Analysis of melt-textured YBCO with nanoscale inclusions

Recently, particles with the chemical composition Y2Ba 4CuMOx where M U, Nb, Zr, etc., and sizes in the range of 50 - 200 nm have been generated within the YBCO matrix of bulk, melt-processed superconductors in order to serve as effective flux pinning sites. By means of AFM and electron backscatter diffraction (EBSD) measurements, we analyse the spatial distribution and the size distribution of these nanoparticles within the superconducting YBCO matrix

Investigation of grain orientations of melt-textured HTSC with addition of uranium oxide, Y2O3 and Y2BaCuO5

Local grain orientations were studied in melt-textured YBCO samples processed with various amounts of depleted uranuim oxide (DU) and Y 2O3 by means of electron backscatter diffraction (EBSD) analysis. The addition of DU leads to the formation of Ucontaining nanoparticles (Y2Ba4CuUOx) with sizes of around 200 nm, embedded in the superconducting Y-123 matrix. The orientation of the Y 2BaCuO5 (Y-211) particles, which are also present in the YBCO bulk microstructure, is generally random as is the case in other melttextured Y-123 samples. The presence of Y-211 particles, however, also affects the orientation of the Y-123 matrix in these samples

EBSD characterisation of Y2Ba4CuUOx phase in melttextured YBCO with addition of depleted uranium oxide

Melt-textured YBCO samples processed with added Y2O3 and depleted uranium oxide (DU) contain nano-particles, which have been identified previously as Y2Ba4CuUOx (U-411). This phase has a cubic unit cell, which is clearly distinct from the orthorhombic Y-123 and Y-211 phases within the YBCO system. In samples with a high amount of DU addition (0.8 wt-% DU), U-2411 particles have sizes between 200 nm and several νm, so identification of the Kikuchi patterns of this phase becomes possible. Together with a parallel EDX analysis, the particles embedded in the Y-123 matrix can be identified unambiguously. In this way, a three-phase EBSD scan becomes possible, allowing also the identification of nanometre-sized particles in the sample microstructure

Anisotropic superconducting strip in an oblique magnetic field

The critical state of a thin superconducting strip in an oblique applied magnetic field H_a is analyzed without any restrictions on the dependence of the critical current density j_c on the local magnetic induction {\bf B}. In such a strip, j_c is not constant across the thickness of the sample and differs from J_c/d, where J_c is the critical sheet current. It is shown that in contrast to the case of {\bf B}-independent j_c, the profiles H_z(x) of the magnetic-field component perpendicular to the strip plane generally depend on the in-plane component H_{ax} of the applied magnetic field H_a, and on how H_a is switched on. On the basis of this analysis, we explain how and under what conditions one can extract j_c({\bf B}) from the magnetic-field profiles H_z(x) measured by magneto-optical imaging or by Hall-sensor arrays at the upper surface of the strip.Comment: 7 pages with 4 figure

Central peak position in magnetization loops of high-TcT_c superconductors

Exact analytical results are obtained for the magnetization of a superconducting thin strip with a general behavior J_c(B) of the critical current density. We show that within the critical-state model the magnetization as function of applied field, B_a, has an extremum located exactly at B_a=0. This result is in excellent agreement with presented experimental data for a YBCO thin film. After introducing granularity by patterning the film, the central peak becomes shifted to positive fields on the descending field branch of the loop. Our results show that a positive peak position is a definite signature of granularity in superconductors.Comment: $ pages, 6 figure

Current density inhomogeneity throughout the thickness of superconducting films and its effect on their irreversible magnetic properties

We calculate the distribution of the current density $j$ in superconducting films along the direction of an external field applied perpendicular to the film plane. Our analysis reveals that in the presence of bulk pinning $j$ is inhomogeneous on a length scale of order the inter vortex distance. This inhomogeneity is significantly enhanced in the presence of surface pinning. We introduce new critical state model, which takes into account the current density variations throughout the film thickness, and show how these variations give rise to the experimentally observed thickness dependence of $$ and magnetic relaxation rate.Comment: RevTex, 9 PS figures. To appear in Phys. Rev.

Characterization of nano-composite M-2411/Y-123 thin films by electron backscatter diffraction and in-field critical current measurements

Thin films of nano-composite Y-Ba-Cu-O (YBCO) superconductors containing nano-sized, non-superconducting particles of Y2Ba 4CuMOx (M-2411 with M = Ag and Nb) have been prepared by the PLD technique. Electron backscatter diffraction (EBSD) has been used to analyze the crystallographic orientation of nano-particles embedded in the film microstructure. The superconducting YBa2Cu3O7 (Y-123) phase matrix is textured with a dominant (001) orientation for all samples, whereas the M-2411 phase exhibits a random orientation. Angular critical current measurements at various temperature (T) and applied magnetic field (B) have been performed on thin films containing different concentration of the M-2411 second phase. An increase in critical current density J c at T < 77 K and B < 6 T is observed for samples with low concentration of the second phase (2 mol % M-2411). Films containing 5 mol % Ag-2411 exhibit lower Jc than pure Y-123 thin films at all fields and temperatures. Samples with 5 mol % Nb-2411 show higher Jc(B) than phase pure Y-123 thin films for T < 77 K

Hydrodynamic Instability of the Flux-antiflux Interface in Type-II Superconductors

The macroturbulence instability observed in fluxline systems during remagnetization of superconductors is explained. It is shown that when a region with flux is invaded by antiflux the interface can become unstable if there is a relative tangential flux motion. This condition occurs at the interface when the viscosity is anisotropic, e.g., due to flux guiding by twin boundaries in crystals. The phenomenon is similar to the instability of the tangential discontinuity in classical hydrodynamics. The obtained results are supported by magneto-optical observations of flux distribution on the surface of a YBCO single crystal with twins.Comment: 12 pages, 3 figures, submitted to Physical Review Letter

Macroturbulent Instability of the Flux Line Lattice in Anisotropic Superconductors

A theory of the macroturbulent instability in the system containing vortices of opposite directions (vortices and antivortices) in hard superconductors is proposed. The origin of the instability is connected with the anisotropy of the current capability in the sample plane. The anisotropy results in the appearance of tangential discontinuity of the hydrodynamic velocity of vortex and antivortex motion near the front of magnetization reversal. As is known from the classical hydrodynamics of viscous fluids, this leads to the turbulization of flow. The examination is performed on the basis of the anisotropic power-law current-voltage characteristics. The dispersion equation for the dependence of the instability increment on the wave number of perturbation is obtained, solved, and analyzed analytically and numerically. It is shown that the instability can be observed even at relatively weak anisotropy.Comment: 10 pages, 5 figures, submitted to Physical Review