with equal probability. For the film on NdGaO3 substrate, many twist boundaries which has a (100) boundary plane parallel to the surface could be observed, while the film on SrTiO3 had only symmetrical tilt boundaries and basal-plane-faced tilt boundaries, which have (013)(0 (bar 1)3) and (001)(010) planes as boundary plane respectively. The difference can be explained from the anisotropic growth rates of the film between c direction and a-b direction.

Laser Deposition of High Quality c-Axis Oriented Superconducting Nd 1+x Ba 2-x Cu 3 O 7-d Thin Films

High quality single phase Nd1+xBa2-xCu3O7-d thin films have been fabricated on SrTiO3 substrates by the laser ablation method. It has been shown that there exists a deposition pressure and temperature onset for the formation of high Tc c-axis oriented films, below which the films show a poor superconductivity due to the oxygen deficiency. The resistivity of the deposited films are usually lower than that of the YBCO films. The Tc(onset) and Tc(R=0) in the optimized thin films are as high as 91 and 88 K, respectively. The rocking curve FWHM of the (005) peak in the thin films is typically 0.05°, and the peak intensity of the strongest peak is substantially higher than those ever reported for the similarly thick YBCO films.

Single Phase a-axis YBCO Films Epitaxially Grown on NdGaO3

YBa2Cu3O7−x (YBCO) thin films on NdGaO3 (110) have been prepared using a newly developed DC-94.92MHz hybrid plasma magnetron sputtering. The hybrid plasma was generated by applying a negative bias voltage to the RF cathode. The YBCO films showed an excellent crystallinity with the a-axis being perpendicular to the plane of the substrate. A full width at half maximum (FWHM) value of 0.027° in the rocking curve through the (200) peak and χmin of 3.8% in the Rutherford backscattering channeling measurement were achieved. The interface between the film and the substrate was found to be very sharp by cross-sectional transmission electron microscopy (TEM). The surface of the films were also very smooth.

New Substrate La1−xNdxGaO3 for High-Tc YBa2Cu3Ox Epitaxial Films

La0.66Nd0.34GaO3(LNGO) single-crystalline boules were successfully grown by the Czochralski method. The boules have no crack and few twins. The structure of LNGO is orthorhombic with the lattice constants; aO=5.49lA, bO=5.492A, cO=7.766A. The length of cell-edges along both and are 3.883A, which are close to the length of b-axis or a third of c-axis length for YBa2Cu3Ox(YBCO). YBCO films grown on a (110) plane of these substrate were epitaxial. The crystallinity of the films was very good; the value of FWHM for YBCO (200) peak was 72sec (0.020deg.), which is comparable with that of the substrate.