We report the fabrication and superconducting properties of ∼0.5 μm thick, fine-grained polycrystalline coatings of MgB2 on single-crystal substrate surfaces. The films exhibit large critical current densities, implying little effect from the grain boundaries. Analyses for thermal activation effects are inconclusive, and evidence is presented that the irreversibility line is dominated by the combined influences of Hc2 anisotropy and polycrystallinity. Comparative studies of the magnetic persistent currents and electrical transport properties reveal excellent agreement over a wide range of temperature and magnetic field. This result is contrary to similar comparisons on high-temperature cuprates, where disparities arise from the effects of large flux creep and the diverse electric field regimes probed by the two techniques. The MgB2 films exhibit extremely sharp voltage-current relations away from the irreversibility line, in qualitative agreement with observed large Jc values and low rates of magnetic flux creep.

All high- Tc superconductors are susceptible to humid environment. The mechanism behind this degradation process is not yet well known or even described. In an attempt to understand the mechanism, we studied the real-time degradation process of HgBa2CaCu2O6+δ (Hg-1212) thin film using scanning probe microscopy (SPM) techniques. A newly developed near-field scanning microwave and optical dual probe was employed to map simultaneously the spatial variation of microwave sheet resistance and optical transmittance of a sample. Mapping was performed at a regular time interval while the sample was exposed to a very high level of humidity (85%-90%). In a separate experiment, atomic force microscope (AFM) was used to monitor the surface roughness of a sample subjected to the same level of humidity. Results indicate that the degradation process of Hg-1212 involves the gradual transformation of the material into a colorless insulator accompanied by continuous surface deformations.

The innovative Combustion Chemical Vapor Deposition (CCVD) process is a non-vacuum technique that is being investigated to enable next generation products in several application areas including high-temperature superconductors (HTS). In combination with the Rolling Assisted Biaxially Textured Substrate (RABiTS) technology, the CCVD process has significant promise to provide low-cost, high-quality lengths of YBCO coated conductor. The CCVD technology has been used to deposit both buffer layer coatings as well as YBCO superconducting layers. A buffer layer architecture of strontium titanate and ceria have been deposited by CCVD on textured nickel substrates and optimized to appropriate thicknesses and microstructures to provide templates for growing PLD YBCO with high critical current density values. The CCVD buffer layers have been scaled to meter plus lengths with good epitaxial uniformity along the length. A short sample cut from one of the lengths enabled high critical current density PLD YBCO. Films of CCVD YBCO superconductors have been grown on single crystal substrates with critical current densities over 1 MA/cm2. Work is currently in progress to combine both the buffer layer and superconductor technologies to produce high-quality coupons of HTS tape made entirely by the non-vacuum CCVD process.

We studied the influence of sample preparation and defects in the superconducting properties samples using atomic ratios of Mg:B=1:1 and Mg:B=1:2. Samples were characterized by SEM, and XRD, and the magnetization properties were examined in a SQUID magnetometer. The presence of Mg vacancies was determined by Rietveld analysis. Most of the samples exhibited sharp superconducting transitions with Tcs between 37–39 K.

We found a strong correlation between the crystal strain and the Tc. This strain was related to the presence of Mg vacancies. In addition, results showed that some samples degraded with time when exposed to ambient conditions. In these samples the Tc did not change with time, but the superconducting transition became broader and the Meissner fraction decreased. This effect was only present in samples with poor connectivity between grains and smaller grain sizes. The degradation was related to a surface decomposition as observed by X-ray Photoelectron Spectroscopy. No correlation was found between this effect and the presence of Mg vacancies.

The vortex dynamics at microwave frequencies in YBa2Cu3O7-δ(YBCO) films has been studied. We observe a peak in the microwave surface resistance Rs vs. T (temperature) plots at 4.88 GHz in some films in magnetic fields (upto 0.8 T) whereas a few films do not show a pronounced peak in their surface resistance. This is associated with the ‘peak-effect’ phenomenon and reflects the order-disorder transformation of the flux-line lattice near the transition temperature. Introduction of artificial pinning centers like columnar defects as a result of irradiation with 200 MeV Ag ion leads to the suppression of the peak in films exhibiting the ‘peak-effect’. Interestingly, a peak is seen to develop in those films that initially did not show a peak in the surface resistance before irradiation. These results and the dynamics of the vortices at microwave frequencies for the unirradiated and irradiated films will be discussed.

Two types of shape and contrast features of superconducting vortices in a Lorentz micrograph were obtained by the newly developed 1-MV field-emission transmission electron microscope on a Bi2Sr2CaCu2O8+8(Bi-2212) thin specimen containing tilted columnar defects. The shape and contrast features could be consistently interpreted by the simulation that some vortices were pinned along tilted columnar defects and others were unpinned. The interesting property for temperature change of vortex core inside the material was also observed.

We have investigated the diffusion mechanism of Hg and Tl cations in cation-exchange process that has been used successfully for fabrication of Hg-based high-Tc superconducting films. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) was employed to map the distribution of Tl and Hg cations in films quenched at different stages of the cation exchange process. SEM/EDS mappings showed that the nonuniform distribution of Hg is visible in micrometer size on the surface of samples quenched after short-time (∼10min) Hg-annealing, but this nonuniformity disappears for longer-time (∼45 min) Hg-annealing samples. This change could be ascribed to different stages in Hg diffusion -- the former is the early stage when Hg is concentrated in the channels and the latter Hg-cations have already diffuse to grains. Our experimental results hence suggest that Hg-cations channel through defects in the films then diffuse into grains along a-b planes and vise versa for Tl-cations. The diffusion mechanism of anions (oxygen) in post annealing has also been discussed. Fluorine-doped Hg-1212 films were post annealed in flowing oxygen at 300 °C for several hours. Magnetic measurement has shown these samples are comprised pure overdoped Hg-1212 phase with smooth and sharp transition below 120 K, but resistivity vs. temperature measurement shows a kink at ∼123 K. This implies that the optimally doped Hg-1212 phase with higher Tc might be surrounded by overdoped Hg-1212 phase and their contribution to magnetization is minimized, thus the possible diffusion mechanism for anions is also through grain boundaries at a much larger time scale compared to the cations.

Inconel, tantalum and a silver-palladium alloy were used to fabricate MgB2tapes. The tapes, made by the Powder In Tube (PIT) method, were heat-treated at 800°C and 900°C in 1 atmosphere of flowing argon. The microstructure and phase composition of the tapes were examined by Scanning Electron Microscopy (SEM) and X-Ray Diffractometry (XRD). Critical temperatures (Tc) and magnetization-applied magnetic field (M-H) curves were measured by SQUID magnetometry. It is found that between 800–900°C inconel and tantalum sheaths have no effect on the Tc,inductive of the material. However, under certain processing conditions, the silver-palladium sheath can decrease the Tcor destroy the superconductivity completely. The inconel sheathed tape was found to have better superconducting properties than the tantalum and silver-palladium sheathed tapes. Consistency checks have been applied to verify the validity of the Bean Model in obtaining the critical current density (Jc) from magnetization data. The inconel sheathed tape is estimated to have a magnetization Jc(5K, ∼0.5 T) of 1.4 × 105A/cm2.

The preparation of non c-axis oriented HTS films with longer coherence length is one of key technologies toward realisation of viable Josephson devices. We reports on the successful growth of (119)Bi-2223 films by MOCVD using a two-step growth temperature procedure and on AFM observation of the film surface. This temperature 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 much improved. The surface morphology observed by AFM showed a mountain-range-shaped one, which strongly reflects the epitaxial relationship between the (119)Bi-2223 film and (110) SrTiO3 substrate.

We have grown La2−xSrxCuO4 single-crystalline films on atetra-axis oriented Zn doped La2CuO4 single crystals by IR-LPE technique. The films show thickness dependence of Tc,onset, but films of thickness below 80μm doesn't show zero resistance down to 4.2K. The I-V characteristics showed reduced voltage jump than usual and small hysteresis. This phenomenon resembles with the characteristics of resistively and capacitively shunted intrinsic Josephson junctions (RCSJ)

A novel x-ray diffractometer has been used to characterize the texture of 2 km of textured tape in segments up to 20 m long. Techniques have been developed for the study of the uniformity of texture and for the detection of second phases, deviations from cube texture, and the sharpness of cube texture, in metal substrates, oxide buffer layers, and YBa2Cu3O7 (YBCO) superconductors.

The presence of a superstructure is identified to play a key role for the modifications in both superconductivity and structure transitions in the Mg1-xAlxB2 system. This superstructure occurs along the c-axis direction, and can be well interpreted by Al-layer ordering. The optimal composition of the superstructure phase is MgAlB4, a superconductor with Tc ∼12K. Raman spectrum of MgAlB4 gives rise to a sharp peak at around 941cm−1. Brief diagrams illustrating the superconductivity and structural features of Mg1-xAlxB2 (0≤x≤1) materials are presented.

The discovery of superconductivity in MgB2 has been followed by many papers reporting attractive thin film properties. In most cases these have involved the deposition of precursor films followed by in-situ or ex-situ post annealing in a Mg-rich atmosphere. Although simple device structures have been fabricated from such films, it is desirable for a number of reasons that a heterostructure device technology be developed. Heterostructure growth is likely to require in-situ growth, preferably without post-annealing. To achieve this, low oxygen and high Mg background pressures are required in the vicinity of the sample. By using a novel heater geometry we have been able to grow superconducting MgB2 films from Mg-rich targets at temperatures below 500 °C. This paper reports the growth method, and structural and electrical characterization of the films.

Surface oxides present on polycrystalline MgB2 were characterized by high-resolution x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). X-ray diffraction (XRD) measurements were used to determine the MgB2 phases. XRD line broadening analysis reveals a grain size of 40 nm. XPS results show that MgO and B2O3 are the major surface oxides. Auger spectra provide further evidence of the presence of MgO. The B 1s and Mg 2p peaks have been used to quantify the amount of the surface oxides.

Sol-gel processing of La2Zr2O7 (LZO) buffer layers on biaxially textured Ni-3 at.% W alloy substrates using a continuous reel-to-reel dip-coating unit has been studied. The epitaxial LZO films obtained have a strong cube texture and uniform microstructure. The effect of increasing the annealing speed on the texture, microstructure and the carbon content retained in the film were studied. On top of the LZO films, epitaxial layers of Yttria Stabilized Zirconia (YSZ) and Ceria (CeO2) were deposited using rf sputtering, and YBa2Cu3Ox (YBCO) films were then deposited using Pulsed Laser Deposition (PLD). A critical current density (Jc) of 1.9 MA/cm2 at 77K and self-field and 0.34 MA/cm2 at 77K and 0.5T have been obtained on these films. These values are comparable to those obtained on YBCO films deposited on all-vacuum deposited buffer layers, and are the highest ever obtained using solution seed layers. The use of all-solution buffers for coated conductor processing has also been explored. A critical current density of 1.1 MA/cm2 at 77 K and self-field was obtained on YBCO films grown be PLD on LZO buffered nickel substrates.

Oxygen nonstoichiometry behaviors of REBa2Cu3Oy (RE123: RE= Nd, Sm, Eu, Gd, Dy. Ho and Y) compounds have been precisely determined by the thermogravimetric measurements. Dependence of oxygen content on temperature and oxygen partial pressure were found to slightly depend on the RE element. Thermodynamic quantities of oxygen, such as hO2 and sO2, also varied with RE element. Relationship between Tc and oxygen content is strongly dependent on the RE species. This suggests that pinnig effect due to the oxygen defects is different in each RE123 compound and, therefore, critical current properties of RE123 must be optimized by precise control of oxygen content as well as selection of suitable RE elements.

SrTiO3 (STO) is a potential buffer layer material for use in YBa2Cu3Oy (YBCO) coated conductors based on the IBAD MgO process. However, the interactions with YBCO are not yet fully understood and little information exists in the way of phase diagrams. With this in mind, the tie-line between STO and SrCuO2 in the pseudo-ternary system SrO-CuO-TiO2 was investigated. Phase assemblages and compositions were determined by x-ray diffraction and electron microscopy in the temperature range of 1000°C to 1100°C in oxygen partial pressures of 1%, 10%, and 100%. Preliminary results showed that an appreciable amount of copper substitutes into the STO crystal structure. Conversely, Ti substitution into the SrCuO2 phase was not detected.

A single crystalline Bi2Sr2CaCu2O8+8 which has columnar defects in its inside are observed by Lorentz microscopy using the newly developed 1-MV field emission electron microscope at the first time. The superconducting vortices are observed with higher contrast than ever. Simultaneous observation of vortices and columnar defect is succeeded in real time.

Recently superconductivity in graphite-sulfur composites was experimentally observed. In this work we have analyzed the electronic structure changes associated with the presence of sulfur atoms in one and two dimensional graphite layers. We have considered ordered and disordered sulfur atoms distributions in many configurations. The density of states (DOS) of these structures were obtained using the negative factor counting (NFC) technique coupled to a tight-binding hamiltonian (Hückel type). Our results indicate that the incorporation of sulfur atoms at edge graphite layers (changing their global geometric curvature and increasing the DOS at the Fermi level) might be in the origin of the graphite superconductivity.

[Ba2CuO2(CO3)]m[ACuO2]n(A=Sr) superlattices containing oxycarbonate blocks as charge reservoir have been prepared on SrTiO3 using the molecular beam epitaxy technique. First, thin films of the oxycarbonate cuprate Ba2CuO2(CO3) have been prepared on SrTiO3(001) using NO2 gas as an oxidant and CO2 gas. The films have been grown at 500°C. At higher substrate temperature or at lower CO2 pressure Ba2CuO3 was formed instead of Ba2CuO2(CO3), and the films becomes amorphous at lower temperature. X-ray diffraction and reflection high-energy electron diffraction observations indicated that (BaxSr1−x)2CuO2(CO3) grew along the [001] crystal orientation on SrTiO3(001) with the following epitaxial relationship: Ba2CuO2(CO3)[100]//SrTiO3[110] and Ba2CuO2(CO3)[110]//SrTiO3 [100]. Depth profile of secondary ion mass spectrometry signals indicated the incorporation of carbon into the films. Secondly, the oxycarbonate cuprates and infinite layers have been alternately stacked. It was confirmed that Ba2CuO2(CO3)was inserted between several unit cells of SrCuO2. Electrical measurements show the as grown films to have a semiconducting behavior.