M. McElfresh

Preferred orientation of BSCCO via centrifugal slip casting

Due to the highly anisotropic properties of BSCCO superconductors, the bulk properties of these materials can be greatly affected by preferential orientation. Substantial [ital c]-axis orientation normal to the desired direction of current flow has been demonstrated by centrifugally slip casting lead-doped BSCCO-2223. The strong preferred orientation developed in the centrifugally slip-cast material demonstrates high critical current potential.

Single variant orientational growth of YBa2Cu3O7−x on vicinal (011) SrTiO3 substrates observed by electron microscopy

Abstract The results of a transmission electron microscopy (TEM) and X-ray microanalysis (EDS) study of YBa 2 Cu 3 O 7− x (YBCO) films grown on vicinal (011) SrTiO 3 substrate are presented. The YBCO films tend to be single crystalline grown in single variant orientation with c -axis ≈45° from the surface. Cracks, second phase precipitates (CuO and Y 2 O 3 ), and a few small YBCO grains in other orientations were revealed in the films.

Biaxially aligned (013) films on vicinal (011)

Epitaxial (013) films have been grown by pulsed laser deposition onto exact and vicinal (011) substrates. X-ray diffraction measurements showed that the films contained two mirror-related orientations when grown on exact (011) surfaces, but that a strong preference for only one of the two (013) orientations developed when the substrate surface was tilted by about toward the direction. The inclination removed the two-fold symmetry of the surface to induce this preference, resulting in a film with (013) film (011) substrate and [100] film [100] substrate. Films with a single (013) orientation may offer some distinct practical advantages over other monocrystalline film orientations.

Correlation length and time in the vortex-glass critical regime in YBa2Cu3O7−x films

Abstract We report on the complex conductivity of a YBa2Cu3O7−x microbridge in the presence of magnetic field 0.5 ≤B≤6.5 T applied parallel to the c-axis. A broad range of frequences 10 ≤ f ≤ 2 × 106 Hz for the transport current passing through the bridge allows us to explore the ac dynamical response of the vortices where conflicting results have been reported, in addition to the static characteristics of the vortex system (dc limit). In consistency with the concept of a correlation length and a correlation time that diverge at the transition temperature Tg(B), we demostrate that a single value for Tg(B) can be identified in both the static and dynamical measurements, within an experimental uncertainty of 1 K. In conflict to previous reports, the real and imaginary components of the ac-linear voltage at T=Tg(B) were found to follow the same power law Vi ∝ Vo ∝ fα, with α=0.82±0.05. Finally, as T→Tg+, the critical scaling laws break down when the diverging vortex correlation length matches the film thickness of our sample and the correlation time exceeds 10 μs.

Effect of Ni and Zn Substitutions for Cu in the 25 K Phase of Bi–Sr–La–Cu–O Superconductors

We investigate the effect of Ni and Zn substitutions for Cu on the lattice parameters, phonon vibrational feature, electronic structure, as well as transport properties in Bi2Sr1.8La0.2CuOy . The experimental data show that both Ni and Zn are soluble in the Bi2Sr1.8La0.2CuOy material up to about 10% of the Cu content. In Bi2Sr1.8La0.2Cu1-x Nix Oy system, the superconducting critical temperature T c decreases linearly with Ni concentration and the rate of T c decrease is around 2.5 K/at.%, while for Bi2Sr1.8La0.2Cu1-x Znx Oy system the dependence of T c on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses the T c much stronger than the Ni substitution. The comparison of the phonon vibrational feature of the apical oxygen atoms in the CuO6 octahedron between the Bi2Sr1.8La0.2Cu1-x Nix Oy and Bi2Sr1.8La0.2Cu1-x Znx Oy systems shows that the Zn substitution leads to a more serious structure distortion than the Ni substitution. In addition, the electronic structures of the two doped systems were compared using XPS spectrum. The results show that the Zn substitution reduces Cu oxidation level more remarkably than the Ni substitution. Some possible explanations for the stronger suppression of T c by the Zn substitution than by the Ni substitution are provided, based on these experimental results.

Evidence for a vortex-solid phase transition in Tl2Ba2CaCu2O8 thin films

Abstract The electric field as a function of current density, the temperature dependences of the resistivity and dc magnetization M(T), as well as the frequency dependence of the ac susceptibility, measured at a series of magnetic fields for an epitaxial thin film of the high-temperature superconductor Tl 2 Ba 2 CaCu 2 O 8 , show strong evidence for a vortex-solid phase transition to a true superconducting state,characterized by a loss of linear resistivity. There is additional evidence of a crossover in behavior at which the dissipation process and magnetization behavior make distinctive changes, which may be associated with the couplingof unbound two-dimensional vortices (pancake-vortex liquid) into a liquid of vortex lines as temperature decreases.

Electronic structure of YBCO as a function of oxygen content: an X-ray emission approach

Abstract The Cu 3 d states in a series a YBa 2 Cu 3 O 7− x thin film samples with varying oxygen content have been investigated by X-ray emission spectroscopy. A decrease in the intensity of the emission attributable to the dpδ ; states is observed when the oxygen concentration is increased. This suggests that charge is transferred mainly to the Cu atoms in the chains upon doping.

Anomalous magnetic field dependence of the critical current density in polycrystalline YBa2Cu3O7

Abstract Magnetotransport as well as local and bulk magnetic measurements were made on polycrystalline samples of the same YBa2Cu3O7 material making it possible to directly compare the various stages of flux penetration with the transport behavior. The bulk of the sample is screened up to the applied field value at which a feature is observed in the magnetization behavior. Above this field, flux penetrates between the grains and then at sufficiently high fileds begins to penetrate into the grains. The critical current density Jc shows an anomalous hysteresis in the applied field Ha dependence. On the initial part of a jc(Ha) curve, Jc decreases rapidly but becomes nearly field independent when flux penetrates into grains. Upon decreasing Ha, Jcreases above the initial curve values.