Fabrication of YBa2Cu3Ox superconductor using Y2BaCuO5, BaCuO2 and CuO

YBa₂Cu₃O _x superconductor was synthesized using Y₂BaCuO₅, BaCuO₂, and CuO powder mixture. Reaction temperatures were identified using differential thermal analysis (DTA) and thermogravimetry (TG) for syntheses of precursor powders and the powder mixtures. Appropriate reaction temperatures for Y₂BaCuO₅ and BaCuO₂ precursor powders were 950 and 930°C, respectively. Two endothermic reactions involving melt formations were identified on the DTA and TG curves of the powder mixture, and the liquid increased the reactivity of the YBa₂Cu₃O _x formation. Powder mixture samples were sintered at various temperatures ranging from 880 to 1000°C. Microstructural and X-ray powder diffraction studies showed YBa₂Cu₃O _x and impurities to be formed in the samples sintered at various temperatures. The samples sintered at 990 and 1000°C showed dense microstructures. The critical temperature was 84 K for the sample sintered at 880°C and rose to 92 K as the sintering temperature increased.     

Standard Gibbs' energies of formation of BaCuO2, Y2Cu2O5 and Y2BaCuO5

The Gibbs' energies of formation of BaCuO₂, Y₂Cu₂O₅ and Y₂BaCuO₅ from component oxides have been measured using solid state galvanic cells incorporating CaF₂ as the solid electrolyte under pure oxygen at a pressure of 1.01×10⁵ Pa BaO + CuO BaCuO₂ G _f,ox ^o (± 0.3) (kJ mol^–1)=–63.4–0.0525T(K) Y₂O₃ + 2CuO Y₂Cu₂O₂ G _f,ox ^o (± 0.3) (kJ mol^–1)=18.47–0.0219T(K) Y₂O₃ + BaO + CuO Y₂BaCuO₅ G _f,ox ^o (± 0.7) (kJ mol^–1)=–72.5–0.0793T(K) Because the superconducting compound YBa₂Cu₃O_7– coexists with any two of the phases CuO, BaCuO₂ and Y₂BaCuO₅, the data on BaCuO₂ and Y₂BaCuO₅ obtained in this study provide the basis for the evaluation of the Gibbs' energy of formation of the 1-2-3 compound at high temperatures.     

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.     

Effects of Y2BaCuO5 on the directional growth of YBa2Cu3Ox superconductor

The effects of Y₂BaCuO₅ on the microstructure and superconducting properties of YBa₂Cu₃O_x samples, textured using the directional growth of Y₂BaCuO₅, BaCuO₂ and CuO powder mixtures, were studied. Y₂BaCuO₅ moved downward as the sample passed up through the hot zone, and this downward movement affected the superconducting properties of the directionally grown samples. The critical temperature increased, and the resistance transition width decreased as the excess 211 content in the powder mixture increased up to a value, beyond which the temperature decreased and the width increased. The critical current densities of the directionally grown samples were scattered because of the presence of uncontrollable severe cracks perpendicular to the growth direction. As the hot-zone temperature increased, the critical current density of the directionally grown sample increased up to a maximum value (>6000 A cm^–2) beyond which the critical current density decreased. Compared to data from other studies, the hot-zone temperature required to produce a maximum critical current density was lower.     

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.     

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.     

Theory of electron distributions and63Cu and17O nuclear quadrupole interactions in YBa2Cu3O7 and YBa2Cu3O6

Ab initio unrestricted Hartree-Fock Cluster investigations have been carried out on the electronic structures of the YBa₂Cu₃O₇ and YBa₂Cu₃O₆ systems. The results of these investigations provide satisfactory explanations of available⁶³Cu and¹⁷O nuclear quadrupole interaction data. The electron distributions obtained rule out the presence of Cu³⁺ ions and are supportive of the presence of Cu²⁺, Cu¹⁺, O¹⁻ ions in the O₇ system and Cu²⁺, Cu¹⁺ and O²⁻ in the O₆ system with actual charges departing significantly from the formal charges, especially in the O₇ system, indicating the importance of covalency effects. Suggestions will be made regarding possible sources that can bridge the remaining gap between theoretical and experimental results for the nuclear quadrupole interaction parameters.     

Electronic specific heat of YBa2Cu3O6+x from 1.8 to 300 K

We have determined for the first time the electronic specific heatγ(x, T) of YBa₂Cu₃O_6+x for 0.16≤x≤0.97 from 1.8 to 300 K. Weakly superconducting behavior betweenx=0.4 and 0.8 progresses rapidly to strong coupling BCS-like superconducting and metallic normal state behavior forx> 0.9. However, the continuous development of the entropyS(x, T) withx andT across the entire series suggests a progressive modification of the low-energy spin spectrum with hole doping rather than a simple band model. Fermi statistics andk-space pairing for allx is indicated by the magnitude andT-dependence ofS(x, T). Pseudogap behavior inS(x, T) is observed over a temperature region aboveT _c , which increases rapidly with oxygen depletion to around 200 K forx～0.7. This loss of entropy reflects normal-state correlations apparently unrelated to the superconducting pairing.     

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.     

Temperature-dependent change of Cu-O bond in La2CuO4 and YBa2Cu3O7

Valence-band spectra of La₂CuO₄ and YBa₂Cu₃O₇ were obtained by using X-ray photoemission varying the temperature of the measurement (35, 260, 300, and 500 C). In La₂CuO₄ the broad band centred around 4eV splits into two peaks at 260 C. At both 35 and 500 C the spectra show almost similar shape. In YBa₂Cu₃O₇ the broad peak centred around 4eV splits into two peaks at 500 C. Below 300 C the spectra show almost similar shape. These splittings of the valence-band spectra may be due to the rearrangement of the crystal structure accompanying the phase transition from the orthorhombic to tetragonal symmetry.     

Vapor pressures of Y,Ba, Cu precursors for the growth of YBa2Cu3O7 by MOVPE

The vapor pressures of theβ-diketonates Y(THD), Ba(THD), and Cu(THD), commonly used as precursors for MOVPE of YBa₂Cu₃O₇, and Y(MCP) were measured at different temperatures. A time-resolved static method recording the pressure vs. time at constant temperature was used, permitting us to deduce the vapor pressure even if the materials tend to decompose. The values of the constants of the Clausius-Clapeyron equation log₁₀p(T)/p ₀=A-B/T are,A=11.7, 8.7, 8.27, 16.6 andB=4359, 2654, 3602, and 6453 K for Y(THD), Y(MCP), Ba(THD), and Cu(THD), respectively withp ₀= 1 Pa and temperatureT in K. The thermal stability of the sources was measured and are discussed.     

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.     

Reaction kinetics in the superconducting YBa2Cu3O7-x and SmBa2Cu3O7x

The compounds under investigation were prepared with nominal composition MBa₂ Cu₃ O_7-x, M being Y or Sm, through solid state reaction starting from Y₂O₃, Sm₂O₃, BaCO₃ and CuO powders. Reactions were investigated by thermo-gravimetric and differential thermal analysis in the temperature range 900–980°C. Reaction kinetics of MBa₂Cu₃O_7-x solid state synthesis were evaluated according to the mathematical relationships theoretically derived for phenomenological models. In the Y-based system the phase boundary model provides a reaction kinetics suitable to describe the process, while a three dimensional diffusion model fits the data in the Sm-based system. The influence of the mixing method on the reaction kinetics was evaluated. Microstructural features of superconducting samples were analyzed.     

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.     

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.