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The temperature dependence of AC susceptibility has been measured as a function of AC amplitude and frequency at zero DC field in MTG-YBa2Cu2.99Li0.01Oy crystals. We try to extract the critical current density near Tc from the real component of AC susceptibility curves. The temperature dependence of critical current density followed Jc(T) = 1.548 × 1010 (1 − T/87.3)1.38 (A/m2), which is good agreement with J c ′ ( T ) = 1.492 × 10 10 ( 1 - T p / 87.3 ) 1.36 ( A / m 2 ) obtained from the traditional “loss-maximum” approach. The effective activation energy near Tc is also discussed.

The nominal composition of YBa 2− x K x Cu 3 O y (YBKCO) cuprates with x ≤0.50 has been synthesized by the standard solid state reaction technique. X-ray diffraction (XRD) and the resistivity measurements are used to characterize the structure and the superconductivity of YBKCO cuprates. There was no impurity phase detected with x ≤0.20. The structure of the main phase (“123”) is orthorhombic with Pmmm symmetry in the whole doping range. With increasing potassium content, the lattice constants and some other structural parameters are almost unchanged. The refined potassium content is consistent with that of the nominal one. The zero resistance temperature T c0 decreases sharply with the increase of potassium content in YBKCO as x ≤0.20, and is almost unchanged as x >0.20. We compared these results with those of La-doped YBa 2− z La x Cu 3 O y cuprates. The difference in superconductivity depression may result from the shift oxygen which transfer conducting carriers from Cu(1)–O chains to Cu(2)–O 2 sheets.

Structural and superconducting behavior of samples prepared with the nominal composition of YBa2−xLaxCu3Oy with x≤0.50 have been studied by X-ray diffraction and resistivity measurements. X-ray diffraction studies show that there is an orthorhombic-to-tetragonal phase transition occurred at about x=0.43. The structural parameters change with the doped La content. The lattice parameters of a and b change in three steps: first a and b remain nearly constant as x≤0.25; then a decreases and b increases rapidly with the increase of La content in the doping range of 0.25≤x≤0.40; and finally a becomes equal to b with x=0.50. The c-lattice parameter and the unit cell volume decrease with the increase of La content monotonically, which may indicate that La exclusively substitute for Ba at Ba-site. The measurements for the resistivity dependence of temperature show that the depression in Tc is more pronounced for La doping in YBa2Cu3Oy at high doping level than that at low doping one. It seems that the suppression of Tc may relate to the decrease of the orthorhombicity of the unit cell.

The microstructures of YBa 2 Cu 3 O 7− δ /PrBa 2 Cu 3 O 7− δ /YBa 2 Cu 3 O 7− δ ramp type Josephson junctions with different ramp slopes fabricated on (100) SrTiO 3 substrates were studied by means of transmission electron microscopy. The results show that for the ramp slope angles of 15°–40°, the epitaxy was still remained through all layers at the ramp region without the formation of big grain boundaries. No amorphous layers and secondary phases were observed at the barrier interfaces. For a gentle ramp junction, small misoriented grains appeared in some portions of the barrier. The substrate ramp formed during the ion etching process had little influence on the growth of the upper layers. For a steep ramp junction, a dimple was created in the substrate, which caused the bending in the film and the nucleation of misoriented grains, although the epitaxy of the barrier was of good quality. The results demonstrate that the slope angle of the junction ramp is an important factor that influences the performance of the Josephson junctions.

Cross-sectional transmission electron microscopic studies have been carried out on (105) oriented YBa 2 Cu 3 O 7−δ thin films. The coherently tilted films were grown on (305) SrTiO 3 substrates prepared by rf magnetron sputtering. Our results showed that the grown films were highly epitaxial, almost single domain and free of twins, except for the presence of some small antidomains near the interface between the film and the substrate. It was found that the YBa 2 Cu 3 O 7−δ film covered the surface steps with the c -axis tilted at an angle of 31° referred to the normal direction of the substrate surface. Planar defects, such as stacking faults, could be observed in the films. Secondary phases were formed mainly due to the surface defects of the substrate, but the further growth of these secondary phase precipitates was effectively restrained by the growth of YBa 2 Cu 3 O 7−δ film with the desired orientation. Such tilted films would be useful for the study of ab -plane anisotropy of YBa 2 Cu 3 O 7−δ .

We report measurements of the anisotropic thermopower in the normal state of YBa 2 Cu 3 O 7 − δ thin films with three different orientations: a -axis perpendicular to the plane of the substrate; c -axis perpendicular; and coherently tilted on a (305) SrTiO 3 substrate respectively. We have also measured the thermopower of magnetically aligned YBa 2 Cu 3 O 7 − δ ceramic samples. The thermopower in the direction of the c -axis is found to be positive and increases as the temperature increases. The thermopower along the b -axis direction has only a small temperature dependence, while the averaged thermopower in the ab -plane in c -axis films decreases as temperature increases. These results are consistent with those made on single crystals.

Calcium- and praseodymium-doped R 1− x−y Pr x Ca y Ba 2 Cu 3 O 7−δ (R = Y, Gd) compounds have been fabricated under identical conditions. The suppression of the transition temperature in R 1− x Pr x Ba 2 Cu 3 O 7−δ with increasing praseodymium concentration x has been found to be correlated to the radii of the rare earth. By introducing non-isovalent ions into the R site, we have studied the origin of the suppression of superconductivity. Apart from this, chemical analysis by energy-dispersive X-ray spectroscopy reveals that there is a solubility limit of calcium in the compounds. This rate, at which the solubility was suppressed, shows a weak dependence on the rare-earth element. The effective valence of the praseodymium ion can be calculated by fitting the transition temperature to an empirical equation. Basic properties of the above mentioned compounds were also studied in detail.

Ultrathin films of YBa 2 Cu 3 O x with good crystallinity and high transition temperatures were prepared by modified off-axis sputtering. The surface morphology of films with thickness down to 2 nm has been studied by scanning electron microscopy and atomic force microscopy. The outgrowths were found to nucleate at the initial growing stage of films. Thus the surface quality of the initially grown layer turned out to be of practical importance in controlling and improving the film-surface morphology. The possible origins of the outgrowths have also been discussed. By employing a substrate with a very good surface and using a suitable deposition temperature, the nucleation of the outgrowths can be greatly suppressed and extremely smooth films up to hundreds nm thickness can be obtained.