Directional solidification of the zone melting method was performed in order to clarify the growth mechanism on YBa2Cu3Ox (123) superconductors from the partial molten state. The effect of a platinum addition for crystal growth was also investigated. Results indicate that a higher G/R ratio was preferable for the continuous growth of 123 crystals, and lead to the idea that the necessary solute for the peritectic reaction is provided from dispersed Y2BaCuO5 (211) particles in liquid to 123 interface through the liquid. Based on this idea, we developed a solidification model which is in good agreement with the experimental results. In the case of a platinum addition, the coarsening rate of 211 particles decreased, and the cell spacing of 123 crystals also decreased. It is tentatively suggested that the platinum addition decreases the products of the diffusivity in liquid and the Gibbs-Thomson coefficients, which include the surface energy between 211 or 123 phase and liquid,(DLΓ211 and DLΓ123).

A high-Tc phase formation-decomposition-recovery process(PFDR) through the use of a short period of melting has been developed for fabrication of Ag-clad Bi-Pb-Sr-Ca-Cu-O (BPSCCO) wires. The weak links and flux pinning have been significantly improved and an extended plateau regime in the Jc-H curve has been observed. The PFDR processed tapes exhibit 3 to 10-fold increase in the Jc at 77 K and 1 Tesla over the normally processed tapes. A Jc of 40000 A/cm2 at 77 K and 0 T and 9000 A/cm2 at 77 K and 1 T have been achieved. The Jc correlates well with the degree of grain alignment which can be measured through the Jc dependence on the orientation of the applied field. The improved Jc - H characteristics is attributed to the desirable microstructures consisting of well aligned (Bi,Pb)2Sr2Ca2Cu3O10+y (2223) grains, fraction of (Bi,Pb)2Sr2CaCu2O8+y(2212) and high density of dislocations produced through PFDR process. The superconducting Ag/Bi-Pb-Sr-Ca-Cu-O wires can be made flexible. The Jc remains unchanged when wires of superconductor thickness 25 μm are bent to a strain rate of 1.5%

Magnetization curves for low external magnetic fields of field-cooled superconducting YBa2Cu3O7 samples show an increase in the diamagnetic signal when the sample temperature is near the transition to the normal state. The Increase of the diamagnetic signal depends on the external magnetic field (H ≤ 13.5 Oe). The effect is observed when the samples are field-cooled, but not when they are zero-field-cooled, that means that it is related with the behavior of the vortex lattice.

The charge transfer in YBa2CU3O7-y system have been analysed by bond valence sum calculation. It is found that there exists an additional effective charge-transfer between the CuO2 planes and the CuO chain which varies nonlinearly with the oxygen stoichiometry and can be expressed as q = 0.151 exp(∼0.007/y4) + 0.35 exp(0.5 – 0.5/y4). This nonlinear charge-transfer is associated with the structural transitions of orthorhombic (I)-to-orthorhombic(II) and orthorhombic(II)-to-tetragonal symmetry, and thus related to the oxygen ordering in the CuO chains. A new relationship between the mobile hole number P(2) the localised hole number P(l) and the total hole number P has been established as P(2) = P - P(1), P(1) = 0.5 - y - q. The results also show that the 90 K and 60 K plateaus in Tc as a function of oxygen stoichiometry correspond to the plateaus in the curve of q versus y, indicating that both the nonlinear variations of Tc and the charge-transfer with the oxygen stoichiometry are intrinsically correlated.

Presently, the rf surface impedance Z of cuprate superconductors is still shrinking with material improvements, which is shown clearly by Z = Z1+Zres still dominated by extrinsic properties summarized in Zres. We present evidence that Zres is due to the large leakage current jbi and the small critical current jcj of weak links. The latter destroys the intrinsic shielding from a λ1 -thin seam λJ deep into the bulk. This causes rf residual losses Rres ≈ (ΩμO)2λJ3 σb1/2. Rres stays finite at T≃O by σb1(T→O)≈σb1(αjb1) being amplified by (λj/λ1>103 as a weighting factor. An appropriate measure of weak links is the grain boundary resistance Rbn(∝ρ(O)) enhancing λj ∝ Rbn and Rres ∝ Rbn2. Thus, Zres is minimal for minimal extrapolated normal conducting resistivity ρ(T→O).

To identify the weak links as new entity the H-field dependence is most helpful, because at very low fields Hc1J∝ 1/λJ Josephson fluxons penetrate into the weak links. These Josephson fluxons show negligible flux flow or flux creep, and enhance Zres by λJ(H, T) ∝ l/√Jjc (H, T). The measured JcJ (H, T) - and Jbl - values explain Zres quantitatively as well as in temperature ∝ ( a+ Tn) (n ≈ 1, T<Tc/2) and in field ∝ (b + Hn) (n≈1, H>Hc1J) dependence being thus a unique method to obtain the different Hc1-values. The strength of the field dependence d Zres/dH ∝ Zres (Hc1J*)/Hc2J(T) is not only a measure of Zres and HC2j(T) but is crucial for nonlinear effects and (fluxon) noise also, which limits the performance of rf devices.

Nb/MgO/Bi2Sr2CaCu2Ox (BSCCO) tunnel junctions using both amorphous and crystalline MgO barriers were fabricated using a BSCCO single crystal in order to compare their superconducting states of interface. Both MgO thin films, 4–8 nm thick and with full coverage, were deposited onto BSCCO single crystals using the MBE method. The junctions using amorphous and crystalline MgO films were found to have gap opening structures at a bias voltage of about 38 mV and 25 mV at 5.2 K, respectively. The gap opening of the junction using an amorphous MgO barrier occurred below 86 K, corresponding very closely with the Tc for the BSCCO single crystal. Its characteristics showed a BCS-like temperature dependence.

Many of the high temperature superconductor phases degrade rapidly when in the presence of water, acids, carbon dioxide or carbon monoxide. In order to foster more rapid developments in the area of high-Tc research, it will be necessary to acquire a more complete understanding of the surface chemistry of these superconducting materials. In this paper, the relative reactivity of the common cuprate phases toward water is reported. X-ray powder diffraction and scanning electron microscopy measurements are utilized here to establish the reactivity trends.

A new layered perovskite vanadium oxide Sr4V3O8-d was synthesized and its crystal structure and magnetic properties were studied. Sr4V3O8-d has a novel crystal structure, containing a V-0 octahedron and a two-fold V-0 fence in a unit cell. It exhibits negative magnetization below 30 K. when cooled in a magnetic field. This peculiar magnetism is attributed to N-type parasitic ferrimagnetism.

This paper reports on the processes used to achieve low resistance silver contacts to YBCO thin films that have either c-axis or a-axis orientation. Characterization by x-ray diffraction and TEM verified that these films are highly oriented with either the a or the c axis oriented perpendicular to the substrate surface. TEM examination of some of the Ag/YBCO interfaces reveals the presence of an amorphous layer. We will describe the effects of ion beam etching and RTA alloying on the contact resistivity for both orientations.

Our study concentrated on the relationship between the proportions of the phases in the precursors used to synthesise the Bi-2223 phase and the ultimate microstructures of the 2223 phase materials, although the overall cation stoichiometry fixed. It was found that the final 2223 phase grains were larger, although the bulk density tended to be lower with the grains loosely packed together, when the major phase in the precursor was 2212 phase. This was proved to be partly caused by the fast growth rate of the 2212 phase grains which were eventually converted into the 2223 phase. When 2223 phase was present in the precursor, the bulk density became higher and 2223 phase grains became closely packed together, although the average grain size became smaller. This was explained by the existence of a liquid phase at higher temperatures during the formation of the 2223 phase and the sintering of the 2223 phase grains.

Tl-2223 phase with a composition of Tl1.7Ca2.3Ba2Cu3O10 was prepared via a solid-state reaction route and systematically annealed under various conditions, including flowing oxygen, sealed quartz in vacuo, high oxygen pressures and hot isostatic press in an attempt to enhance the superconducting properties in the Tl-Ca-Ba-Cu-0 system. An almost monophasic sample was obtained by sintering at 910 °C for 3 hours in flowing oxygen. Most anneals enhance the Tc by 2 – 7 K, depending on the environment. When the sample is post-annealed at 820 °C for 16 hours in flowing oxygen it results in an optimal Tc of 127 K, as confirmed by magnetic susceptibility and electrical resistance measurements.

Single crystals of R1Ba2Cu3O7–8, (R=Y, Eu and Gd), have been irradiated with 0.4–1.0 MeV electrons in directions near the c-axis. An incident threshold electron energy for producing flux pinning defects has been found. In-situ TME studies found no visible defects induced by electron irradiation. This means that point defects or small clusters ( ≤ 20 Å) are responsible for the extra pinning. A consistent interpretation of the data suggests that the most likely pinning defect is the displacement of a Cu atom from the CuO2 planes.

Epitaxial YBCO/YSZ thin films have been successfully grown on ultrathin Si(100) wafers as part of an effort to fabricate infrared detectors which are both fast and highly responsive. Single crystal Si wafers with thicknesses ranging from 400 μm to 3μm are used as growth substrates. Detectors are fabricated by patterning the YBCO films into meanderline structures and their IR response is evaluated as a function of substrate thickness. So far, we have achieved a responsivity of 1750 V/W with a 10 Hz chopping rate on the 25 μm substrate, and rise/fall times as fast as 3 msec on the 3 μm substrate. We have also measured the low temperature IR transmission from 15 μm to 300 μm of both the YSZ buffered Si substrate, and the complete YBCO/YSZ/Si sample. This study indicates that silicon is a superior substrate for the fabrication of high performance infrared focal plane detector arrays.

Atomic Layer-by-Layer Molecular Beam Epitaxy (ALL-MBE) of high Tc superconducting films can be used to grow defect-free and flat multi-layer structures in which superconducting molecular layers are stacked with molecular layers having other electronic properties. In particular, tri-layers consisting of c-axis Bi2Sr2CaCu2O8 base and counter electrode layers, each several hundred angstroms thick, have been grown separated by single molecular layers of metastable compounds such as Bi2Sr2Can-1CunO2+4 where n ranged from 5 to 11. Furthermore, the electronic properties of such barrier layers have been modified by doping with trivalent cations. Using such structures, tri-layer Josephson junctions have been fabricated which exhibit hysteretic I - V characteristics. Other doping schemes have given insulating layers which dominate c-axis transport, demonstrating the ability of this technique to grow thin, 25 Å, single molecular layers that are free of pinholes.

We have carried out finite-inductance calculations of the critical current vs, flux(Ic-φ) and voltage vs, flux (V-φ) characteristics of superconducting interferometers with many Josephson junctions (JJ's) in parallel. At least two features of our calculations suggest that interferometers with many junctions, which we call Superconducting QUantum Interference Gratings, or SQUIG's, might be advantageous for the detection of magnetic flux. First, the voltage noise can potentially be reduced significantly as compared to a dc SQUID with the same overall voltage-to-flux transfer coefficient - a feature which might reduce 1/f noise and enhance the magnetic flux sensitivity of both low and high Tc superconducting (HTS) devices. In addition, nonuniformity of the junction critical currents appears to have little adverse effect on the predicted diffraction-grating like enhancement and narrowing of the peaks in the Ic-φcharacteristic, suggesting that flux uniformity, rather that critical current uniformity, is of primary importance.

The transition temperature and the critical current density of a bulk YBa2Cu3O7−x-Ag microcomposite superconductive wire, prepared by powder processing followed by warm extrusion, have been measured employing the standard AC four probe technique. Measurements were performed, at 15K and zero applied field, with varying distance between the voltage contacts, while the current contacts remained a constant distance apart. It is observed that the resistance-temperature behavior remained identical in all the cases. Interestingly, the critical current density, determined from the experimentally recorded current-voltage characteristic by applying a constant electric field criterion, is seen to increase significantly, smoothly and steadily with increasing voltage tap length. However, the critical current density remains virtually unchanged if a constant voltages criterion is applied for its determination. This paradoxical dependence of critical current density, a material property, on measurement length has been explained with the help of a simple theoretical treatment, taking into account the nature of current-voltage chracteristics as well as the phenomenon of current transfer through the metallic to the superconducting ceramic phase. It is suggested that the constant electric field criterion may not be an appropriate one to use in the evaluation of critical current density of metal-ceramic superconductive composites.

Quantitative high resolution transmission electron microscopy and x-ray diffraction have been used to study films of YBa2Cu3O7-δ grown on LaAlO3 substrates at low substrate temperatures. Based on analysis of high-resolution micrographs, it is asserted that the films are b-axis oriented near the film-substrate interface, and switch to a-axis oriented at some distance away from the interface, in a manner which varies from sample to sample. Thus, the films undergo a change in orientation as a function of distance from the substrate. X-ray diffraction confirms that these films contain both a-axis oriented and b-axis oriented components normal to the plane of the substrate, consistent with the high-resolution microscopy data.

The flux-pinning forces in high temperature superconductors were found to be weakened in an ac field as the applied field strength or the frequency increases. In the weakly pinned regime, flux dynamics could be described widi the concept of magnetic diffusion. Flux-motion-induced finite resistivity could lead to significant skin-effect which was reflected in the ac screening effectiveness. The frequency dependence of the relative local field within a superconducting hollow cylinder were used to deduce the flux-motion resistivity (ρ). For superconducting YB2Cu3O7−x compounds at 77K it was found that ρ≈ 1μΩ-cm. The corresponding magnetic diffusion coefficient was ≈224 cm2/s. At 750 Hz the skin depth is around 1.54 mm as compared with the 2-mra sample wall thickness.

Thin film rings with microbridge were constructed using high Tc Y-Ba-Cu-O super-conducting thin films which were made by laser ablation on the single crystal MgO(100) substrates. The thickness of the films is 5000 Å. The weak links were biased with an AC induced current. The resulting electron pair charges on the weak links were monitored by an output voltage. The charge stored on the link (as a function of the input bias voltage) is consistent with that of a “macroscopic quantum pendulum”

We report systematic studies of the dc transport properties in proton-irradiated Y-Ba-Cu-O single crystals. We find that the onset of vortex dissipation in moderately irradiated samples is associated with the occurrence of a second-order vortex-solid melting transition. In addition, the decreasing zero-field transition temperature and increasing critical current density with the increasing defects reveal the effects of disorder on reducing the electron mean-free-path and on increasing the pinning density.