Epitaxial Ba2YCu3O7-δ (BYCO) films grown by the ex situ BaF2 process are comparable to single crystals both in crystalline integrity (RBS/ion channeling χmin < 3%) and the value and temperature dependence of the critical current (Jc) in an applied magnetic field in the BYCO (001) direction of Ha = 0.9T. With the appropriate dose of either 2 MeV H+ or 135Xe+, we are able to enhance Jc by a factor of 2 in H = 0.9T with little effect on Tc. The ability to change Jc by such a large factor in these films is a prerequisite for isolation and study of the induced defects and study their properties.

Thin films of YBCO were prepared using a mass-spectrometer controlled coevapora-tion/MBE process with atomic oxygen. Under the conditions of low pressure, necessary for mass-spectrometer rate control, it is shown that an atomic oxygen beam source can be utilized in order to grow high quality thin films of YBCO as well as multi-layer structures involving YBCO and Y2O3. Values of Tc = 88.5 K and Jc = 6–106 A/cm2 at 77 K were determined for a strip with a width of 10 μm of YBCO deposited on a LaAlO3 substrate. Film structure is analyzed by XRD and rocking curve measurements. Magnetic characterization of films and multi-layer structures are reported.

We have deposited YBa2Cu3O7 (YBCO) films on LaAlO3 substrates up to two inches in diameter using 90° off-axis magnetron sputtering from a single stoichiometric target. With this same method, epitaxial bilayers and trilayers of insulating SrTiO3, LaAlO3, MgO, metallic PrBa2Cu3O7 (PrBCO), or superconducting La2−xSrxCuO4 (LSCO) were deposited in combinations with YBCO. In the resulting structures with c-axis orientations, the YBCO had Tc's of 90K or higher over the entire surface area. Maximum resistivity values of 6 × 109 Ω-cm in YBCO/insulator/YBCO structures were obtained in vertical transport measurements with SrTiO3. An important cause for degraded electrical properties in the structures incorporating epitaxial insulators was the presence of copper oxide particles on the YBCO surface. Methods used to minimize the number of these particles are discussed.

Dense Bi2Sr1.7CaCu2Ox (2212) superconductor pellets were made by hot isostatic pressing in an inert atmosphere. The pellets exhibited a small amount of preferred orientation of the grains. The transport critical current density (Jc) values were very low for all specimens. Dislocations, planar faults, and intergrowths of the Bi2Sr2CuOx phase, which were produced during the pressing, caused the intragranular Jc to increase substantially.

We report on the studies of low-field magnetic properties in the well-characteristic superconducting LaCaBaCu3Oy, YBa2Cu3Oy, YaBa4Cu8Oy, (La1–85Sr0–15)CuO4-y, (Tl0–5Pb0–5)Ca2Sr2Cu3Oy and (Bi2−xPbx)Ca2Sr2Cu3Oy oxides. The studies of magnetic properties include ac zero-field cooled (ZFC), field-cooled (FC), and ac susceptibilities, and their dependences on the applied magnetic field and temperature. As the applied dc magnetic field is low enough, the FC susceptibility varies flat with the increasing of temperature from low temperature, but the diamagnetic signal becomes increasing at some temperature and reaches to the maximum values, and then continously decreases with temperature, at last the diamagnetism rapidly transfers to paramagnetism with temperature going to above Tc. The magnetization reduction at low temperature significantly appears at the low dc magnetic field and exists within a lot of single-phase high-Tc superconductors. However, this phenomenon is not observed in the measurements of low-frequency ac complex susceptibility for the ac applied fields from 0.001 G to 10 G and disappears for higher dc applied field. But, there exists a two-step drop in ac and ZFC susceptibilities curve and the drop correlates with the de FC magnetization dip. The magnetic field range of the dip occurrence is dependent on specimens. The strange magnetic property may be regard with superconducting coupling link in grain boundaries and interpreted by the grain-cluster model associated with Josephson effect.

Temperature dependences of DC magnetization with low magnetic applied field (< 160 G) have been studied on layered superconductors. Two-step behavior below Tc are seen in most of the bulk samples, when the field is smaller than a few tens of Gauss. A grain-cluster model which taking both inter- and intragrain regions into consideration as two components of the present system is used to interpret the observed phenomena. The intergranular component is conjectured as coming from Josephson tunneling supercurrent between grains. A two-critical-states model including the surface-critical-state is extracted to outline the transition behavior of the shielding currents. The two-step phenomenon of the “bulk” magnetization is well explained with appropriate selections of the coupling strength and phase for the intergrains and of those critical current densities for the intragrains.

The modifications of the laser ablated target surface morphology and the particulates on YBa2Cu3O7−x (YBCO) thin films have been investigated. As the number of laser pulse per site increases, the ablated target surface changes to conical structures. After the texturing of the target surface, the ablated surface morphology changed identically, independent of the initial surface roughness of the polished targets. The YBCO thin films deposited using the stationary target have an almost particulate-free surface. However, in the case of the YBCO films deposited using the rotating target, there is a considerable increase in the density of particulates on the film surface.

Using a novel melt-spinning technique, we have produced highly disordered NdBa2Cu3O7−x and GdBa2Cu3Oy−x materials. Samples which were melt-spun in an O2 environment consist of nanocrystals with the tetragonal REBa2Cu3O7−x structure: samples which were processed in an N2 environment consist of an amorphous matrix with small amounts of crystalline BaCu2O2, as shown by x-ray diffraction and electron microscopy. High temperature XRD studies indicate that the BaCu2O2 is eliminated during heating to 500°C in O2 and that the REBa2Cu3O7−x Phase recrystallizes directly from the amorphous matrix at temperatures below 800°C. Preliminary magnetization measurements show that higher temperature heat treatments are needed to restore superconductivity.

Low surface resistance epitaxial Tl2CaCu2O8 thin films were prepared on orthorhombic (001) NdGaO3 and pseudocubic (100) LaAlO3. The films were c-axis oriented on both substrates, with [100] Yl2Ba2CaCu2O8 parallel to [110] NdGaO3 or pseudocubic [100] LaAlO3. The orientation was particularly good on NdGaO3, where the rocking curve full width at half maximum was 0.19°. Films on LaAlO3 exhibit high Tc and exceptional microwave properties: inductive superconducting Tc of 106 ± 2 K, onset of flux exclusion at 107.6 ± 0.5 K, and 10 GHz surface resistance of 130 ± 20 μΩ at 77 K. These results represent the highest reported Tc of any Tl2Ba2CaCu2O8 films and the lowest surface resistance at 77 K of any films of any phase on any substrate reported to date. On NdGaO3, in spite of a substrate-film reaction, films with high Tc and good microwave properties were also produced: inductive Tc of 105 ± 2 K, onset of flux exclusion at 104.0 ± 0.5 K, and 10 GHz surface resistance at 77 K of 600 ± 100 μΩ.

A series of samples with a nominal composition of R3CaBa3Cu7Oy (abbreviated as R3137), where R = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm, was prepared by a similar process as for an 80-K superconductor, La3CaBa3Cu7Oy. Most of the R3137 (except for R = Ce, Tb) had a tetragonal triple-perovskite phase like La3137 identified from the XRD patterns, and they were superconducting except Pr3137. For R3+ radius larger than Dy3+ (0.908 Å), a single phase R3137 was observed, and their Tc(zero)' probably related to the rCu-o distance in the copper-oxygen layer, was spread from 85 to 76 K with a maximum for the Eu3137. Oxygen stoichiometry of these R3137 was slightly decreased with decreasing lanthanide radii. The hole concentration, (Cu-O), was found between 0.47 and 0.38/unit cell. For Smaller R3+ ions, residual resistivity was found in the superconducting samples due to the existence of second phase. From the structural study of La3137, it was known that part of the lanthanide ions had to be fitted into the bigger Ba-site. Because ionic radii of the heavy lanthanides (R3+ < Ho3+ = 0.894 Å) were too small to be located at the Ba-site, it was rather difficult to prepare these single phase R3137.

Anisotropie electrical resistivity is studied in epitaxial superlattice films of YBa2Cu3O7-δ/PrBa2Cu3O7-ε grown in situ by laser ablation on SrTiO3 surfaces aligned slightly away from the [100] direction. Layer thicknesses of each compound range from two to eight atomic cells. Electrical resistivity of these superlattice films always shows a peak at some cryogenic temperature that decreases with increasing PrBa2Cu3O7-ε layer thickness. Clear evidence is seen of vortex pinning or of supercurrent blocking by the step edges or by twin boundaries. The ratio of electrical resistivities for current directions parallel and perpendicular to these boundaries shows a large step-like change just above the superconductive critical temperature. The possibility is discussed that the Kosterlitz-Thouless transition explains this anisotropy change.

In addition to the well known I4/mmm and Bmab phases, recent low temperature structural studies of certain T-type (K2NiF4-related) La2CuO4 analogues have revealed two additional structural modifications, one with Pccn and the other with P42/ncm symmetry. Via appropriate iso- and aliovalent substitution chemistry, regions of T-type phase field have been mapped out and the influence of these phase transitions on superconductivity studied.

The synthesis of superconducting Tl-Ba-Ca-Cu-O thin films on metal foils (Au and Ag) by metal-organic chemical vapor deposition (MOCVD) has been investigated. Ba-Ca-Cu-O-(F) films are first prepared via MOCVD using fluorinated “second generation” metal-organic precursors. After an intermediate anneal with water vapor-saturated oxygen to promote removal of F, Tl is introduced by annealing in the presence of a mixture of oxides (Tl2O3, BaO, CaO, CuO) of a specific composition. Characterization of the thin films by scanning electron microscopy, EDX, x-ray diffraction, and variable temperature magnetization measurements has been carried out. High temperature superconductor (HTS) films of Tl2Ba2Ca1Cu2O8−x on Au foil exhibit a magnetically derived Tc = 80K and a high degree of texturing with the crystallite c-axes oriented perpendicular to the substrate surface as evidenced by enhanced (000 x-ray diffraction reflections. Thin film coverage on Ag foil becomes non-contiguous during the (Tl2O3, BaO, CaO, CuO) mixture anneal.

Ba2YCu3Ox superconducting films were prepared by Nd:YAG laser ablation, equipped with a second (ArF excimer) laser for irradiation onto the growing film surface. The irradiation onto the film during the deposition were delayed for various delay times against the ablation of Ba2YCu3Ox target. The experiment showed that the second laser irradiation within several tens us around the ablation event induces a change of the crystal orientation. This result suggests that the crystal growth for the laser ablation is determined mainly around this time scale.

Composite YBCO/BSCCO films on MgO(lOO) substrates were fabricated by using laser deposition. The structures and transport properties of these composite films were studied. The model of 3D percolating network in YBCO thin films will be discussed.

In contrast to sintered Bi2Sr2Ca1Cu2Oy (Bi-2212), melt processed material exhibits nonzero intergrain supercurrents at T < 77 K. By means of partial melting and long term annealing we achieved nonoriertted material with Jc(77) - 3500 A/cm2 (1μV/cm criterion) and Tc = 96 K. Zone melting leads to well textured bulk material with the c-axis of the crystallites oriented perpendicular to the pulling direction R, and thus to an anisotropie critical current density. The anisotropy factor is between 2 and 5. At low temperatures the melt textured ceramic exhibits higher critical currents than the nonoriented material. This is not true at high temperatures, since the latter has a higher critical temperature.

At 77 K and small fields both oriented and nonoriented ceramics exhibit no transition in their current-voltage characteristics, but obey the power law E = Eo(j/jo(H)) α(H) over a large range in E (10”−3-10”−3 V/cm). Measurements on the textured ceramic with magnetic fields parallel and perpendicular to the pulling direction demonstrate that the superconducting properties of these materials are only affected by the field components parallel to the c-axes of the crystallites. For fields parallel to the pulling direction the measured values of α(H), Jo(H) and of the reversible magnetization Mrev can be attributed to the misorientation of the Bi-2212 platelets which is in average about 14°.

Semi-coherent Y2O3 (yttria) inclusions have been observed in high quality, c-axis oriented YBa2Cu3O6+×,(YBCO) Alms. The inclusions were studied by transmission electron microscopy (TEM), and were found to be embedded in the YBCO matrix without disturbing its structure seriously. Their number density is estimated to higher than 2 × 1016 cm−3 in epitaxial YBCO films, having transport critical current densities in excess of 1 × 106 A cm−2 at 77 K. Magnetization measurements indicate a possible correlation between a high density of yttria inclusions and high pinning strength. The effects of the inclusions on film growth and microstructure is discussed.

In an ultra-high vacuum equipment, YBa2CU3O7−x (YBCO) thin films were deposited by the ozone-assisted co-evaporation method and their surfaces were investigated by LEED, XPS and ISS without breaking the vacuum. A LEED pattern was observed for the first time at an as-deposited surface. A clean, crystalline and superconducting surface was preserved to the top of the film. Only Cu atoms were detected at the first monolayer by ISS, suggesting that clean surfaces are stabilized by Cu-O layers.

The critical current density inferred from magnetization hysteresis for epitaxial Tl2Ca2Ba2Cu3O10 films on LaAlO3 exhibits a pronounced increase with film thickness in the high-field, high-current regime. This thickness dependence does not extend to comparatively “thick” single crystals, however, suggesting that the grain boundaries play a major role in collective vortex pinning for Tl-based superconducting thin films.

We used a microwave technique in which a small strip of YBCO film is induced to self-resonant in order to measure the surface impedance at 21 GHz. Both the amplitude and phase of the transmission coefficient, S21, as a function of frequency was measured at different temperatures. The surface resistance, Rs, was obtained from at resonant frequency, where Z0is the waveguide impedance. The surface reactance, Xa was obtained from the measurement of frequency shift with temperature. We found that Ra changed about 3 orders of magnitude as T decreased from 90K to 80K reaching a low value of 4.9 × 10−4μ. From Ra and Xa we deduced the London penetration depth to be λ ∼ 1800Å at 4K and 8000Å at 86.5K.