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The temperature T variation of the normalized magnetization relaxation rate S in high-temperature superconductors (HTS) with strong vortex pinning exhibits a maximum in the low-T range. This was reported for various HTS, and the origin of the faster relaxation at low T appearing in standard magnetization relaxation measurements was usually related to specific pinning properties of the investigated specimens. Since the observed behaviour seems to be characteristic to all HTS with enhanced pinning (generated by random and/or correlated disorder), we show that the S(T) maximum can be explained in terms of classic collective vortex creep. The influence of thermo-magnetic instabilities in the low-T range is also evidenced. The collective (elastic) creep regime is generated by the T dependent macroscopic currents induced in the sample during standard magnetization measurements.

We attempted to grow non-c axis heterostructures of substrate-IS type with S = BSCCO and I = (Sr,Ca)CuO2. A careful selection of the films-substrate relationship is revealed as a key condition toward growth of layered heterostructures with different orientations. However this powerful principle, that is also a mandatory condition, is not sufficient in order to obtain high quality structures. The reasons are discussed looking for further solutions. Our comparative analysis is expanded to other heterostructures we reported in other works trying to reveal some of the more general problems of the non c-axis heterostructures growth.

In order to study the mechanism of the small n-value for Bi2212, E- J characteristics were measured carefully for various kinds of Bi2212 tapes and round wires in high magnetic fields. Some of wires were heat-treated in a magnetic field of 5 T. We found that the n -values of all the samples increase by the in-field heat-treatment at 5 T. In addition, we analyzed the E- J properties by the combination of the percolation model based on the local Jc distribution and the two directional Jc distribution model. The improvement of the n-value by the in-field heat treatment can be understood by the change of the aspect-ratio of the grains. Therefore, the aspect-ratio of the grains plays an important role in the current transport influencing not only Jc but also n-value.

Four-cation nanograined strontium and magnesium doped lanthanum gallate (La0.8Sr0.2) (Ga0.9Mg0.1)O(3-delta) (LSGM) and its composite with 2 wt% of ceria (LSGM-Ce) were prepared. Morphologically homogeneous nanoreactors, i.e., complex intermediate metastable aggregates of desired composition were assembled by spray atomization technique, and subsequently loaded with nanoparticles of highly energetic C3H6N6O6. Rapid nanoblast calcination technique was applied and the final composition was synthesized within the preliminary localized volumes of each single nanoreactor on the first step of spark plasma treatment. Subsequent SPS consolidations of nanostructured extremely active LSGM and LSGM-Ce powders were achieved by rapid treatment under pressures of 90-110 MPa. This technique provided the heredity of the final structure of nanosize multimetal oxide, allowed the prevention of the uncontrolled agglomeration during multicomponent aggregates assembling, subsequent nanoblast calcination, and final ultra-rapid low-temperature SPS consolidation of nanostructured ceramics. LaSrGaMgCeO(3-delta) nanocrystalline powder consisting of approximately 11 nm crystallites was consolidated to LSGM-Ce nanoceramic with average grain size of approximately 14 nm by low-temperature SPS at 1250 degrees C. Our preliminary results indicate that nanostructured samples of (La0.8Sr0.2)(Ga0.9Mg0.1)O(3-delta) with 2 wt% of ceria composed of approximataley 14 nm grains can exhibit giant magnetoresistive effect in contrast to the usual paramagnetic properties measured on the samples with larger grain size.

In this work we focus on the microstructural and magnetic characterization of wires with different architectures (design and reinforcement). The microstructural characterization was performed using scanning electron microscopy. AC magnetic susceptibility was measured with field applied both parallel and perpendicular to the wire axis. The heat treatment performed to form the A-15 superconducting phase leads to partial spheroidization followed by coarsening of the Nb filaments in the reinforcement material. The differences concerning the microstructure of the reinforcement material among the investigated wires were reflected in the broadening of the superconducting transition of Nb, more evident for a field applied parallel to the wire axis. From the magnetic data the wires were also compared in terms of the superconducting volume fraction.

The tensile strain dependence of the upper critical field Bc2 for Nb3Sn wires was measured using a new apparatus for applying tension at low temperature. The critical current Ic calculated from the strain-dependent Bc2 measurements is the almost consistent with the measured Ic. In the measurements by a new apparatus, the axial and the lateral applied strains were examined by the strain gauges attached onto the wire. The deviatoric strain epsivdev and the hydrostatic strain epsivhyd were estimated by the axial and the lateral strain. We found the deviatoric strain dependence of Bc2 is different between the tensile and compressive strain states. This means that the hydrostatic strain also should be taken into account in order to understand the strain effects on the superconducting properties of Nb3Sn.

The transport properties for in-field heat-treated monocore and multicore Bi2Sr2CaCu2O8 (Bi2212) tapes and multicore Bi2212 round wires were measured in detail. We found that the in-field heat-treatment enhances not only the critical current density Jc but also the n-value for all samples used in this study. These enhancements for the monocore tapes were more remarkable, compared in those of the multicore tapes and wires. Therefore the in-field heat-treatment is more effective for the Bi2212 tapes with a large core size. Microstructure observation shows that the grain size of the monocore tapes decreases as the result of in-field heat treatment. The analysis based on the percolation model related to the local Jc distribution suggests that the in-field heat-treatment increases the m-value, which determines the shape of the local Jc distribution and is related to the n-values. The two Jc distribution model can explain that the increases in n- and m-values are due to the decreases in the aspect ratio of the grains during in-field heat treatment.

We investigated MOCVD growth of c-axis two-layer and three-layer stacked heterostructures on (100)SrTiO3 of SI, IS or SIS type. S denotes high temperature superconductor thin films of YBa2Cu3O7 (Y-123), Bi2Sr2Ca2Cu3O10 (Bi-2223) or Bi2Sr2CaCu2O8 (Bi-2212), while I stands for insulating thin films of (Ba,Ca)CuO2 or (Sr,Ca)CuO2. X-ray diffraction, atomic force microscopy and measurements of resistivity versus temperature were performed after the deposition of each layer. The best results, suitable for further device fabrication, were obtained for the SIS structure (100)SrTiO3/Bi- 2223/(Sr,Ca)CuO2/Bi- 2212. Similar structures with I = (Ba,Ca)CuO2 have shown higher impurity phases and lower superconducting quality of the top superconducting layer. For two-layer or three-layer structures, Bi-2223 as a top layer could not be obtained and, instead, a phase with Bi-2212 formed. Structures with (100)SrTiO3/Y- 123/(Sr,Ca)CuO2/Y- 123 or (100)SrTiO3/Y- 123/(Ba,Ca)CuO2/Y- 123 were also promising, but a current leak often occurred due to greater roughness. Structures with Y-123 are less sensitive to growth temperature than those with BSCCO.

High density (more than 90% of the theoretical density) bulk samples with relatively large size (2.0 cm in diameter) of MgB2, pristine and doped with 5 mol% SiC and B4C were obtained by the field-assisted sintering technique (FAST) from commercial MgB2 powder. The superconducting properties in the magnetic field of the doped samples show depressed upper critical field Hc2 and irreversibility field Hirr when compared with pristine MgB2. The reason for better Hc2 and Hirr in the undoped sample is not clear. Most probable is that this is due to the limited diffusion time which restricts the substitution of carbon for boron in this short-time processing method as well as due to the way grain boundaries are formed in the FAST process. However, all our samples have shown higher values of Hc2 and Hirr than the values reported previously for FAST-MgB2 samples. The data were further investigated through scaling analysis. Data suggest that both additions of SiC and B4C stabilize two well defined pinning regimes in FAST-processed MgB2. At low temperatures, the pinning occurs at the grain border whereas at high temperatures, T≥24 K, there is mixed pinning (H≤5 T).

Relaxation of the thermal residual strain in Nb3Sn wires is realized through cycles of multiple torsion loadings ('pre-torsion') at room temperature. As a consequence, the critical current, Ic, is enhanced. This effect is stronger than previously reported mechanical treatments of multiple bending ('pre-bending'). The maximum Ic is attained for complex mechanical treatments of pre-torsion and pre-bending. Complex treatments allow efficient and relatively independent control of residual strain relaxation over all three directions of the wire, resulting in enhancement of Ic towards the theoretical limit of the material. Our findings have an immediate positive impact on the performance of Nb3Sn wires and, hence, on their applications (e.g. react-and-wind coils).

Two practical Nb3Sn composite wires (short samples) were investigated from the points of view of critical current, Ic , and cracks evolution versus number of bending load cycles. Bending was applied manually at room temperature using a block-form with curved surface corresponding to applied bending strains of 0.5% and 0.8%. It was applied in plane, alternatively in one direction and in the opposite one up to 130 times. We shall use for this mechanical treatment term pre-bending. The wires had approximately the same diameter but a different architecture: one was of conventional type (standard US-Japan wire, Hitachi Cable) and the second one was CuNb reinforced wire (Furukawa Electric Co. Ltd.) with reinforcement located near-the-edge. It was found that critical current maximizes gradually with the number of bendings and the maximum value of Ic is attained for a number of bending loadings, Npb , of about 15. Beyond this optimum critical current is constant. Optimum Npb is likely not dependent on the wire, applied field or pre-bending strains. Results suggest that Cu is important in the work hardening process. Work hardening during pre-bending is imposing limitations in application of this technically convenient mechanical treatment for release of the residual strain and further enhancement of Ic . Semi-quantitative analysis of the electron microscopy observations shows that for our experimental conditions the density of cracks is approximately constant being in agreement with described Ic behavior. Other possible consequences of our results are discussed. Among the most interesting issues is possibility of the 3D independent control of yielding and hence of residual strain release. Several new practical ideas that will need future confirmation are proposed: e.g. application of loading-unloading cycles of torsion treatment in combination with tensile or pre-bending treatments might be useful.

Sr 2 FeMoO 6 was synthesizes by soft chemistry route using citric and/or oxalic acid as complexing agents. This method provides good compositional control with promising results as a base for further improvements. The material shows complex temperature- and field- (magnitude and orientation) dependent electro-magnetic behavior as probed by magnetic and transport measurements. The zero-field cooling curves of magnetization versus temperature taken at different magnetic fields, show two temperature regions separated by a transition temperature T cups ZFC . When H increases, T cups ZFC values decrease. At 5 K and 5 T the saturation moment is 3.55 μ B per formula unit, and the coercitive force is 0.021 T. The relative value of the magnetoresistive effect (i.e. 100 ( ρ H − ρ 0 )/ ρ H , %, with ρ being resistivity) at 5 K is 68% and 36% when applied field H is parallel and perpendicular the measuring current I , respectively. The results are discussed in relation to magnetic sublattices and the interaction that produces anti-site defects and spin-dependent electron anisotropy.

First-order branched tree structures of MoO3 belts on sillimanite fibers (refractor wool) were grown by a vapor transport method. At longer growth times, MoO3 micro-belts on micro-belts form a flower-like structure. Most belts are of α-MoO3, and they grow in the two opposite directions along the c-axis. They show a high level of crystal quality, clean surfaces, sharp edges, and a triangular-shape tip with certain angles. The most frequent are angles of 47° and 94°. The growth mechanism is of vapor−solid type. In the initial stages of growth, elements specific for spontaneous spread and island formation on the surface of the sillimanite fibers described in the literature are observed. Significant enhancement of the thickness suggests that a layer-by-layer 2D growth is also probable. It is thought that responsible for such growth is direct vapor deposition of the Mo−O vapors on the already formed belts, this being similar to thin film growth. Curved surfaces of the substrate (in our case of the sillimanite ...

For the fabrication of new devices taking advantage of interface phenomena, growth of thin films with different orientations is necessary. Epitaxial thin films of c-axis type (001) Bi-2201, (001) Bi-2212, and (001) Bi-2223 and of non-c-axis type (115) Bi-2201, (117) Bi-2212, and (119) Bi-2223 were grown by spin coating and subsequent annealing on single-crystal substrates of SrTiO3, MgO, or LaAlO3 with (001) and (110) orientations, respectively. We show that lattice matching relationship between the substrate and the film is the key condition for the orientation control in Bi–Sr–Ca–Cu–O thin films obtained by this route. This is similar to Bi–Sr–Ca–Cu–O films grown by vapor deposition methods. Therefore, it is concluded that the growth method is not essential for the orientation control of the film. However, for the epitaxial growth it is necessary to ensure a high mobility of the atomic species at the substrate-film interface and for the spin-coated films this criterion suggests the presence of a liquid at the interface during annealing. Our films have shown specific structural features and occurrence of impurity phases or orientations. Further research is required.

We review the current status of the growth, characterization and applications of Bi-Sr-Ca-Cu-Ο (BSCCO) whiskers from the materials science point of view. Our analysis leads to the conclusion that reconsideration of the importance of the whisker studies is necessary; whiskers can play a major role in studies of the growth mechanism and defects, properties control and their understanding, and new devices and applications. Arguments favourable to the idea that BSCCO whiskers can possibly have a significant impact on future synthesis of nano-objects in this system or other high-T c superconducting systems, as well as in investigation and understanding of the growth processes under elevated magnetic fields, are discussed. Emphasis is made on superconducting whiskers, but non-superconducting ones are also briefly addressed.

We report successful growth of high-quality thin films with clean and completely precipitate-free surface, suitable for device applications, by applying a new concept and method to the substrates. The concept consists in generation of artificial steps of controlled height and width, and desired shape on the surface of the substrate. The width of the step is chosen so that it is equal to the double of the migration (surface diffusion) length of the atomic species in the growth process of the film. If precipitates occur, they will be selectively gathered to the step edge where the free energy is lowest. Using this new method, we have successfully obtained by MOCVD high-quality precipitate-free Bi-2223 and Bi-2223/Bi-2212-superlattice thin films on (100) SrTiO3 substrates with artificial steps of controlled width and height. These as-grown films have been further used to fabricate patterned intrinsic Josephson junctions. Completely precipitate-free films offer a strong advantage for integration, and generate new possibilities for the device fabrication.

High quality superconducting thin films of HTS have been grown by MOCVD on substrates with artificial steps of predefined height and width. The surface of the films grown on the steps having width equal to the 'double of the migration length' of the atomic species depositing on the substrate is totally free of precipitates: precipitates are gathered at the step edges where the free energy is lowest. The method has several advantages: it is simple, universal (it is independent of the materials, substrates, deposition technique or application) and allows control of precipitates segregates so that the quality and growth conditions of the films are the same as for the films grown on conventional substrates. The method is expected to result in new opportunities for the device fabrication, design and performance.

The critical current, upper critical field and critical temperature of bronze route Nb3Sn commercial wires are enhanced by applying a repeated bending strain at room temperature, i.e., "prebending strain". In order to investigate the prebending effects from a viewpoint of a residual strain, axial and lateral residual strains were evaluated directly by neutron diffraction at room temperature. We found that the axial residual strain changes from -0.10% to 0.02 % but the lateral one is unchanged by applying a prebending strain of 0.5% for an ordinary bronze route (Nb,Ti)3Sn wires without reinforcement. Hence, in the case of the ordinary Nb3Sn wires without reinforcement, the prebending treatment modifies only the axial residual strain states independently to the lateral one, although it may depend on the wire structure. The critical current properties under the axial tensile strain suggest that the axial residual strain is reduced by about 0.11% but the radial residual strain unchanged by the prebending treatment of 0.5%. This is consistent with the results of the neutron diffraction

In this paper we focus on supersaturation and migration of the atomic species involved in the growth of oxide films in order to search for new efficient methods for growth of flat, uniform and precipitate-free complex thin films with high quality. In this respect we have applied MOCVD to HTS thin films, 'interrupted growth' (consisting of on/off vapour deposition cycles, keeping the temperature and atmospheric conditions constant), similar to 'pulsed laser interval deposition' reported in the literature. We show that MOCVD can be easily and successfully applied to suppress island growth, contrary to the currently accepted idea that this approach is unique to PLD. This suggests that principles of supersaturation–migration–supersaturation cycles are universal for the films' growth. By this approach, high-quality Bi-2223 thin films with a uniform flat surface, having roughness less than half the c-axis unit cell, were grown by MOCVD. Using substrates with artificial steps of predefined width equal to 'twice the migration length' totally precipitate-free thin films were obtained: precipitates–segregates of impurity phases (Cu rich for HTS materials) are gathered at the step edges where the free energy is lowest. As-prepared films were used to fabricate intrinsic Bi-2223 and Bi-2212/Bi-2223 SIS-type Josephson junctions (IJJs).

Six Nb3Sn composite wires with different architectures ('central and near-the-edge reinforcement') were repeatedly in-plane bent at room temperature (in-plane 'pre-bending'). Breakage behaviour was revealed from scanning electron microscopy observations by semi-quantitative analysis of the filament crack formation and evolution. Cracks are formed in the transversal and longitudinal directions. Transversal cracks show some tolerance to the applied bending strain due to the fact that filaments are composite materials; residual Nb?core can arrest development of a partial transversal crack initiated in the Nb3Sn outer part of the filament. Together with the density of cracks C and the evolution of this parameter with pre-bending strain, ?pb, in different regions of the wire, R??pb curves are important to understand breakage behaviour of the wires. R is the ratio (number of full transversal cracks)/(number of full transversal cracks?+?number of partial transversal cracks). Parameters?C and R allow us to reveal and satisfactorily understand the wire architecture?breakage?critical current decay relationship when pre-bending treatment is applied. As a consequence, breakage criteria necessary to minimize Ic?decay were defined and the positive influence of the reinforcement in preventing breakage was observed. It was also found that, in this regard, more Nb in the CuNb?reinforcement, for the investigated wires, is better, if the heat treatment for the wire synthesis is performed at 670??C for 96?h. A different heat treatment, 650??C for 240?h, is less efficient in preventing filament breakage. Our results suggest the possibility of control and improvement of the breakage susceptibility of the filaments in the wires and, hence, of the bending Ic?decay, through the wise design of the wire architecture (i.e.?by correlating design with the choice of composing materials and heat treatments).

Thin films of MgB2 on r-cut Al2O3 substrates have been grown by pulsed-laser deposition (PLD) using a Nd–YAG laser (fourth harmonic—266 nm) instead of the popular KrF excimer laser. The growth window to obtain superconducting films is laser energy 350–450 mJ and vacuum pressure with Ar-buffer gas of 1–8/10 Pa (initial background vacuum 0.5–1 × 10−3 Pa). Films were deposited at room temperature and post-annealed in situ and ex situ at temperatures of 500–780 °C and up to 1 h. Films are randomly oriented with maximum critical temperature (offset of resistive transition) of 27 K. SEM/TEM/EDS investigations show that they are mainly composed of small sphere-like particles (≤20 nm), and contain oxygen and some carbon, uniformly distributed in the flat matrix, but the amount of Mg and/or oxygen is higher in the aggregates-droplets (100–1000 nm) observed on the surface of the film's matrix. Some aspects of the processing control and dependences on film characteristics are discussed. The technique is promising for future development of coated conductors.

Growth of such non-superconducting whiskers [Sr2.24Ca0.4Al2Ox, (Ca0.8–0.85Sr0.15–0.1)2CuO3,CuO, or Bi2.44Sr2Ca1.3–2Cu6.9–9.95Al0.35–0.46Ox (Cu-rich whiskers)] formed during the growth of Bi-2212 superconducting whiskers from powder or glassy substrates, is discussed. These whiskers are likely to grow from the bottom end, and there is a tight relationship with the growth of the Bi-2212 whiskers. A general reaction-path model for the whisker growth in the BSCCO system, independent of the type of the catalytic impurity and substrate, is proposed. When whiskers are grown under magnetic fields, up to 10 T, changes in the whisker size, aspect ratio, and morphology are observed.

Fe-sheathed tapes of MgB 2 with addition of SiC have been prepared by the powder-in-tube (PIT) method using a powder mixture mechanically milled in an argon or hydrogen atmosphere. The reactivity of the component powders in the mixtures, Mg, B and SiC, was significantly enhanced by mechanical milling, so that MgB 2 formed faster than in tapes fabricated from a hand-milled mixture. At the same time the number of impurity phases was higher, and the impurity phases of Mg 2 Si and Fe 2 B were observed only in these tapes. Mechanical milling also induced a lower quality of MgB 2 , especially for tapes produced from powders milled in a hydrogen atmosphere, with a significant influence on the values of the critical temperature T C and critical field, J C .

Recently we have found superconductivity in a cubic antiperovskite-like compound Li 2 BPd 3 . A new pseudo-binary complete solid solution Li 2 B(Pd 1- x Pt x ) 3 , x =0–1 with similar structure has been synthesized and observation of superconductivity in the entire x -range is reported. Our results strongly suggest that superconductivity is of bulk type. Critical temperature T c is decreasing approximately linearly with amount ( x ) of Pt from 7.2–8 K for Li 2 BPd 3 to 2.2–2.8 K for Li 2 BPt 3 . From isothermal magnetization ( M – H ) measurements, lower critical fields H c1 (138 Oe/ x =0 , 38 Oe/ x =1 ), upper critical fields H c2 WHH (3.4 T/ x =0 , 1 T/ x =1 ), coherence length ξ(0) (9.8 nm/ x =0 , 17.9 nm/ x =1 ) and penetration depth λ(0) (190 nm/ x =0 , 364 nm x =1 ) were estimated and shown to follow approximately linear dependencies with x , either. Structure and superconducting similarities with MgCNi 3 , viewed as a bridge between low and high T c superconductors are increasing the expectations t...

In this paper, we report on the effects of using LiH as a starting material or and hydrogen atmosphere on the synthesis processes and electrical resistivities of LiBC compound. We observed that, in argon atmosphere, the use of LiH was effective in synthesizing the LiBC-samples of high phase purity. In this case, the obtained LiBC phase is non-stoichiometric, Li-deficient. On the other side, samples prepared from LiH as starting material, under different hydrogen partial pressures (PH2=0.01, 0.10 and 1.00 MPa), have shown a different behavior. Namely, sample prepared under PH2=1.00 MPa has significantly lower concentration of impurity phases and the c-axis lattice parameter of LiBC is as of the stoichiometric phase (without Li-deficiency) that is usually observed and reported in the literature. The other samples prepared under lower hydrogen pressure (PH2=0.01, 0.10) have an appreciable amount of impurity phases and Li-deficiency probably caused by the Li evaporation. Electrical resistance vs. temperature was measured for two almost single-phase LiBC samples with Li-deficiency and without Li-deficiency and it is about one order of magnitude lower for the sample with Li-deficiency, due to hole-doping effect. Both samples show semiconducting behavior and do not exhibit superconductivity down to 2.0 K, although superconductivity is predicted theoretically for the Li-deficient LiBC compound.

Superconducting whiskers of Bi-2212 have been grown by an airtight crucible method in elevated magnetic fields, up to 10 T. Growth behavior with and without application of a magnetic field, as well as the SEM detailed study of the growth defects of the whiskers allow to emphasize some aspects of the growth mechanism. The conclusion is that whiskers are growing mainly through continuous crystallization at the base-end and from the so-called ‘micro-crucible’ formed by the glassy or powdered substrates. The supply of cations by a vapor transfer is very probable, but is taking place to the micro-crucible reaction site rather than to the droplet, as in the classic vapor–liquid–solid (VLS) growth mechanism. Therefore, whiskers grow by a modified VLS–microcrucible mechanism (MVLS–MC). Application of magnetic fields during growth has proven to be a powerful tool to investigate and influence some processes. Observation of stoichiometric Bi(Pb)-2212, CuO and Ca 2 CuO 3 whiskers is also reported.

Bi2Sr2CaCu2Ox whiskers have been grown by a modified airtight two-crucible method, employing radial and vertical thermal gradients resulting from the relative position of the crucibles. A heating profile with a constant heating rate applied from 842 to 846 °C allows formation of whiskers up to 2 cm in length for a certain level of Al impurity in the glass. A cation-rich atmosphere is found to be a key intrinsic parameter. Our data confirm the importance of the thermal gradients, but this parameter is a somewhat extrinsic one and from a practical point of view should be correlated to other intrinsic parameters that influence growth atmosphere (i.e. to total and partial pressures). As-grown whiskers have composition Bi:Sr:Ca:Cu = (2.9–3.5):2:(1.7–2.05):(2.4–3.0) (normalized to Sr2), zero-resistance critical temperature Tc (R = 0) = 76–98 K and maximum transport critical current density above Jc = 104 A cm−2 at 77 K in self-field.

We report the in-plane resistivity and magnetic susceptibility of the layered cobalt oxide Na$_{0.35}$CoO$_{2}{\cdot}1.3$H$_{2}$O single crystal. The temperature dependence of the resistivity shows metallic behavior from room temperature to the superconducting transition temperature $T_{c}$ of 4.5 K. Sharp resistive transition, zero resistivity and almost perfect superconducting volume fraction below $T_{c}$ indicate the good quality and the bulk superconductivity of the single crystal. The upper critical field $H_{c2}$ and the coherence length $\xi$ are obtained from the resistive transitions in magnetic field parallel to the c-axis and the $ab$-plane. The anisotropy of $\xi$, $\xi_{ab} / \xi_{c} =$ 12 nm/1.3 nm $\simeq$ 9.2, suggests that this material is considered to be an anisotropic three dimensional superconductor. In the field parallel to the $ab$-plane, $H_{c2}$ seems to be suppressed to the value of Pauli paramagnetic limit. It may indicate the spin singlet superconductivity in the cobalt oxide., Comment: 4 pages, 4 figures

AC susceptibility measurements have been performed on Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox (Bi-2223) samples doped with different Li-based compounds and prepared by the solid-state method. As-prepared samples and/or samples annealed in oxygen or argon, ground or unground, have been investigated in detail in order to understand the occurrence, nature and evolution of the anomalous peaks observed in curves versus the measuring parameters; we have detected up to four peaks instead of the usual two peaks observed in the non-doped samples. It was found that intrinsic physical–chemical properties, such as the melting temperature of the doping compound, are no less important for the final properties of the superconductor than their insertion properties into the crystal lattice of Bi-2223. Doping compounds with melting temperatures below or close to the phase formation temperature of the Bi-2223 phase can act as flux, changing the growth conditions. Intensification of some processes against others (e.g. decomposition–recovery of the Bi-2223 phase, solubilization–precipitation of the secondary phases, changes in the properties of the liquid phase, etc) can lead to the formation of the Bi-2223 phase with different properties than for the non-doped superconductor. Generally, when using flux-type doping compounds, anomalous AC losses peaks are detected and the intra-granular critical current density is enhanced.

(Cu,C)Ba2Ca3Cu4Ox superconductor with addition of y mol LiF has been synthesized by a high-pressure method. For the same synthesis conditions it was found that (almost) single-phase Cu, C-1234 samples can be synthesized for yLiF = 0–0.1 if the amount of z mol AgO oxidizer is increased linearly from zAgO = 0.45 to 0.73 and for yLiF = 0.1–0.2 if zAgO = 0.73 = constant. Transport measurements (ρ(T) and room-temperature Seebeck coefficient) have shown that these samples are overdoped: LiF is an effective addition for synthesis of overdoped Cu, C-1234 with a controlled level of carriers. LiF addition continuously decreases Tc. The critical point at yLiF = 0.1 is discussed as the solubility limit of LiF and/or the point where the doping mechanism changes. It is proposed that the reason is the reaction of extra Li with C and O to form Li2CO3, inducing a lower concentration of C in Cu, C-1234/LiF crystals, and at the same time a possible substitution of Li not only for the Cu site but also for the Ca site, resulting in formation of a higher amount of residual Ca0.828CuO2 (for yLiF>0.1). LiF induces the formation of a liquid phase and acts as a flux promoting the formation of Cu,C-12 (n−1)n with . LiF modifies to some degree the grain growth from a 3D to a 2D type (thinner platelike grains have been observed in the LiF added samples).

MgB2 is an intermetallic compound and is a promising superconductor for practical applications due to its high TC value of 39 K. The production cost of MgB2 wires is expected to be low compared to the conventional superconducting wires, since it is composed of cheap and available elements. However, the critical current density JC in applied magnetic fields is not yet at practical level and, hence, the improvement of high field performance remains one of the most important issue to be solved urgently.Recent advances are showing that the addition of SiC is effective to improve the high field JC, while the use of MgH2 instead of Mg as raw material is enhancing MgB2 crystal growth. In both situations the mechanisms of these effects are still unclear.This paper reports the synthesis of MgB2 superconductor in high-pressure hydrogen-atmosphere, which is an extreme reducing atmosphere. The results are compared with those for inert atmosphere i.e. argon-atmosphere, in order to provide useful information for wire processing.Samples were prepared by reacting the mixture of Mg (or MgH2), B and nano-SiC powders in a high-pressure (up to 3.5 MPa) steel vessel, at temperatures below 900°C. Samples that are fabricated in the pressurized hydrogen atmosphere show higher TC, bigger a -axis lattice parameter and larger grain size than for the samples synthesized in Ar-atmosphere, in the same conditions. It turns out that the pressurized hydrogen atmosphere is useful to produce MgB2 with good crystallinity. Nevertheless, the JC in magnetic field is higher for the samples synthesized in argon atmosphere and it is considered that the lattice distortion in a -plane, smaller grain size and fine precipitates, such as MgO, are responsible for the stronger flux pinning. SiC addition is effective regardless the atmosphere and the highest JC was obtained for the sample starting from MgH2+2B+SiC powder mixture heat treated in argon atmosphere.

Precursor powders of Ba/Zr=1:1 chloride have been prepared by freeze drying (FD) and conventional mixing. A simple, laboratory-made apparatus for sublimation of the frozen water from the cryo-particles is presented. Both powders have the same phase formation behavior. Up to 500–600 °C, Zr chloride transforms into ZrO 2 . At high temperatures, BaCl 2 starts melting and decomposing and two subsequent processes take place: (a) chlorination of the Zr atoms from the oxide and (b) formation of BaZrO 3 phase. These processes depend on treatment atmosphere and reactivity of the starting powders. Freeze-dried powders with higher reactivity and/or O 2 atmosphere are suitable to obtain high content of BaZrO 3 phase. In the freeze-dried powder, decomposed in O 2 atmosphere at 1190 °C for 20 h, 90% of BaZrO 3 phase content was attained. No new phases were observed during decomposition of the freeze-dried powder. In the as-synthesized freeze-dried powder, some XRD lines remained unidentified.

From ac susceptibility measurements of the [Bi(Pb)]2Ca2Sr2Cu3Oz (Bi-2223) samples doped with LiCl, we have found and report for the first time anomalously suppressed superconductivity (Tc is anomalously decreasing) in the slightly under-doped region. For our BPSCCO/(LiCl)y samples this region is situated around y=0.07. Hole concentration (p) per number of Cu–O planes of the Bi-2223 unit cell, determined from room temperature thermopower (S 300 K ) measurements, is approximately 0.15 when y is within anomalous region. Literature data shows that for the La-based cuprates, similar suppression of superconductivity was observed at p∼0.12 (so-called “1/8 problem”). This discrepancy would appear because charge ordering phenomenon (probably induced by Li known as a pair breaker as e.g. Zn) may be accompanied by changes in oxygen content or oxygen ordering effects.

By very short time rf sputtering in certain deposition conditions we have grown three-dimensional Ag nanodots on the substrate prior to the growth of Tl-based superconducting films. These nanodots create pinning centers, leading to an increase in the critical current density about 10 times. From the frequency dependence of the critical current density we estimated also the activation energy for the flux jumps, which resulted to be several times higher. The rate of the thermally-activated flux jumps decreases several orders of magnitude. We suggest that our method can be used for the reduction of thermal noise in high-T/sub c/ dc SQUID's.

In high temperature superconductors (HTS) the coherence length along non-c-axis directions is longer. This feature can be useful when designing electronics devices based on HTS. Therefore, growth and characterization of non-c-axis oriented thin HTS films is of great interest. In this paper we present our comparative results on preparation by MOCVD and characterization by atomic force microscopy, X-ray diffraction and transport measurements of as-grown (1 1 9) Bi-2223 and (1 0 3) Y-123 thin films on (1 1 0) SrTiO3. The films have shown the same type of surface morphology (grains with mountain-range shape, aligned in parallel chains to the [0 0 1] substrate’s azimuth). This morphology is the result of the orientation relationship between the (1 1 9) Bi-2223 or (1 0 3) Y-123 and (1 1 0) SrTiO3. The transport measurements with temperature revealed strong anisotropy of the films in the normal state region when the measuring current has been applied in-plane, parallel and perpendicular to the edges (from the basal plane) of the mountain-range grains. Higher values for Tc0 have been obtained for the Y-123 films (42 K) than for the Bi-2223 ones (31 K). The both films are as-grown and non-oxygenated ones.

Superconducting (0 0 l) thin films of SmBa2Cu3Oy have been grown by MOCVD on (0 0 1) SrTiO3 and (0 0 1) LaAlO3 substrates. The films on both substrates are smooth with RMS of 1.428 and 1.037 nm respectively, but have different morphology (revealed by atomic force microscopy) for the same synthesis conditions. The films grown on SrTiO3 have shown rectangular- or square-type terraces. The height between the neighboring terraces was corresponding to the c-axis length of one unit cell. This type of morphology is due to the two-dimensional nucleation and growth mechanism. In the case of the films on the LaAlO3 the surface morphology consisted of irregular grains (close to spherical). For both situations we have not found any evidence for a spiral growth mechanism. Therefore, our films are suitable for electronics applications of integrate type and not only. The as-grown films were oxygen deficient and due to this exhibited rather poor superconducting properties. Oxygen annealing at temperatures of 500–600 °C increased Tc(R=0) up to 87.8 K.

Thin films of (Ca 1− x Ba x )CuO 2 ( x =0–1) have been grown on (0 0 1) SrTiO 3 by metal organic chemical vapor deposition. These films are potential candidates as barrier layer for SIS and/or SNS Josephson junctions. For this purpose, the films should be flat and smooth. In this regard we have investigated the possibility of decreasing the films roughness by the control of the Ba/Ca ratio, the substrate temperature and oxygen partial pressure during the growth of the film. The minimum mean square root roughness and regular morphology are obtained for the x =0.3–0.5. In the samples with x =0.5 the highest intensity of the X-ray diffraction peaks for the (Ca 0.5 Ba 0.5 )CuO 2 phase and the lowest for the impurity phases were attained at a substrate temperature of 750–780 °C and oxygen partial pressure of 15 Torr.

TlBa2Ca2Cu3Oy (Tl-1223) superconducting films were prepared under identical conditions, on MgO substrates with different morphologies resulting from heat treatments at temperatures between 600 and 1350 °C. The superconducting films have almost the same morphology, in-plane alignment and composition, but different critical current densities Jc. Critical current density Jc determined at 77.3 K differs for films with various annealed MgO substrates, by a factor of 5 in zero field and 10 in 1 T. The behavior of Jc is discussed in relation with the flatness of the MgO surface, and with pinning effects induced by the Ca-segregates. For the present work, the best films were obtained for the un-annealed (as-received) substrates and for substrates treated at 1350 °C. In these samples, the substrate’s flatness and morphology with regular steps are essential for high quality of the superconducting films. In the films grown on MgO annealed between 800 and 1200 °C and showing rough surface and Ca-segregates, Jc was lower. However, Jc was increasing with heat treatment temperature of the substrate, possibly due to the Ca-segregates inducing or acting as pinning centers. The effects of Ca-segregates are of interest for maximization of the quality of the high-temperature superconducting films. In the literature, up to now, these effects have not been considered and therefore Ca-segregates removal has been recommended.

Ag2O-doped (1.2% wt.) nitrate freeze-dried powders (Bi : Pb : Sr : Ca : Cu = 1.7 : 0.3 : 2 : 2.5 : 3.5) were processed under an external electrical field and 17.5 MPa pressure at 800°C, for 4 min in vacuum. Final heat treatments (HT) were applied at 835–850°C for 70 h. in air (Bi, Pb)2Sr2CaCu2O x (2212-phase) was formed by electrical field processing in just 4 min. Electrical field application enhanced (Bi, Pb)2Sr2Ca2Cu3O y (2223-phase) formation during final HT. Ag2O additions to field sintered BSCCO ceramics increased the amount of 2223-phase and the zero resistance critical temperature (Te(R=0)) by ∼ 4 k.

Application of active technology factors (for example, the introduction of Bi 2 Sr 2 Ca 2 Cu 3 O y , (2223) phase additive) and corresponding optimization of synthesis parameters of Bi(Pb)–Sr–Ca–Cu–O (BSCCO) ceramics have allowed reducing the time of oxide precursor sintering from 250 to 25 h. Conditions of positive influence of 2223 phase additives have been investigated. Intensification of BSCCO ceramics synthesis was the basic purpose, but the authors have deliberately conducted broader examinations of the influence of technological parameters on the properties and structure of this material. The complex of the obtained effects (similarly to the solution of an equation set with several unknowns) helps increasing the probability of exact comprehension of the system: technology––processes occurring under ceramics synthesis––phase composition and structure––properties of BSCCO.

Thin films of Cu m −1 Ba m (Sr,Ca) n Cu n +1 O 2 m +2 n +1 , i.e., [BaCuO 2 ] m /[(Sr,Ca)CuO 2 ] n superlattices have been grown by layer-by-layer growth method with rf magnetron sputtering on NdGaO 3 (1 1 0) substrate. Two targets with the compositions BaCuO 2 and (Sr,Ca)CuO 2 , which form the charge reservoir and superconducting layers, respectively, were used. [BaCuO 2 ] m /[(Sr,Ca)CuO 2 ] n superlattice was grown at 460–520 °C in a gas mixture of Ar and N 2 O by alternatively depositing the charge reservoir layer and superconducting layer. The superlattice modulation is close to that of high pressure synthesized bulk materials. By means of the layer-by-layer growth method, a system of Cu-based artificial lattice structures is formed. Electrical transport properties of these superlattices were also investigated.

Superconducting samples have been synthesized from starting compositions Cu 0.5 Tl x Ba 2 Ca 3 Cu 4 O 12+ y , x Tl =0.35–1 in the sealed Au-tubes. Starting oxygen content was y =0 and/or y =±0.225. Samples contain non-superconducting phases and only one main superconducting phase, 1234. The energy dispersive spectroscopy grain composition was Tl 0.87–1.3 Ba 1.9–2.05 Ca 2.55–3.1 Cu 4 O x (normalized to Cu=4). The starting Tl and oxygen content do not influence the occurrence of the phases, grain size, morphology and composition. On the other hand the phase content strongly depends on Tl content and almost does not depend on oxygen content. Both Tl and oxygen content are important for the transport properties of the superconducting grains and it seems that also of the non-superconducting matrix. T c of the samples was in the range 119.1–111.3 K.

We report the growth of TlBa 2 Ca 2 Cu 3 O y superconducting thin film on CeO 2 buffered r-cut sapphire substrate by amorphous phase epitaxy method, wherein an amorphous phase of a composition, TlBa 2 Ca 3 Cu 4 O y was deposited by rf magnetron sputtering and subsequently crystallized by annealing at high temperatures in a closed system. There exists an optimum thickness of about 200 A of CeO 2 buffer layer that gives a smooth buffer layer surface. At the temperature around 840 °C we get only Tl-2212 phase and for a longer annealing time it results in Tl-1212 phase. At higher temperatures Ba reacts with Ce to form BaCeO 3 . To avoid this reaction process we introduced another amorphous buffer layer, TlSr 2 CaCu 2 O y onto CeO 2 layer. By this method we could prepare a good quality Tl-1223 thin film which showed a T c of 104 K and a J c = 0.3 MA/cm 2 at 77 K and 0.1 T field.

Orientation control of high T c superconductors is essential for superior device performance, because the coherence length is longer along the non- c -axis directions than along c -axis direction. In this study, we report on successful preparation of (1 1 9) oriented Bi 2 Sr 2 Ca 2 Cu 3 O 10 (Bi-2223) films by metalorganic chemical vapor deposition using (1 1 0) SrTiO 3 substrates. Atomic force microscopy observations of the (1 1 9) Bi-2223 films are reported for the first time. Our films have an in-plane aligned mountain-range-shaped surface morphology. This morphology is resulting from the epitaxial relationship between the (1 1 9) Bi-2223 film and (1 1 0) substrate. The obtained films exhibited a large in-plane anisotropy as revealed by resistivity measurements and surface morphology observations.

The cuprates seem to exhibit statistics, dimensionality and phase transitions in novel ways. The nature of excitations [i.e. quasiparticle or collective], spin-charge separation, stripes [static and dynamics], inhomogeneities, psuedogap, effect of impurity dopings [e.g. Zn, Ni] and any other phenomenon in these materials must be consistently understood. Zn-doped LSCO single crystal were grown by TSFZ technique. Temperature dependent Polarized XANES [near edge local structure] spectra were measured at the BL13-B1 [Photon Factory] in the Flourescence mode from 10 K to 300 K. Since both stripes and nonmagnetic Zn impurities substituted for Cu give rise to inhomogeneous charge and spin distribution it is interesting to understand the interplay of Zn impurities and stripes. To understand these points we have used Zn-doping and some of the results obtained are as follows: The spectra show a strong dependence with respect to the polarization angle, $\theta$, as is evident at any temperature by comparing the spectra where the electric field vector is parallel with ab-plane to the one where it is parallel to the c-axis. By using the XANES [temperature] difference spectra we have determined T* [experimentally we find, T* $\approx$ 160-170 K] for this sample. The XANES difference spectra shows that the changes in XANES features are larger in the ab-plane than the c-axis, this trend is expected since zinc is doped in the ab-plane at the copper site. Our study also complements the results in literature namely that zinc doping does not affect the c-axis transport., Comment: To appear in Physica C [ISS2001 Special Issue], related talk presented at ISS2001 as PC-16, 10 pages revtex and 7 pages of figures (pdf)

The preparation of non c-axis oriented high Tc superconductor films with longer coherence length is one of the key technologies toward realization of viable Josephson devices. For the first time, we have succeeded in growing in-plane aligned (1 1 9)Bi-2223 films on (1 1 0)SrTiO3 substrates by metalorganic chemical vapor deposition using a two-step growth temperature procedure. This procedure was found to be effective for orientation control and phase control in comparison with a fixed growth temperature procedure. As a result, superconducting properties were improved. The surface morphology observed by atomic force microscopy showed an in-plane aligned mountain-range shape, which strongly reflects the orientation relationship between the (1 1 9)Bi-2212 film and (1 1 0)SrTiO3 substrate.

A slice of the (Cu,C)Ba2Ca3Cu4O12−y bulk superconductor grown by high-pressure synthesis was polished down to a thickness of 50 μm, and then irradiated with high-energy (240 MeV) Au15+ ions (1011 cm−2). Since the length of the ion tracks was estimated to be 12.2 μm, our sample consists of two different types of grain, with and without columnar defects, having very different critical current densities. Using the third-harmonic response of the sample, we were able to separate the small dissipation in the irradiated grains from the much larger dissipation in the non-irradiated grains, in a certain experimental range of temperature, field and current density. The critical current density was estimated from critical state models. For temperatures in the range of 85–105 K, we observed a peak effect at high dc magnetic fields, suggesting the possibility of a record irreversibility field at 77.3 K.

Temperature and time dependence of the phase formation in the (Cu0.5,Tl0.5)-1234 composition have been studied. The liquid–vapour deposition mechanism for the formation of 12(n − 1)n superconducting phases is presented. XRD, SEM, EDS and weight loss data are discussed. Samples were superconducting with maximum Tc and Jc (at 60 K and 0.5 T), determined from M(H) loops of 118 K and 4 × 105 A cm−2, respectively.

The effects of residual carbon in precursors on phase formation of single Tl–O layer superconductors of TlBa 2 Ca n −1 Cu n O y ( n =3 and 4) have been investigated. The samples were prepared from Tl 2 O 3 and precursors of Ba 2 Ca n −1 Cu n O 2 n +1+ δ (C x ) ( n =3 and 4) with various residual carbon contents ( x =0.02–0.19). Even a small amount of carbon promotes the formation of double Tl–O layer phases of Tl 2 Ba 2 Ca n −1 Cu n O y ( n =3 and 4). Using the carbon reduced precursor ( x =0.02), a TlBa 2 Ca 2 Cu 3 O y phase is synthesized directly in a silver-sheathed tape without such elements as Pb, Bi and Sr.