E.A. Early

Two new electron cuprate superconductors, Pr1.85Th0.15CuO4−y and Eu1.85Ce0.15CuO4−y, and properties of Nd2−xCexO4−y

Abstract We report the first observation of superconductivity in the Pr 2− x Th x CuO 4− y and Eu 2− x Ce x CuO 4− y systems. Superconducting resistive onset temperatures of ∼ 23 K and ∼ 13 K were observed for Pr 1.85 Th 0.15 CuO 4− y and Eu 1.85 Ce 0.15 CuO 4− y , respectively; the corresponding transition midpoints are T 0.5 c = 19.5 and 8 K . Magnetic susceptibility data indicate that the superconductivity is a bulk property. Transition temperatures higher than previously reported for Pr 1.85 Ce 0.15 CuO 4− y and Sm 1.85 Ce 0.15 CuO 4− y were also observed. Oxygen content analysis reveals that the superconducting Ce-doped specimens have quite small oxygen deficiencies. Specific heat data for Nd 2 CuO 4− y ( y ≈0.07) and Nd 1.85 Ce 0.15 CuO 4− y ( y ≈0.02) display a large peak, presumably associated with magnetic ordering of Nd 3+ ions, Which decreases in magnitude with Ce doping. Pressure-dependent electrical resistivity measurements reveal an increase of T c with applied pressure for Nd 1.85 Ce 0.15 CuO 4− y ( y ≈0.02), at a rate of approximately d T c /d P ≈0.025 K/kbar.

Low temperature specific heat of Ln2CuO4 (Ln=Pr, Nd, Sm, Eu and Gd) and Nd1.85M0.15CuO4−y(M=Ce and Th)

Abstract Specific heat measurements in the temperature T range 0.5 K ⩽ T ⩽ 30 K were performed on insulating Ln 2 CuO 4 compounds (Ln = Pr, Nd, Sm, Eu and Gd) and on superconducting Nd 1.85 M 0.15 CuO 4 − y compounds (M = Ce and Th). The specific heat of Eu 2 CuO 4 and Pr 2 CuO 4 were analyzed to obtain electronic contributions and Debye and Einstein lattice contributions. The data for the compounds Nd 2 CuO 4 , Sm 2 CuO 4 and Gd 2 CuO 4 show magnetic ordering of the trivalent Ln ions at temperatures T M of 1.7 K, 5.94 K and 6.66 K, respectively. Magnetic ordering is also observed in Nd 1.85 M 0.15 CuO 4 − y at T M = 1.2 for both M = Ce and M = Th. The data for doped and undoped Nd 2 CuO 4 were compared to derive an electronic contribution to the specific heat of Nd 1.85 Ce 0.15 CuO 4 − y , with γ ≈ 53 mJ/mol K 2 .

Increase in the flux‐pinning energy of YBa2Cu3O7−δ by shock compaction

Magnetic flux relaxation data on a composite specimen of YBa2Cu3O7−δ +Ag shock compacted at 167 kbar show that the flux‐pinning energy at 70 K and 10 kOe is enhanced by a factor of 2–3. This enhancement persists after annealing at 890 °C in oxygen for 24 h. The increased pinning energy is apparently caused by shock‐induced 〈100〉 and 〈110〉 line defects having a density up to 1012 cm−2. Annealing replaces line defects with (001) extrinsic stacking faults having displacement vectors of 1/6[031]. The average separation of the stacking faults in the [001] direction is 0.04 μm, which is comparable to the separation between fluxoids at 10 kOe. These results indicate that intragranular critical current densities can be increased significantly in bulk materials by shock processing. High densities of flux‐pinning sites could be induced efficiently in industrial‐scale quantities of bulk high Tc oxides by explosives.

Crystallographically oriented superconducting Bi2Sr2CaCu2O8 by shock compaction of prealigned powder

Highly textured specimens of Bi2Sr2CaCu2O8 were obtained by shock compaction of powder at 30–140 kbar and by sintering. The compacts were characterized by x‐ray diffraction, magnetic susceptibility, electrical resistance, optical microscopy, and scanning electron microscopy data. Preferential orientation with the a‐b plane in the plane of the disk‐shaped compacts was achieved by tapping platelet‐shaped powder with a controlled size distribution, followed by shock compaction or by sintering. The shocked compacts were annealed to induce metallic resistance while maintaining preferential crystallographic orientation. The electrical and magnetic data demonstrate that shock compaction heterogeneously heats and bonds grain boundaries.

Demonstrating superconductivity at liquid nitrogen temperatures

This article describes two demonstrations of superconductivity at the boiling temperature of liquid nitrogen (77 K) using the 90 K superconductor YBa2Cu3O7−δ(δ≊0.2). Both demonstrations involve the repulsion of a permanent magnet by a superconductor due to the expulsion of the magnetic field from the interior of the latter. In the first demonstration, the repulsion is manifested in the separation of a permanent magnet and a superconductor that are suspended from separate threads, while in the second it results in the levitation of a permanent magnet above a flat superconducting disk.

Effect of pressure on critical parameters of electron-doped superconductors Ln2−xMxCuO4−y (Ln=Nd, Sm, Pr; M=Ce, Th)

Abstract The effects of pressure on the temperature of the superconductive transition and the critical field of irreversibility for n-type polycrystalline high- T c compounds Ln 2−x M x CuO 4−y (Ln=Nd, Sm, Pr; M=Ce, Th) was investigated. The pressure derivatives of critical parameters are less in magnitude than these for p-type high- T c compounds. This fact may be accounted for by structure peculiarities of electron-doped HTSC.

Preparation and properties of high-Tc superconducting oxide-silver composites by oxidation of a ductile alloy precursor

Abstract We have prepared alloys of metallic Y, Ba, Cu, and Ag and oxidized these precurors to form superconducting oxide - silver composites. The alloy precursors are ductile, and the superconducting oxides have high transition temperatures.

Superconducting glass behavior and frustration in polycrystalline Sm2−xCexCuO4−y

Abstract A magnetic field was used to separate contributions to superconductivity from islands and inter-island Josephson coupling in polycrystalline samples of Sm 2− x Ce x CuO 4− y (0.14≤x≤0.17). The electrical resistivity below the Josephson coupling temperature T cJ is significantly larger in the zero-field-cooled state than in the field-cooled state up to 1 kOe, implying metastable configurations. As the field is increased, T cJ decreases rapidly up to 1 kOe and saturates at higher fields. These observations indicate a glass state in a disordered Josephson array.

Properties of alloy precursors and the resulting superconducting YBa2Cu3O7−δ‐Ag composites

A significant obstacle to the widespread application of high‐temperature superconductors are the poor mechanical properties of these oxides. A novel processing technique was investigated that attempts to circumvent this problem using ductile alloy precursors to superconducting composites. Alloys of composition Y‐2Ba‐3Cu‐Ag were prepared by arc‐ and induction melting the metals. A variety of mechanical tests show that alloys with 90 wt % Ag are ductile, and differential thermal analysis indicates a solidus temperature of 703 °C. Using thermogravimetric analysis, the oxidation rate of Y‐2Ba‐3Cu‐90 wt % Ag alloys was studied. This rate does not follow that which is expected based upon the theory of internal oxidation, nor is there a transition to external oxidation upon lowering the partial pressure of oxygen. The superconducting properties of the oxide in the resulting composite are similar to those of polycrystalline samples, with a Tc of 90 K. Superconductivity is achieved only when the alloy is oxidized ...

Shock consolidation of crystallographically aligned Bi2Sr2CaCu2O8 powders

Abstract Bulk samples of crystallographically aligned Bi 2 Sr 2 CaCu 2 O 8 were fabricated by shock-consolidation of tapped micaceous powders. Basal plane alignment was verified by means of optical microscopy, X-ray diffraction and magnetic measurements. We have explored the dependence of this crystallographic alignment as a function of both shock pressure (from 35 kbar to 135 kbar) and powder particle size (from