FINITE SIZE EFFECT ON YBCO/PBCAO AND YBCO/PBCGO NANOMETER MULTILAYERS

We have fabricated (110) epitaxy YBa 2 Cu 3 O 7-δ (YBCO), PrBa 2 Cu 0.8 Al 0.2 O 7 (PBCAO), PrBa 2 Cu 0.8 Ga 0.2 O 7-δ (PBCGO) nanometer-thin films and YBCO/PBCAO, YBCO/PBCGO multilayers of a variety of film thicknesses. Electrical resistivities measured from these systems were plotted against temperatures and film thicknesses and are presented in this paper. Superconducting onset temperature Tc of the YBCO films was estimated and plotted against YBCO film thickness. Superconducting coupling length was deduced. Finite size effect and 2D to 3D transition are also discussed.

PREPARATION AND SUPERCONDUCTIVITY OF NEW 214-TYPE CUPRATES (La1.85−xCa0.15Ax)CuO4−y (A=Bi, Pb or Tl)

Single-phase 214-type cuprates (La1.70Ca0.15Bi0.15)CuO4−y and (La1.70Ca0.15Pb0.15) CuO4−y have been successfully synthesized and identified by X-ray diffraction analyses. The former has a superconducting onset temperature of about 35 K which is higher than that of (La1.85Ca0.15)CuO4−y, whereas the latter does not show superconductivity down to 20 K. Further introduction of Tl into La-site in (La,Ca)2CuO4-type cuprates can reduce the formation temperature of this kind of compounds, but neither (La1.50Ca0.15Bi0.15Tl0.20)CuO4−y nor (La1.50Ca0.15Pb0.15Tl0.20)CuO4−y shows superconductivity.

Evaluating the Chemical Compatibility of Potential Substrate Materials for Bi-Sr-Ca-Cu-O Films

Potential substrate and buffer layer materials for Bi2Sr2CaCu2Ox (BSCCO) were surveyed using bulk ceramic processing techniques. Of the materials tested, only Ag was inert with respect to BSCCO. MgO slightly degrades the superconducting onset temperature and volume fraction. LaGa03 may be a feasible ceramic substrate for BSCCO, although some reaction appears to occur.

Structural and Magnetic Properties of Untwinned Yba2Cu3O6+x Single Crystals

We have conducted structural and magnetic investigations on thermomechanically-detwinned YBa2Cu3O6+x single crystals. Single crystal x-ray diffraction studies on a fully untwinned crystal with a superconducting onset temperature of 54 K have revealed that oxygen atoms in the basal plane are offset from the crystallographic mirror plane in the a direction, leading to “zig-zag” Cu-O chains. Magnetic measurements on untwinned and twinned crystals at 77 K indicate low levels of flux pinning in both crystals, with a slightly larger amount of pinning in the twinned crystal.

Synthesis and Properties of YBaSrCu3O6.9 Prepared from Soluble Precursors

ABSTRACTSolution chemistry routes have been used to prepare single-phased superconducting ceramics of YBaSrCu3O6.9 (1113). Resistivity measurements on air annealed samples showed a superconducting onset temperature of 85 K with zero resistance at 78 K which improves to 81 K upon oxygen annealing. Magnetization data show ca. 78% flux exclusion at 5 K for 1113. X-ray diffraction experiments revealed that an orthorhombic to tetragonal transition occurs near 590°C when 1113 was heated in air. This work has demonstrated that such techniques can be utilized to prepare solid solution material with properties similar to those observed in YBa2Cu3Ox (123).

High Temperature Superconductors in the La1+xBa2-xCu3Oy System

ABSTRACTIn the perovskite-related La/Ba/Cu oxide system, the lanthanum ions can substitute on the barium sites due to a close match in the ionic radius of the two. Thus it is possible to make a solid solution La1+xBa2-xCu3Oy where O ∼ x ≤ 0.5 and y ∼ 7. For the Ba-rich end member composition (x ∼ 0). the structure is orthorhombic. but as the La content increases and La begins to substitute on the Ba sites (disordered), the crystal symmetry changes from orthorhombic to tetragonal. In addition, the superconducting properties vary with increasing La content. In particular, the superconducting onset temperature is found to decrease linearly from 80 to 45 K over the compositional (x) range 0.0 → 0.375. In the La-rich limit (x = 0.5). 1/4 of the Ba atoms are replaced by La and superconductivity is lost. A correlation between oxygen ordering (annealing) and high Tc is observed for orthorhombic LaBa2Cu3Oy and inferred in the case of tetragonal La1.25Ba1.75Cu3Oy.