David B. Pulling

Superconductivity in PrBa2Cu3O7

Abstract Inhomogeneous granular superconductivity has been observed in PrBa 2 Cu 3 O 7 films probed by microwaves, and in powders studied by electron spin resonance - althought both the films and the powders have insulating dc resistivities. The transition temperatures are ≈ 90 K with 90%-10% transition widths of ≈ 20 K or less. We interpret this as meaning (i) that small regions of PrBa 2 Cu 3 O 7 superconduct, (ii) that perfect PrBa 2 Cu 3 O 7 superconducts in the vicinity of the CuO chains, (iii) that cuprate-plane models of superconductivity are invalid, and (iv) that Ba-site Pr ions break pairs and destroy superconductivity.

Magnetism and superconductivity in Sr \(\)YRu \(\)Cu \(\)O \(\) and magnetism in Ba \(\)GdRu \(\)Cu \(\)O \(\)

Abstract:We report magnetization, surface resistance (), and electron spin resonance (ESR) for non-superconducting Ba2GdRu1-uCuuO6, and find that all three magnetic ions (Gd, Ru, and Cu) are ordered at low temperatures. Both ESR (Gd sublattice) and weak ferromagnetic resonance (dopant Cu) are observed, while no magnetic resonance due to either paramagnetic or ordered Ru is detected. In addition, for superconducting ( K) Sr2YRu1-uCuuO6, resistivity, muon spin rotation (SR), and 99Ru Mössbauer absorption are reported. None of the O6 materials (e.g., Sr2YRu1-uCuuO6) have cuprate planes, although Cu is employed as a dopant. In Sr2YRu1-uCuuO6, the Ru moments order at a temperature (23 K) below that for the resistive onset of superconductivity, while the Cu orders at a higher temperature, 86 K. Therefore at low temperatures, this material exhibits magnetic order, coexisting with diamagnetism. The only non-magnetic layers in the superconducting O6 structure, the SrO layers, carry holes and exhibit diamagnetic screening characteristic of type-II superconductivity.

Evidence of superconductivity in PrBa2Cu3O7

Abstract The surface resistances of PrBa 2 Cu 3 O 7 films provide evidence of patches of superconductivity with transition temperatures of ∼92 K. The same samples have insulating dc resistivities. We interpret this as evidence that PrBa 2 Cu 3 O 7 is intrinsically superconducting, but contains Ba-site Pr ions that break pairs and destroy macroscopic superconductivity. The theoretical implications are discussed.

Flux-flow and phase-slip dissipation in crystalline Bi2Sr2CaCu2O8+δ and YBa2Cu3O7−gd

Abstract We have employed a microwave technique to determine the temperature dependences of dissipative mechanisms is textured polycrystallin Y123 and in single phase crystalline BSCCO. The measurements were carried out over the temperature range of 5–100 K, with the applied magnetic field typically varying to a maximum of 1.9 T, applied parallel to the c -axis. The RF current was in the c -plane. In addition to small flux-flow signals, a thermally activated field dependent signal, which apparently arises from phase-slip in a network of weak links, was identified. Phase-slip resistivity is the dominant loss mechanisms near T c . The data have been successfully fit to an extension of a theory of Tinkham, yielding estimates of the phase-slip pinning energy and H c2 .

Twin-boundary pinning and flux flow in zone-melt-textured YBa2Cu3O7-δ crystals

Using a microwave (13 GHz) technique which is sensitive to field-induced changes in the surface resistance, we have observed twin-boundary pinning in zone-melt-textured crystals of YBa2Cu3O7-δ. With the rotation of a static 1.9-T field applied in thea-b plane, the surface resistanceRs exhibits minima at 90° intervals. The twin-boundary minima are shown to be connected to the orientation of the crystalline axes of the sample. The resistance minima are observed superimposed on a flux-flow response over the temperature range ofTc to the lowest temperature attempted. Substantial flux mobility is observed to the lowest temperatures. The angular dependence of the flux-flow response at low temperatures is consistent with the “brick” model for local intergranular conductivity, which provides for an in-plane isotropic component.

Prediction that Cm2−zThzSr2Cu2NbOx and Gd2−zCezSr2Cu2TiOx will superconduct

Abstract Properly fabricated and doped Cm 2− z Th z Sr 2 Cu 2 NbO x and homologues of Gd 2− z Ce z Sr 2 Cu 2 TiO x , both with x ≈ 10, are predicted to superconduct, based on the charge-reservoir oxygen model of high-temperature superconductivity. In cuprate-plane models, (i) since neither CmBa 2 Cu 3 O 7 nor Cm 2− z Th z CuO 4 superconducts, Cm 2− z Th z Sr 2 Cu 2 NbO x is not expected to superconduct; and (ii) Gd 2− z Ce z Sr 2 Cu 2 TiO x is expected not to superconduct since a similar compound, Nd 2 CeSr 2 Cu 2 TiO x , did not.

Nd substitution in Y/Ba sites in melt processed YBa 2 Cu 3 O 7-d through Nd 2 O 3 additions

YBa2Cu3O7− δ (Y123) samples with excess Nd2O3 and Y2O3 additions in the same molar ratios were melt textured in air. In the Nd-doped samples, in addition to Y ion site substitution, partial substitution into the Ba2+ sites is anticipated because of the similar ionic sizes of Nd3+ and Ba2+. The microstructure, Tc, and magnetic properties of Nd-doped samples were analyzed and compared with undoped Y123 and samples with excess Y2O3. The Nd2O3 additions lead to significant magnetization improvements, likely due to both rare earth- and Ba-site substitution by the doped Nd3+ ions, and to increases in Tc. Y2O3 additions resulted in no marked property enhancement.

Superconductivity and magnetism in Sr2Y(Ru1-uCuu)O6 and in Ba2Gd(Ru1-uCuu)O6

Abstract Muon spin rotation and electron spin resonance data on sintered samples of superconducting Sr 2 Y(Ru 1− u Cu u )O 6 and non-superconducting Ba 2 Gd(Ru 1− u Cu u )O 6 are reported, both for u =0.1. In the case of Sr 2 Y(Ru 1− u Cu u )O 6 , the SrO layers are found to be p-type and to exhibit an onset for superconductivity at ≈45 K – a temperature considerably lower than the spin-ordering temperature of the Cu ions (≈86 K), indicating that the Cu ions themselves do not play a significant role in the superconductivity. Below T c , the fluctuating Ru moments begin to slow down and freeze, so that at about ≈29.3 K a spin-glass state is observed, which gives way to ferromagnetic ordering of the Ru ions in the Y(Ru 1− u Cu u )O 4 planes, with the magnetization alternating direction in the a – b plane from one magnetic layer to the next. These data confirm our earlier discovery that fluctuating moments (in this case, Ru moments) interfere with pairing. Ba 2 Gd(Ru 1− u Cu u )O 6 shows no evidence of superconductivity, which we interpret as due to pair breaking by the L =0 magnetic Gd ions, which are not crystal-field split.

Flux-flow in sintered polycrystalline YBa2Cu3O7−δ

Abstract We have observed flux-flow resistivity in superconducting sintered polycrystalline Y123 over the temperature range of 5–89 K, using a contact-less microwave technique, in an applied magnetic field varying to a maximum of 1.9 T. Flux-flow is observed superimposed on an isotropic background resistivity, and is at maximum several degrees below the transmission temperature. Flux-flow resistivity, a consequence of the Lorentz force on a fluxoid, varies as sin 2 (φ), where φ is the angle between the applied magnetic field and the RF current.

Iodometric titration of copper-oxide superconductors and Tokura's rule

Abstract Iodometric titration measurements, which are often interpreted as obtaining the excess valence on Cu atoms in Y(La y Ba 1− y ) 2 Cu 3 O x , are reinterpreted as determining concentrations of “neutral” or hypocharged oxygen instead. Tokuras Rule, an empirical relationship between the critical oxygen content x c and the critical La content y c at the onset of superconductivity, is derived on the basis of a hypocharged-oxygen model.