Using single-target off-axis sputter deposition, high quality superconducting films of YBa2Cu3O7−δ were made in situ. These films have properties which are distinctly different from those of bulk ceramics and of post-deposition annealed films. Their superconducting resistive transitions remain sharp regardless of the value of Tc between 75 and 86 K. Normal-state conductivities are as high or higher than the best single crystals. Critical current densities as high as 6×107 A/cm2 at 4.2 K. Tc (R=0) falls off with film thickness down to 10 K for 35–40 Å films. All of the above properties are relatively insensitive to compositional variation. The results can be explained if the in situ growth results in well-formed CuO2 planes with defects occurring elsewhere.
YBa(2)Cu(3)O(7) films have been grown epitaxially on SrTiO(3) (100) and LaAlO(3) (100) substrates with nearly pure a-axis orientation and with transition temperature T(c) (R = 0) of 85 K. A unique feature of these films is their smooth surface. These smooth surfaces enable the growth of short-period superlattices with well-defined modulations. The films are untwinned and the grains grow with their c-axis along one of two perpendicular directions on the substrate ([100] or [010]). The fabrication of sandwich-type Josephson junctions with good characteristics may now be possible because unlike c-axis-oriented films, the superconducting coherence length of these smooth films is appreciably large perpendicular to their surfaces.
Dopant site occupancies in YBa2Cu3 -"M"O7 -s with Af Fe (x 0.3 and x 0.5), Co (x 0.2 and x=0.5), Ni (x 0.3), and Zn (x 0.3) have been found using differential anomalous x-ray scattering. The Ni and Zn atoms were found to occupy the Cu(1) ("chains") site and the Cu (2) (" planes" ) site in a nearly random distribution. The Fe and Co atoms were found to occupy the Cu(1) site predominantly at low x, with an increasing fraction on the Cu(2) sites as the total amount of dopant increases. In all cases, our results appear to have high statistical significance, with very little sensitivity to expected uncertainties in oxygen content, total dopant content, anomalous corrections to the atomic scattering factor of the dopant, and to relative atomic coordinates assumed in the modeling. %e have also discussed the results in the context of existing extendedx-ray-absorption fine-structure and neutron-diffraction results, thermogravimetric analysis, Mossbauer spectra, and T, and Hall-effect studies.
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