National Institute Of Materials Physics - Romania

Hg-based multi- and super-multi-layered cuprates HgBa2Can-1CunOy (Hg:12(n-1)(n)) (n >= 4) are composed of the charge reservoir layer (CRL) HgBa2Ox, and of CuO2 planes: outer planes (OP) with pyramidal oxygen coordination and inner planes (IP) with square oxygen coordination, with different charge distribution. This fact gives rise to very interesting phenomena, like coexistence of superconductivity (SC) in OPs with antiferromagnetism (AF) in IPs in Hg:1245. The dependence of T-c on the number n of CuO2 planes is also interesting: for n increasing from 4 to 5 and to 6, T-c decrease from 125 to 108 and then to about 103 K; further increase in n keeps T-c constant at about 103 K. In Hg:1245 we show that the AF ordering at T-N of 60 K has no influence on vortex dynamics, and there is a quite robust c-axis supercurrent. Vortex melting lines of various samples showed that Hg-based multi- layered cuprates (n = 4, 5) behaves in the usual way (increasing n resulted in decrease of Josephson coupling (JC) and a less sharp melting line) and are well described by the usual anisotropic GL-based theory. Samples with n >= 6 have all the same melting line, suggesting that, in this case, the short-range JC becomes much less important than the long-range magnetic coupling (MC). However, since the JC between two pancakes in OPs of adjacent unit cells (pancakes separated by the thin CRL) is large, we suggest that the vortex matter in Hg-based super-multilayered cuprates is composed of magnetically-coupled pancake-vortex-molecules.

We observed two dissipation peaks in the field cooled AC susceptibility measurements on aligned multilayered cuprate Superconductor, (Cu,C)Ba2Ca2Cu3Oy [(Cu,C)-1223], and demonstrated that it is the direct experimental indication of the two-component vortex matter in the multilayered cuprates. We propose that, the second loss peak at lower temperature is due to the additional degree of freedom of the rotation of 'vortex molecule' composed of two fractional vortices, originated at two components, mediated by an i-soliton bond. To probe the dynamics of this vortex molecule, we measured frequency dependence of AC susceptibility response on aligned crystallites of (Cu,C)-1223 at different temperatures and DC fields. In second peak region, it gives resonance peak. The observed frequency dependence patterns of chi ''(T), for 0.5 T H-DC, are rescaled with the resonating frequencies and show tail at low frequency region indicating that the dissipation is due to rotation and twisting of vortex molecule. The temperature dependence of the average relaxation time shows that the vortex molecule rotation/twisting glass state follow critical slowing down process.

Field-assisted-sintering technique (FAST) has been employed to obtain polycrystalline MgB2 samples, pristine and doped with 5 mol % SiC and B4C, using commercial MgB2 powder. The mass density of the samples is around 90% of the theoretical value. The response in magnetic field shows that the upper critical field and the irreversibility field are depressed by comparison with pristine MgB2. The latter has an upper critical field H-c2=14.37 T close to the values reported for the single crystals. Scaling analysis of the field dependence of the pinning force suggests that both addition of SiC and B4C stabilizes two well defined pinning regimes in FAST-processed MgB2. At low temperatures, T < 24 K (H <= 5T), the pinning occurs at the grain boundaries whereas at higher temperatures there is a mixed pinning. These effects are supposed to be the combined result of relatively low level of C diffusion into the lattice of MgB2, specific features in formation of the grain boundaries during FAST processing and the influence of the added materials on morphology. At high fields (above 8.5T) and 4.2K, doped samples are superior to the pristine one from the critical current density viewpoint.

Materials may show anisotropic properties on different crystal directions and this is also the case of High Temperature Superconductors (HTS). To take advantage of the materials anisotropy one concept of interest is "orientation engineering" in thin films. This can be realized through the control of the film-substrate lattice relationship. Some examples in this regard are presented in this work and through comparative analysis we try to evaluate the viability of this approach and of he entcountered problems. It is expected that in the future this approach will generate new nano composite materials with new properties and effects leading to development of new devices with new or improved functionality.

Room temperature mechanical treatment consisting of multiple torsion loadings (named pre-torsion) was applied to Nb3Sn composite wires with different architecture to reduce thermal residual compressive strain experienced by the superconducting Nb3Sn filaments. Due to this effect all investigated wires have shown enhancement of the critical current density, I-c up to 56% at 15T and 4.2K. Enhancement of I-c was larger for the reinforced wires than for the wires without reinforcement suggesting that reinforcement is useful in strain relaxation during pre-torsion. The best results were obtained when the position of the reinforcement was located in the outer region of the wire. Pre-torsion is similar or more efficient than cycles of bending loadings (named pre-bending). For pre-torsion important parameter is rotation angle per length of the wire and optimum conditions for short wires are valid for long wires. Uniformity of Ic along the length of a 99cm wire after pre-torsion was within +/-2.1A.

ac susceptibility measurements were performed on pure and SiC-doped MgB2 prepared by field assisted sintering technique in the temperature range between 5 and 50 K. The activation energy for flux-creep U for both samples was determined at dc-fields of 0, 0.5, and 5 T, respectively, from the frequency dependence of the imaginary part of the ac-susceptibility chi '' data measured in the range 17-9997 Hz. It was found that at 0 and 0.5 T, the activation energy of the SiC-doped sample is almost half the energy of the pristine sample whereas at 5 T the activation energies of the two samples become almost equal. A similar behaviour show the H-p(T) lines as obtained from susceptibility data in the range 0-9 T. For fields lower than 7.5 T, the H-p field of the doped sample is lower than for the undoped MgB2. At similar to 7.5 T, the H-p(T) for SiC doped MgB2 crosses over the H-p(T) of pure MgB2 and for higher fields the doped samples exhibits a higher irreversibility H-p. This field dependence was extracted from the evolution of the. chi(1)''(T) data. This behaviour is a consequence of the morphology of the sample with a large spread in grain size.

Superconducting c-axis thin films of Bi2Sr2Ca2Cu3O10 (Bi-2223) were grown by MOCVD on (001) and (110) MgO substrates. Films on both substrates show, as revealed by AFM investigations, similar morphology composed of regular rectangular grains and relatively low roughness of about two or three half c-axis unit cell order. AFM images also suggest that films might have in-plane epitaxy. The films have zero resistance critical temperatures of 95.1K for the (001) MgO substrate and 75K for the (110) MgO substrate, respectively. Considering very large films-substrate lattice mismatch between (110) MgO and (ab)-plane of the superconducting phase the growth of indicated films is a surprise.

Fe clusters have been synthesised in ultrahigh-vacuum chamber using a gas-stabilized cluster aggregation method that ensures good control of the cluster size and naturally oxidized in order to obtain Fe/Fe oxide core-shell nanoparticles. The morphology of an individual nanoparticle, as revealed by transmission electron microscopy, consists of a Fe core of an average diameter of 4.4 nm surrounded by an oxide shell of uniform thickness of about 1.2 nm in average. The nanoparticles may be assimilated with a ferro-/antiferromagnetic (FM/AF) system. The morpho-structural features have been correlated with magnetic measurements on the core-shell nanoparticles. A significant exchange bias effect has been measured, when the sample was field-cooled under an applied field of 3 T. As the morphology of core-shell nanoclusters is much more complicated than in FM/AF bilayers of regular thickness due to the particular geometry of the coronal AF layer, the shape and surface anisotropy have to be taken into account for a correct interpretation of the magnetic data.

The laser pyrolysis is a powerful and a versatile tool for the gas-phase synthesis of nanoparticles. In this paper, some fundamental and applicative characteristics of this technique are outlined and recent results obtained in the preparation of gamma iron oxide (gamma-Fe2O3) and titania (TiO2) semiconductor nanostructures are illustrated. Nanosized iron oxide particles (4 to 9 nm diameter values) have been directly synthesized by the laser-induced pyrolysis of a mixture containing iron pentacarbonyl/air (as oxidizer)/ethylene (as sensitizer). Temperature-dependent Mossbauer spectroscopy shows that mainly maghemite is present in the sample obtained at higher laser power. The use of selected Fe2O3 samples for the preparation of water-dispersed magnetic nanofluids is also discussed. TiO2 nanoparticles comprising a mixture of anatase and rutile phases were synthesized via the laser pyrolysis of TiCl4- (vapors) based gas-phase mixtures. High precursor concentration of the oxidizer was found to favor the prevalent anatase phase (about 90%) in the titania nanopowders. Copyright (C) 2008 R. Alexandrescu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.