P. Marsh

Preparation of superconducting thin films of calcium strontium bismuth copper oxides by coevaporation

Superconducting films of Ca‐Sr‐Bi‐Cu oxides have been prepared by coevaporation of CaF2, SrF2, Bi, and Cu, followed by post‐oxidation in wet O2. The films were characterized by four‐probe resistivity measurements, Rutherford backscattering, transmission electron microscopy, x‐ray diffraction, and Hall measurements. Zero resistance was achieved at ∼80 K, although evidence of traces of superconductivity at higher temperatures was seen in resistivity and Hall data. The critical current at 4.2 K was 1.0×106 A cm−2. The films were epitaxial on 〈100〉 and 〈110〉 SrTiO3 substrates. The electrical and structural properties of the films were insensitive to film composition over a wide range of stoichiometries.

Stoichiometry and superconductivity in single layer Bi2+xSr2−yCuO6±δ

An insulating polymorph with a composition close to Bi2Sr2CuO6 complicates the synthesis of the single copper layer superconductor Bi2+xSr2−yCuO6±δ. We concentrate on the region where the material is single phase in air synthesis (2.18 < x < 2.29, 1.44 < y < 1.7). The physical properties from insulating to marginally conducting as the composition moves toward x = y = 0. Like to two-layer compound, the material has a superlattice due to a buckling of the planar structure, however the superlattice coherence is degraded by defects at finite values of x and y. Density measurements indicate that the nonstoichiometry of Bi2+xSr2−y CuO6±δ is not accompanied by the formation of a large number of vacancies. The insulating polymorph, Bi2Sr2CuO6, can be suppressed by synthesis in modest oxygen pressures (above 5–10 atm). The result is a bulk superconductor (Tc ≈ 10 K).

Preparation of high Tc and Jc films of Ba2YCu3O7 using laser evaporation of a composite target containing BaF2

High Tc and Jc superconducting films of Ba2YCu3O7 are prepared on SrTiO3 〈100〉 substrates using pulsed excimer laser evaporation of a composite target containing BaF2, Y2O3, and CuO, followed by annealing in wet oxygen. High transition temperatures (R=0 from 89.5 to 91 K) and high critical current densities (Jc≥7×105 A cm−2) are obtained. The electrical transport properties of these films are significantly better than films previously grown via laser ablation of Ba‐Y‐Cu‐O targets.

Observation of a halide (F/Cl) stabilized, new perovskite phase in superconducting Y2Ba5Cu7Ox films

A new, ordered perovskite structure was stabilized as a majority phase in superconducting Y2Ba5Cu7O20−δ’(F/Cl)δ‘ films epitaxially grown on SrTiO3(100). The new phase was formed via fluorination or chlorination introduced during the post‐deposition O2 furnace anneal for recrystallization. The structure is determined by x‐ray diffraction to be an orthorhombic unit cell [a∼b=3.86(1) A, c=27.24(6) A] with reflection conditions consistent with a fully twinned A‐centered cell. Films containing primarily the new phase show a lower resistivity ρ(300 K)∼177 μΩ cm, a comparable ratio of ρ(300 K)/ρ(100 K)∼3.0, and a lower superconducting temperature of 78–74 K when compared to Y1Ba2Cu3O7.

Bulk superconductivity in single CuO layer Bi-Sr-Cu-O ceramics

Despite early recognition that a relatively low‐temperature superconductor occurs in samples of approximate composition Bi2Sr2CuO6, studies of this material have been plagued by difficulties in the preparation of single phase material with a large superconducting fraction. We report the first preparation of ceramic samples in the Bi‐Sr‐Cu‐O system that show bulk superconductivity at 4–15 K. Structural, chemical, and electronic characterizations of superconducting and nonsuperconducting ceramics of composition Bi2.05Sr1.95CuO6 are described.

Preparation Of High Tc And Jc Films Of Ba2YCu3O7_8 By Laser Evaporation And Observation Of Superconductivity In A 27Å Phase

The remarkable discovery of high Tc superconducting ceramics[1] has been followed by intense efforts to realize their potential in the form of devices, films, and wires. A number of techniques have been shown to be effective for growing thin films of these superconductors including electron beam/thermal co-evaporation,[2-41 RF and DC sputtering, [5-8] laser [11-12] evaporation,[9-10] and spin-on/pyrolysis. Of all of these techniques, the e-beam/thermal co-evaporation methods have been shown to give the best electrical characteristics in Ba2YCu3O7 films, particularly with respect to the critical currents which have reached 106 A/cm2 at 77K. [6,13]