Spray Pyrolysis of MgO Templates on 321-Austenitic Stainless Steel Substrates for YBa2Cu3O7 Deposition by PLD

Spray pyrolysis was used to deposit MgO films on polycrystalline 321-austentic stainless steel substrates using magnesium nitrates and magnesium acetates as precursors. The MgO films deposited from the nitrate precursors were amorphous; however, MgO (200) oriented films were obtained when the acetates precursors were used. The texture of the films was improved with increasing the concentration and the deposition temperature. To evaluate the performance of the MgO buffers, PLD was used to deposit YBCO on MgO-buffered 321 substrates. Only the smoothest MgO films were found as good buffers for the deposition of c-axis oriented YBCO films. The superconducting transition temperature was broad and the T _C onset was 83.6 K. Austenitic 321 steel is an alternative for C276 as a substrate for thin film deposition.     

Superconducting properties of YBa2Cu3O7-Au composites

By X-ray diffraction and photoelectron spectroscopy, we have demonstrated that Au is incorporated in the YBCO lattice up to a concentration 0.1. An XPS study gave evidence for an electronic valence state Au¹⁺. Its replacement in the Cu(1) site appears questionable due to the large difference in ionic radii (Cu²⁺0.073 nm; Au¹⁺0.137 nm). The so called irreversibility lines determined by AC susceptibility measurements are strongly affected by the doping. Up to the limit of solubility in the grains, an improvement in intra and intergrain coupling and flux pinning energy is observed, showing the beneficial interest of YBCO-Au composites.     

Pulsed Laser Deposition of a Single Buffer Layer of LaMnO3 for YBa2Cu3O7−x Coated Conductors

A single buffer layer of LaMnO₃ for YBa₂Cu₃O_7−x (YBCO) coated conductors has been grown on textured Ni substrate using pulsed laser deposition (PLD). Detailed X-ray diffraction (XRD) studies showed that the LaMnO₃ single buffer layer had a good out-of-plane and in-plane texture with a full width at half maximum of 5.2° and 9.9°, respectively. The scanning electron microscopy image revealed a dense, smooth, and uniform surface. The single LaMnO₃ buffer layer has been proved to be a very effective diffusion barrier layer against Ni and oxygen diffusion and a good template for subsequent growing high critical current density YBa₂Cu₃O_7−x films. PACS: 74.78.Fk, 74.78.Bz, 74.72.Bk, 81.15.Fg.     

Fabrication and Characterization of YBa2Cu3O7 − x Thin Film Step Junctions on MgO Substrates

We have investigated the tunneling properties of proximity-effect superconductor-normal-superconductor junctions based on high-temperature superconductor YBCO thin films and normal Ag layer. The systematic analysis of the properties of the films and the fabrication process which leads to a good-quality junctions is discussed in detail. The properties of the tunneling barrier are investigated by dc and rf measurements: the dc properties reveal that our junctions fully behave as expected from the de Gennes model. Regular Shapiro steps in the current–voltage characteristics are obtained as a result of microwave irradiation, indicating reliable rf properties of the junctions.     

Characterization of YBa2Cu3O7/PrBa2Cu3O7 superlattices

We investigated the structural and superconducting properties ofc-axis oriented (YBa₂Cu₃O₇) _nY /(PrBa₂Cu₃O₇) _npr superlattices with thicknesses of the individual layers down to one unit cell (10≥nY≥1; 18>nPr≥ 1). By transmission electron microscopy and X-ray diffraction we find an excellent structural quality of the samples, though the quantitative analysis shows the existence of defects. In superlattices with decoupled YBa₂Cu₃O₇ layers of two unit cell thickness we find a highT _c value of 75 K. We probed the flux line structure in the superlattices by measurements of the critical current density in magnetic fields. The experiments show that the flux-line dynamics is dominated by the movement of pancake vortices.     

Specific heat of YBa2Cu3O7

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearT _c , on a number of YBa₂Cu₃O₇ samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu²⁺ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the “linear term” is presented and discussed.     

Low-temperature specific heat of YBa2Cu3O7, YBa2Cu3O6, Y2BaCuO5, YBa3Cu2O7, BaCuO2, and CuO

We have measured the low-temperature specific heat (1.3T20 K) and the dc magnetic susceptibility (100T250 K) of eight samples of the high-T _c superconductor Y _x Ba_3–x Cu₃O_7– (x=0.9, 1.0, 1.1) and of two samples of nonsuperconducting YBa₂Cu₃O₆₊. We have also performed specific heat measurements on the possible impurity phases: YBa₃Cu₂O₇, Y₂BaCuO₅, CuO, and BaCuO_2+x . The superconducting samples all have a nonzero, sample-dependent linear term * and an upturn inC/T at very low temperature. We show that this anomalous behavior is at least partly due to the presence of a small amount (1%) of BaCuO_2+x impurity phase in the measured samples. This is evidenced by the correlation between * and the Curie component of the susceptibility, which is proportional to the amount of paramagnetic impurities.     

Glassy behavior of low-temperature energy relaxation in YBa2Cu3O7 and YBa2Cu3O6

Measuring the power release after rapid cooling a YBa₂Cu₃O₇ sample (m=42.85 g, T_c=91 K) from the equilibrium temperature T₁ (2.35 KT₁15.1 K) to T₀=1.5 K, we observed a time dependence typical of a glass: is proportional to t^–1. The results allow us to determine the linear term of the heat capacity (0.8 mJ/mole · K²) due to the two-level systems. While the low-temperature heat capacity anomaly noticeably decreases, the power release is essentially unchanged after oxygen reduction of the sample.     

Superconducting and non-superconducting PrBa2Cu3O7

Superconducting properties of Pr123 and microscopic structural differences between the superconducting and non-superconducting Pr123 crystals are reviewed. It is shown that the variety of physical properties of Pr123 is closely related to Cu deficiency at the chain site and our experimental results are consistently explained with a model that the Pr 4f-O 2p hybridization forms a narrow band and carriers are localized by a kind of atomic disorder in non-superconducting crystals.     

Low-Frequency Relaxation Rate in the Superconducting YBa2Cu3O6.7

The submillimeter-wave 3 cm ^–1 < < 40 cm ^–1 complex conductivity of the reduced YBa ₂ Cu ₃ O _6.7 film, (T _C =56.5 K) was investigated for temperatures 4 K < T < 300 K. The frequency dependence of the effective quasiparticle scattering rate 1/*() was extracted from the spectra. 1/* is shown to be frequency independent at low frequencies and high temperatures. On decreasing temperature the scattering rate increases with increasing frequencies. Finally, at 6 K it follows a power-law, 1/* ^1.75±0.3.     

Raman scattering studies of ultrathin-layer YBa2Cu3O7/PrBa2Cu3O7 superlattices

We present Raman scattering studies ofc-oriented ultrathin-layer superconducting (YBa₂Cu₃O₇) _m /(PrBa₂Cu₃O₇) _n superlattices. For the superlattice with (m=2,n=1) sequence, Raman spectra reveal a new line in the spectral region around 320 cm^?1. It is interpreted as a mode representing a combination of IR optical phonons of the Y-sublayers with an admixture of aB _1g type Raman active vibration in the Pr sublayers. This new line, which is similar to those from the interior of the Brillouin zone of the original lattice, does not exhibit superconductivity-induced self-energy effects, although its counterpart in the pure substance does. No additional line is found in the (m=1,n=2) superlattice in the same region, supporting our interpretation for the (m=2,n=1) sample.     

Electronic structure of YBa2Cu3O7 and YBa2Cu3O6—charge-self-consistent band structure calculations

Charge self-consistent LCAO band structure (CSCBS) calculations are reported for orthorhombic YBa₂Cu₃O₇ and tetragonal YBa₂Cu₃O₆ assuming ordered vacancy models. The effective atomic charges are used to study the charge transfer. In YBa₂Cu(1)₁Cu(2)₂O₇, the two types of copper atoms have their energy bands almost overlapping with effective valency of each copper as 7/3 (or effective valency of each oxygen as approximately — 13/7), so that electron hopping can take place without any loss or gain of energy while in YBa₂Cu(1)₁Cu(2)₂O₆, Cu(1)₁ is monovalent and Cu(2)₂ are divalent with significant difference in their bands. Therefore, YBa₂Cu₃O₇ should conduct much better compared to YBa₂Cu₃O₆. This corroborates the experimental observations that YBa₂Cu₃O₇ is a (super)conductor while YBa₂Cu₃O₆ is not. The calculated effective charges and DOS support the above view.     

Field modulated microwave absorption in YBa2Cu3O7/PrBa2Cu3O7 multilayers

We describe field modulated microwave absorption measurements on YBa₂Cu₃0₇ / PrBa₂Cu₃O₇ multilayers with PrBa₂Cu₃O₇ thickness in the range 2.5 to 10 nm and with a fixed YBa₂Cu₃O₇ thickness of 10 nm. The Fourier spectrum of the reflected microwave power reveals one fundamental frequency which appears at T_c and even harmonics of the modulation field frequency at lower temperatures. The determination of the irreversibility line near T_c for different values of the PrBa₂Cu₃O₇ thickness and inferred from the vanishing of the nonlinear response signal at 2 in the presence of superimposed dc and modulation fields is reportedit.     

Laser surface refinement of YBa2Cu3O x superconductor

A novel laser-processing technique that produces bulk YBa₂Cu₃O _x (1 2 3) plates has been developed. Through the application of a square CO₂ laser beam with uniform energy density distribution to the surface of 1 2 3 powder compact, a single piece of ribbon-like plate is produced. This plate may be separated from the powder compact after laser scanning. The width of the plate is 6 mm, while its thickness is 0.1–0.2 mm. Powder X-ray diffraction indicates that laser-treated samples contain both orthorhombic and tetragonal 1 2 3 phases, as well as Y₂O₃ (2 0 0), Y₂BaCuO₅ (2 1 1), BaCuO₂ (0 1 1), and CuO (0 0 1) phases. Scanning electron microscopy reveals a pattern of phase segregation along the transverse cross-section after solidification of the plate. After oxygen annealing of a single ribbon piece, T _c is found to be 90 K. This technique may be applicable to the mass production of 1 2 3 bulk superconductor by continuous melting of 1 2 3 powders. In addition to its potential for practical applications, the laser technique also helps to explain the complex phases and microstructure formation during melting and solidification of laser-melted 1 2 3 liquid. A model relating the microstructure to the thermal history inside the laser-affected region and to the phase diagram of incongruently melting 1 2 3 material has been developed to analyse phase formation during laser melting and solidification processes. Reasonable correspondence between theoretical analysis and experimental results was obtained.     

Critical current in YBa2Cu3O7 ceramics

Measurements of critical currents using a four-point direct current (dc) method and an alternating magnetic field method have been performed on several superconducting YBa₂Cu₃O₇ ceramics at 77 K. In the presence of a constant magnetic field, the critical currents obtained with the alternating field method are several orders of magnitude larger than the critical currents measured by the dc method. Also, we observed a minimum in the dc critical current as a function of applied transverse magnetic field. Several authors have suggested that these ceramics behave as individual superconducting grains coupled by Josephson junctions. In this paper, we explain the two observations above using that model.     

Nonmonotonous Variation of the Superconducting Parameters of Neutron Irradiated Li-Doped YBa2Cu3O7−x

The effect of thermal and epithermal neutron irradiation on the superconducting critical temperature and critical current density of some Li-doped YBa₂Cu₃O_7?x samples was studied. The critical temperature exhibits a peak and the critical current density a valley in their dependence on neutron fluence, for moderate dose. A simple model, based on the Van Hove scenario and the kinetics of the defect production, is used to describe both phenomena.     

High-Quality YBa2Cu3O7−x Films with CeO2/YSZ Buffer Layers on 2-Inch Si Wafers Deposited by Pulsed Laser

The pulsed laser deposition (PLD) process is shown for in situ reproducibly fabricating YBa₂Cu₃O_7?x (YBCO) superconducting films with yttrium-stabilized zirconia (YSZ) and CeO₂ buffer layers, nonsuperconducting crystalline YBa₂Cu₃O_7?x (YBCO*) passivation layer, and silver contact film on 2-inch silicon wafers. Variations of less than ±7% in film thickness have been obtained for this multilayer growth over the whole wafer. The YBCO films on 2-inch silicon wafers have homogeneous superconducting properties with zero resistance temperature T _c0 from 88.4 K to 88.9 K. and critical current density J _c at 77 K and zero field from 2.5 × 10⁶ to 7× 10⁶ A/cm². The YSZ, CeO₂ and YBCO layers grow epitaxially on silicon wafers. Full widths at half maximum (FWHMs) of (113) reflections of 40 nm thick YBCO layer from φ-scan patterns are only 1.71° and 1.85° corresponding to the center and edge of the wafer, respectively. These results are very promising for developing high-quality high-T _c superconducting devices on large-area silicon wafers.