Polarized and unpolarized triple-axis neutron measurements were performed on an underdoped crystal of YBa 2 Cu 3 O 7−x (x = 0.4 ± 0.02, T c = 62.7 K).Our results indicate, contrary to earlier evidence, that the spin excitations in the superconducting state are essentially the same as those in the fully doped material except that the unusual 41 meV resonance is observed at 34.8 meV. The normal state spin excitations are characterized by a weakly energy-dependent continuum whose temperature dependence shows the clear signature of a superconducting gap at T c . The enhancement at the resonance is accompanied by a suppression of magnetic fluctuations at both higher and lower energies. (below which no magnetic intensity can be found) which is much less than 2∆. They also reported a complicated temperature dependence of χ ′′ (q, ω), but found no resonance coupled directly to T c .We report here the first polarized neutron scattering measurements on an underdoped single crystal of (123)

The effect of ambient oxygen pressure on the synthesis of epitaxial YBa2Cu3O7−x films on (100) SrTiO3 substrates by post-deposition annealing of amorphous precursor films was studied for oxygen partial pressures pO2 between 1.0 and 8.0×10−5 atm and annealing temperatures between 890 and 650 °C. A pO2−1/T diagram containing recent literature data regarding YBa2Cu3O7−x oxygen stoichiometry, phase stability, and liquid-phase formation was compiled to provide guidance for the selection and interpretation of annealing conditions. The results evidence a strong dependency of growth properties on the oxygen pressure with enhanced c-oriented epitaxy at lower pO2 values. A particularly interesting result is the formation of predominantly c-oriented films at 740 °C and pO2=2.6×10−4 atm (0.2 Torr). Similar to YBa2Cu3O7−x films produced by in situ laser ablation at the same temperature and oxygen pressure, the films exhibited low ion channeling yields (χmin<0.1) and a dense (smooth) surface morphology, while critical currents at 77 K were well in excess of 1 MA/cm2. From the observed systematic variation of structural film properties with synthesis conditions, annealing lines were derived indicating (T-pO2) combinations for either c- or a-oriented epitaxial growth. A comparison is made between these lines and synthesis conditions for in situ film growth as compiled recently by Hammond and Bormann [Physica C 162–169, 703 (1989)].

The entire ** data base for the dependence of the nonstoichiometry, x, on temperature and chemical potential of oxygen (oxygen potential) was retrieved from the literature and represented. This data base was interpreted by least-squares analysis using equations derived from the classical thermodynamic theory for the solid solution of a solute in a solvent. For hyperstoichiometric oxide at oxygen potentials more positive than -266700 + 16.5T kJ/uiol, the data were best represented by a [U0 2 ]-[U3O7] solution. For 0/U ratios abo/e 2 and oxygen potentials below this boundary, a J.UO 2 ]-[U 2 Oi t m 5] solution represented the data. The data were represented by a [UO 2 J-[U2/3] solution. The resulting equations represent the experimental ln(pQ )-ln(x) behavior and can be used in thermodynamic calculations to predict phase boundary compositions consistent with the literature. Collectively, the present analysis permits, for : the first time, a mathematical representation of the behavior of the total data base.

. The resulting equations well reproduce the large amount of oxygen potential-temperature-composition data for the mixed oxide system, all of which were also retrieved from the literature. These models are the first that appear to display the appropriate oxygen potential-temperature-composition and phase relation behavior over the entire range of existence for the phases.

The dependence of the nonstoichiometry of (YBa,Cu,O,-,) (solid) has been studied over 5 orders of magnitude in oxygen pressure and from 573 to 1173 K. Hydrogen-reduction methods fpr determining the absolute oxygento-metal ratio were developed. The resulting data were used to derive a chemical thermodynamic representation of the experimental variables. The data were also compared with several other investigations to identify the selfconsistent sets of data. The present data and thermodynamic data from the literature were correlated on an Ellingham diagram. [