In order to unveil the mechanism responsible for the large decrease of Tc ((Delta) Tc approximately equals -30 K) found in YSr2Cu3O6+x with respect to YBa2Cu3O6+x, we compare structural and electronic properties of these two cuprates. We report X-ray, neutron diffraction, resistivity and ac susceptibility data and results of ab initio electronic structure calculations carried out within the local density approximation. Main structural differences between the two phases are: (1) the absence of long CuO chains in YSr2Cu3O6+x possibly due to a large orthorhombic distortion predicted by calculations; (2) the strong compression of the CuO5 pyramids along the z- direction, which reduces the metallic character of the bond between the chain copper and the apical oxygen. This is expected to hinder the hole transfer from the CuO chains to the superconducting CuO2 planes, thus reducing the effective doping of the planes with respect to the optimum doping level. Both the disorder associated with short CuO chains and the reduced hole transfer would qualitatively account for the observed reduction of Tc.
Since the discovery of superconductivity in 1911, most of the tailoring of new superconducting materials have been based on empirical criteria such as educated substitutions inspired by the periodic table. This task has become very complicated with the advent of high Tc oxide superconductors. In particular, it would be difficult to envisage new superconducting systems different from cuprates and bismuthates. By analyzing as to how some superconducting materials were discovered, four strategies are proposed. These are based on: (1) band structure calculations, which may lead to systems different from known ones; (2) high resolution electron microscopy studies, which may lead to different structural arrangements within the same systems; (3) systematic studies of phase diagrams; (4) in-situ high pressure studies of the critical temperature of known systems, which may lead to cation or anion substitutions simulating at ambient the structure stable under pressure and responsible for the higher Tc. An example of the latter strategy is discussed in detail.
We report a combined analysis of resistivity and x-ray diffraction rocking curve measurements on c-axis oriented YBA2Cu3O6.9 films epitaxially grown on (100) SrTiO3 and LaAlO3 by ion-beam sputtering. We find that the growth-induced reduction of long-range lattice order in the films begins to depress superconductivity and normal conductivity at a critical value of lattice coherence length of approximately equals 10 and 5 nm for the two above types of substrates respectively. Evidence for disorder-induced localization is given by a deviation from linearity of the temperature-dependence of the resistivity which scales as the reduction of superconducting critical temperature. Similar nonlinear dependence observed in slightly reduced or lightly Co-doped samples suggests that the disorder in our films significantly affects the CuO chains. Our analysis of the paraconductivity term in the films gives evidence for the enhancement of the superconducting fluctuations by the disorder.
We report measurements of surface impedance in YBa2Cu3O7 (YBCO) ultra-thin films and YBCO/PrBa2Cu3-xGaxO7 superlattices deposited by laser ablation on SrTiO3 with different thickness d of the YBCO sublayer. We find that the Tc is unchanged in relaxed or strained structures while the width follows two distinct regimes: it is narrow and slightly decreases with d in the former and is broad and follows the law (Delta) Tc approximately (root)d in the latter. We propose a simple argument which explains these two regimes in terms of statistical fluctuations of the number of carriers respectively in single relaxed crystalline domains and in the whole strained sublayer.
Superconducting YBa2Cu3O7-delta (YBCO) thin films have been grown in situ by ion beam sputtering on Si and GaAs substrates with intermediate, conducting Indium Tin Oxide (ITO) buffer layers. Uniform, textured YBCO films on ITO exhibit Tc onset at 92K and Tc0 at 68K and 60K on Si and GaAs substrates respectively, the latter value is the highest Tc reported on GaAs. YBCO/ITO films exhibit metallic resistivity behavior. In situ YBCO films on SrTiO3 show Tc onset = 92K and Tc0 = 90.5K, transition widths are less than 1K. A simple optical bolometer has been constructed from YBCO films on SrTiO3. Tunnelling measurements have also been carried out using the first YBCO-Pb window-type tunnel junctions.
This short overview briefly summarizes the most important parameters for successful preparation and associated properties of thin films of YBa2Cu3O(7-delta) (YBCO) superconductors. The principles are illustrated by using the example of monotarget ion beam sputtering technique: YBCO films grown in situ on SrTiO3 show Tc(onset)
Superconducting YBa2C u 3 0 7 (YBCO) thin films were grown on Si with
transparent, conducting Indium Tin Oxide (ITO) buffer layers The onset
temperature at 92K and zero resistance at 68K were measured. Both, ITO and
YBCO films were deposited by ion beam co-deposition. The YBCO/ITO films
exhibit metallic resistivity with positive slopes (r0.055 1K). The YBCO is
uniform, textured and polycrystalline. The relevance for hybrid opto-electronic
device structures is briefly discussed.
Superconducting YBa2Cu3O 7 (YBCO) thin films were grown on Si with
transparent, conducting Indium Tin Oxide (ITO) buffer layers The onset
temperature at 92K and zero resistance at 68K were measured. Both, ITO and
YBCO films were deposited by ion beam co-deposition. The YBCO/ITO films
exhibit metallic resistivity with positive slopes (''O.O55 □/K). The YBCO is
uniform, textured and polycrystalline. The relevance for hybrid opto-electronic
device structures is briefly discussed.
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