K. Verbist

Microstructure and flux pinning properties of melt textured grown doped YBa2Cu3O7−δ

Abstract The physical properties and the microstructure of highly textured YBa 2 Cu 3 O 7 /Y 2 BaCuO 5 ceramics, doped with platinum and/or cerium have been investigated by high resolution electron microscopy (HREM), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The Ce, Pt and Ce + Pt doped samples show different J c behaviours as function of the applied field and a systematic comparison of the microstructure at different scales has been performed. The combination of both dopants leads to a drastic improvement of the critical current density either under zero field or with an external magnetic field (fishtail effect). A critical current density up to 56 000 A/cm 2 has been observed, which remains 43 000 A/cm 2 under 1 T in a sample doped with 2 wt% cerium oxide and 0.5 wt% platinum oxide. We suggest that a slight introduction of Ce in the YBa 2 Cu 3 O 7−δ matrix locally changes the superconducting properties, and enhances the flux pinning properties. Micron size BaCeO 3 particles, which appear with a preferential orientation in the YBa 2 Cu 3 O 7−δ matrix, are also potential candidates for strong pinning. Other secondary phases, formed during the growth and the presence of a nanocrystalline phase in the plate like gap of the Ce + Pt doped samples are other admixtures. Finally a high density of twins is observed in all samples with some differences between the cerium and the platinum doped samples. They most probably contribute to pinning under intermediate field.

Y2O3 inclusions in YBa2Cu3O7−δ thin films

Nanoprecipitates in YBa2Cu3O7‐δ (YBCO) thin films have been identified by high resolution electron microscopy (HREM) as Y2O3 inclusions; they correspond to two different types of epitaxial relationships namely [001] or [110] parallel to the YBCO c‐axis. The [001] precipitates are situated near the YBCO surface, in the bulk and on the YBCO film/substrate interface. The [110] precipitates have only been observed at the surface. Literature data have been reinterpreted.

Combination of CeO2 and PtO2 doping for strong enhancement of Jc under magnetic field in melt-textured superconductor YBaCuO

Abstract A combination of CeO 2 and PtO 2 doping has been studied in melt-processed YBa 2 Cu 3 O 7−x . This study was carried out using an optimized well established MTG process. The cerium-platinum doped samples exhibit a high fishtail effect with a J c of 4.3 × 10 4 A/cm 2 under an applied field of 1 T. Microstructural and nanostructural studies have been performed. The 211 and BaCeO 3 inclusions are a few micrometers wide and the formation of a metastable nanocrystalline phase (Y 4 Ba 4 )Cu 8− x (Ce,Pt) x O 20±σ related to the perovskite is detected. The 123 grains show no specific extended defect generated by the doping process. The relationship between structure and properties are discussed.

Phase competition between Y2BaCuO5 and Y2O3 precipitates in Y-rich YBCO thin films

Abstract Segregation and competition of Y-based impurities in Y-rich biepitaxial YBCO samples is investigated. The analysis of distribution and microstructure of the precipitates is performed by high resolution electron microscopy (HREM). Y 2 O 3 precipitates are found in (103) YBCO films grown on (110) SrTiO 3 (STO) substrates. On the same biepitaxial samples, Y 2 BaCuO 5 (Y211) precipitates are found in (001) YBCO films deposited on an intermediate (110) MgO seed layer. Agglomeration of Y211 precipitates is observed near the edge in the MgO seed layer. The experimental data are discussed in the context of phase competition, nucleation, epitaxy and surface migration in the Y–Ba–Cu–O system. It is deduced that the change of supersaturation and surface energy, also governed by the favorable epitaxy of the secondary phase with the substrate or the YBCO matrix, plays a major role in determining the segregation of either Y211 or Y 2 O 3 precipitates.

Ga segregation in DyBa2Cu3O7-δ/PrBa2Cu3-xGaO7-δ/DyBa2Cu3O7-δ ramp-type Josephson junctions

Ramp-type Josephson junctions with highly doped PrBa2Cu3-xGaxO7-δ barrier layers (x=0.7, 1.0) have been investigated by high-resolution electron microscopy. A Ga-rich intergrowth and Ga diffusion in the ion-milled SrTiO3 substrate are observed. The Ga segregation is responsible for the deviating electrical behavior as expected from extrapolation of low doping levels (x=0.1-0.4). The Ga diffusion in the ion-milled substrate, and possibly in the base electrode, may hamper Josephson junction fabrication using substituted REBa2Cu3O7-delta materials (RE=rare earth)

Josephson phenomenology and microstructure of YBaCuO artificial grain boundaries characterized by misalignment of the c-axes

Abstract YBa 2 Cu 3 O 7− δ (YBCO) grain boundaries characterized by a misalignment of the c -axes (45° c -axis tilt or 45° c -axis twist) have been obtained by employing a recently implemented biepitaxial technique. Junctions based on these grain boundaries exhibit good Josephson properties useful for applications. High values of the I C R N product and a Fraunhofer-like dependence of the critical current on the magnetic field, differently from traditional biepitaxial junctions, have been obtained. The correlation between transport properties and microstructure has been investigated by Transmission Electron Microscopy (TEM), which was also performed on previously measured junctions. The presence of atomically clean basal plane (BP) faced tilt boundaries, among other types of interfaces, has been shown. The possibility of selecting these kinds of boundaries by controlling film growth, and their possible advantages in terms of reproducibility and uniformity of the junction properties are discussed. The possibility of employing these junctions to explore the symmetry of the order parameter is also discussed.

Low- or high-angle Ar ion-beam etching to create ramp-type Josephson junctions

The dependence of the ramp geometry on high- or low-angle ion-beam etching, used to structure ramp-type Josephson junctions based on high- superconductors, is investigated by cross-section transmission electron microscopy. The surface quality, interfaces and crystal defects are analysed by high-resolution electron microscopy. Technical difficulties to reproducibly obtain the desired slope angle and shape make high-angle ion-beam etching less interesting although the surface quality is comparable and no systematic differences in electrical properties are observed.

Structural properties of Zn-substituted epitaxial thin-films

We optimized the deposition of thin-films using inverted cylindrical magnetron sputtering and report here a detailed structural study, especially in relation to crystal growth, associated surface morphology, precipitation and other secondary phases important for flux pinning. We find that the epitaxial quality of the Zn-substituted films is decreased compared with high-quality pure films prepared under identical conditions. The pure films have smoother surfaces, while those of Zn-substituted films contain pinholes and outgrowths. Secondary phases and a-axis grains were observed in the Zn-substituted films. precipitates with typical dimensions of 50 - 100 A have been found in both pure and Zn-substituted samples. However, their density of about , observed in the pure films, is significantly reduced in the Zn-substituted films when increasing the Zn concentration up to 4%.

YBa_{2}Cu_{3}O_{7-x} Josephson junctions and dc SQUIDs based on 45^{\text{\textdegree}} a-axis tilt and twist grain boundaries : atomically clean interfaces for applications

YBa2Cu3O7-x artificial grain boundary Josephson junctions have been fabricated, employing a recently implemented biepitaxial technique. The grain boundaries can be obtained by controlling the orientation of the MgO seed layer and are characterized by a misalignment of the c-axes (45° a-axis tilt or 45° a-axis twist). These types of grain boundaries are still mostly unexplored. We carried out a complete characterization of their transport properties and microstructure. Junctions and dc SQUIDs associated with these grain boundaries exhibit an excellent Josephson phenomenology and high values of the ICRN product and of the magnetic flux-to-voltage transfer parameter respectively. Remarkable differences in the transport parameters of tilt and twist junctions have been observed, which can be of interest for several applications. A maximum speed of Josephson vortices as calculated from the voltage step values of the order of 2 × 106 m s-1 is obtained.These devices could also have some impact on experiments designed to study the symmetry of the order parameter, exploiting their microstructure and anisotropic properties. High-resolution electron microscopy showed the presence of perfect basal plane faced boundaries in the cross sections of tilt boundaries.

YBa2Cu3O7-x Josephson junctions and dc SQUIDs based on 45° a -axis tilt and twist grain boundaries: atomically clean interfaces for applications

YBa2Cu3O7-x artificial grain boundary Josephson junctions have been fabricated, employing a recently implemented biepitaxial technique. The grain boundaries can be obtained by controlling the orientation of the MgO seed layer and are characterized by a misalignment of the c-axes (45° a-axis tilt or 45° a-axis twist). These types of grain boundaries are still mostly unexplored. We carried out a complete characterization of their transport properties and microstructure. Junctions and dc SQUIDs associated with these grain boundaries exhibit an excellent Josephson phenomenology and high values of the ICRN product and of the magnetic flux-to-voltage transfer parameter respectively. Remarkable differences in the transport parameters of tilt and twist junctions have been observed, which can be of interest for several applications. A maximum speed of Josephson vortices as calculated from the voltage step values of the order of 2 × 106 m s-1 is obtained.These devices could also have some impact on experiments designed to study the symmetry of the order parameter, exploiting their microstructure and anisotropic properties. High-resolution electron microscopy showed the presence of perfect basal plane faced boundaries in the cross sections of tilt boundaries.