Search

The dependence of properties in YBa2Cu3O7 (YBCO) films grown by pulsed laser deposition at different distances from the target to a substrate has been systematically studied. The target to substrate distance DTS is shown to enable simple means for tunability of the critical current density and microstructure of the YBCO films. The variation of structural and electromagnetic properties in the films as a function of DTS has been explained in terms of laser plume propagation model and thin film formation mechanism. [ABSTRACT FROM AUTHOR]

The vortex pinning model based on the presence of the large number of edge dislocations in high quality YBa2Cu3O7 (YBCO) films and multilayers has been refined. By introducing the pinning potential of a chain of individual edge dislocations, we have been able not only to describe the critical current density dependence on the applied magnetic field over its entire range, but also to extract the microstructural parameters in the films, such as interdislocation spacing and average domain size, without employing sophisticated microstructural analysis. The model applicability and its results have been verified with the help of microstructural characterization combined with magneto-optical imaging in YBCO films and multilayers with different properties. [ABSTRACT FROM AUTHOR]

MgB2 superconductors were made with two processes in this work. In one case, the SiC doped MgB2 pellets were sintered in excessive Mg medium. It is found that the samples sintered in excessive Mg medium present Mg-rich composition comparing with the one sintered in Ar gas. The density is higher and the resistance is lower due to the penetration of the Mg into the pores and grain boundary of MgB2, which is beneficial to the Jc improvement. In the other case, a two-step process was employed to make MgB2. Jc, of the sample with the composition of MgB4 is very low after the first sintering. The Jc. was dramatically improved after the second sintering, but J. of the two-step made sample is still poorer than the MgB2 made with general one step sintering due to the connectivity deterioration caused by the pulverization prior to the second sintering. [ABSTRACT FROM AUTHOR]

Much denser YBa2Cu3O7 (YBCO) superconducting films at least 1 μm thick with smooth surfaces have been obtained after introducing (Y/Nd)BCO multilayered structures. A significant enhancement of current carrying abilities has been achieved in these pulsed laser deposited films. The critical current density (Jc) has been increased by a factor of up to three in the entire field range. The imitation of the (Y/Nd)BCO multilayered structure by using YBCO ‘interlayers’ has also exhibited a considerable improvement in the current carrying ability. Similarities and differences between these two ‘multilayering’ approaches, leading to the observed effects, are discussed. The presence of interfaces even without element variation is likely to be the key to the improvements observed. In order to further promote Jc enhancement, small ‘multilayered’ additions of silver (Ag) to the films and multilayers provide various results: (i) rather strong enhancement for rather thin films and low addition levels, (ii) considerable degradation for large addition levels, and (iii) much less pronounced effect for thicker films. The enhancement of Jc is observed in zero and low magnetic fields, but has almost no effect at higher fields. All these experimental observations are discussed in terms of pinning, transparency and structural modifications introduced during the deposition of the film structures. [ABSTRACT FROM AUTHOR]

Significant differences in superconducting and microstructural properties between two types of MgB2 films prepared by pulsed laser deposition were determined. A very high Hc2 — T slope of 1.1 T/K was achieved in the in situ film. The Jc — H curves of the in situ film also show a much weaker field dependence than that of the ex situ film. The magneto-optical (MO) images show that at 4 K the flux penetrates the in situ MgB2 film through random paths, while for the ex situ film, the flux penetration pattern is mostly repeatable, indicating a defect-controlled flux penetration. Microstructural study (transmission electron microscopy and atomic force microscopy) revealed a relatively big grain size in the ex situ film. The correlation between the super- conducting properties, microstructure and preparation conditions is discussed with regard to the two types of films. [ABSTRACT FROM AUTHOR]

A novel method is proposed to increase the MgB2 core density and, as a consequence, the critical current density of Fe-sheathed MgB2 wires. This method involves two additional stages compared to the usual wire fabrication: post-annealing mechanical deformation and a consecutive heat-treatment. Using this method we obtained denser core structures in 7-filament wire having Jc values larger by a factor of about 4.3 than that found in the initial “in situ” prepared mono-core wire. Independent of the manufacturing procedure, the wires showed a transport property degradation effect after being exposed to the air for 4 months. [Copyright &y& Elsevier]

Much smoother surfaces and significantly improved superconducting properties of relatively thick YBa2Cu3O7 (YBCO) films have been achieved by introducing a multilayered structure with alternating main YBCO and additional NdBCO layers. The surface of thick (1 μm) multilayers has almost no holes compared to YBCO films. Critical current density (Jc) has been drastically increased up to a factor >3 in 1 μm multilayered structures compared to YBCO films over entire temperature and applied magnetic field range. Moreover, Jc values measured in thick multilayers are even larger than in much thinner YBCO films. The Jc and surface improvement have been analyzed and attributed to growth conditions and corresponding structural peculiarities. [ABSTRACT FROM AUTHOR]

The glass-like vortex distribution in pulsed laser deposited YBa2Cu3O7 − x thin films is observed by scanning superconducting quantum interference device microscopy and analysed for ordering after cooling in magnetic fields significantly smaller than the Earth's field. Autocorrelation calculations on this distribution show a weak short-range positional order, while Delaunay triangulation shows a near-complete lack of orientational order. The distribution of these vortices is finally characterised as an isotropic vortex glass. Abnormally closely spaced groups of vortices, which are statistically unlikely to occur, are observed above a threshold magnetic field. The origin of these groups is discussed, but will require further investigation. [ABSTRACT FROM AUTHOR]

Abstract: Structural and magnetic properties of (NdBa)MnO3 thin films grown on several perovskite substrates by pulsed laser deposition are presented. A high crystal quality epitaxial film with smooth surface and low level of internal strain may be grown up to thicknesses of 70nm. The in-plane distortion of the film lattice by the substrate-introduced compressive strain promotes corresponding changes in the out-of-plane lattice constant, following the volume-preserving dilation. The films under tensile strain are growing totally relaxed. Different oxygenation conditions during depositions induce correlated changes of structural and magnetic properties of the material. The temperature of ferromagnetic transition of the films is around 100K and increases with expansion of the unit cell. The magnetic moment in saturation is usually higher along the longest lattice constant of the film. An increase of the bi-axial compressive strain introduced by the substrate results in a rapid increase of the magnetic moment of the films. The observed effects may be explained by a high level of local distortions due to difference in the ionic radii of Nd and Ba. [Copyright &y& Elsevier]

The glass-like vortex distribution in pulsed laser deposited YBa2Cu3O(7-x) thin films is observed by scanning superconducting quantum interference device microscopy and analysed for ordering after cooling in magnetic fields significantly smaller than the Earth's field. Autocorrelation calculations on this distribution show a weak short-range positional order, while Delaunay triangulation shows a near-complete lack of orientational order. The distribution of these vortices is finally characterised as an isotropic vortex glass. Abnormally closely spaced groups of vortices, which are statistically unlikely to occur, are observed above a threshold magnetic field. The origin of these groups is discussed, but will require further investigation.