B.W. Veal

Time-dependent structural phenomena at room temperature in quenched YBa2Cu3O6.41 : local oxygen ordering and superconductivity

Abstract The superconducting transition temperature, T c , of YBa 2 Cu 3 O 6.41 quenched from 500°C into liquid nitrogen increases from 0 to 20 K while annealing at room temperature during the first few days following the quench. Using neutron powder diffraction and Rietveld refinement we show that this time-dependent increase of T c is accompanied by changes in the structural properties. The a -and c -axes contract by 0.04%, while the percentage shortening of the b -axis is three times smaller. We attribute this behavior to charge transfer between the chains and planes, with the smaller contraction along the b -axis arising from oxygen ordering in the chains. Clear evidence for this charge transfer is seen in the relevant Cu-O and Cu-Cu distances. No significant changes in the average occupancies of oxygen sites are observed. We attribute the increase in T c to local ordering of oxygen atoms around the Cu(1) atoms with no change in the average site occupancies.

Superconducting Gap in Bi-Sr-Ca-Cu-O by High-Resolution Angle-Resolved Photoelectron Spectroscopy

Detailed studies indicate a superconducting gap in the high-temperature superconductor Bi2Sr2CaCu2O8. Photoemission measurements with high energy and angle resolution isolate the behavior of a single band as it crosses the Fermi level in both the normal and superconducting states, giving support to the Fermi liquid picture. The magnitude of the gap is 24 millielectron volts.

Superconductivity in YBa2−xSrxCu3O7−δ

We report structure, resistivity, and Meissner effect measurements on YBa2−xSrxCu3O7−δ for 0<x<2.0. We find a region of solid solubility extending at least to x=1.0 and a monotonic depression of Tc with x. Using arguments based on structural changes with Sr doping, we speculate that the depression of Tc is due to the local distortion of the lattice in the neighborhood of the Sr site and the introduction of additional oxygen vacancies.

Pressure-induced charge transfer and dTc/dP in YBa2Cu3O7−x

Abstract Subtle pressure-induced structural changes in YBa2Cu3O6.93 and YBa2Cu3O6.60 have been measured by neutron powder diffraction for samples in a hydrostatic helium-gas pressure cell. Small, but significant, differences in the compression of particular Cu-O bonds (notably Cu(2)-O(4)) are observed. However, when the charges on the two copper sites are calculated, requiring overall charge conservation versus pressure, it is found that the net pressure-induced charge transfer of holes from Cu(1) to Cu(2) is essentially the same for both systems. We conclude that the much smaller value of dTc/dP for YBa2Cu3O6.93 results from the fact that, in the 90 K superconductor, the Tc has already reached its optimum value and the introduction of additional hole carriers cannot further increase Tc.

Atomic short-range order in oxygen deficient YBa2Cu3O7−σ

Abstract Low field magnetization measurements are presented for a set of oxygen deficient single crystals of YBa 2 Cu 3 O 7−σ . It is demonstrated that the superconducting transition temperature T c not only depends on the oxygen concentration but also on the degree of oxygen vacancy ordering of the samples. Various degrees of ordering were obtained by annealing crystals with reduced oxygen stoichiometries at various temperatures below 350°C and fast quenching them to cryogenic temperatures. Depending on oxygen stoichiometry and annealing temperature, changes in T c of up to 27 K are observed. The observed annealing and quenching behavior is explained in terms of the kinetics of vacancy short-range order in the Cu-O chains. One major conclusion is that the so-called “60 K plateau” in the plot of T c versus σ is completely absent when the crystals are in their most disordered state, i.e., with the smallest degree of vacancy short-range order.

Oxygen vacancy ordering and superconductivity in YBa2Cu3O7−x

Abstract The superconducting properties of YBa 2 Cu 3 O 7−x have been shown to depend markedly on the overall oxygen stoichiometry and on the ordering of oxygen vacancies on the available sites. YBa 2 Cu 3 O 7−x has been studied in situ by high-temperature neutron powder diffraction in controlled oxygen atmospheres in order to learn the structural properties of the system in thermodynamic equilibrium. Additionally, metastable, oxygen-deficient samples produced by quenching into liquid nitrogen from high temperature and various oxygen partial pressures have been studied at room temperature or low temperature in order to correlate the structural and superconducting properties. Such experiments have shown that a systematic relationship exists between the superconducting transition temperature, T c , the number of oxygen vacancies, and the degree of oxygen vacancy ordering. This brief paper reviews our work in these areas.

Dependence of hole concentration on oxygen vacancy order in YBa2Cu3O7−δ: A chemical valence model

Abstract Using a simple chemical model, the hole concentration is calculated for YBa 2 Cu 3 O 7−δ as a function of oxygen stoichiometry δ. For the calculations, it is assumed that 2-coordinated Cu is monovalent while 3- and 4-coordinated Cus are nominally divalent. Because monovalent Cus bind more electrons than do divalent Cus, the degree of hole doping of the conduction band depends on the oxygen vacancy configuration. We examine extreme ordering conditions and apply the chemical valence model to an extended phase field of the double-cell (Ortho II) structure. For δ > 0.2, the dependence of the calculated hole concentration on δ, obtained for the (defect) double-cell structure, is similar to the observed dependence of T c on δ suggesting a simple relationship between T c and hole concentration. If T c is assumed to depend quadratically on hole concentration (as observed for other oxide superconductors), then the calculated T c versus δ corresponds closely with experiment. Low temperature aging behavior is also simulated in the model.

Occurrence of van Hove singularities in YBa2Cu4O8 and YBa2Cu3O6.9

Abstract We report seeing a saddle point in the energy band, known as a van Hove singularity. We measure an exact position of the singularity in both YBa 2 Cu 4 O 8 (Y124) and YBa 2 Cu 3 O 6.9 (Y123). In Y123 the van Hove singularity is at binding energy of less then 10 meV, closer ot E F , than in Y124, which has a singularity at 19 meV. The singularity has and extended character along the Γ-Y direction. Another clearly resolved band exhibits a saddle point singularity at binding energy of 115 meV. Similar singularities are observed in the vicinity of the X point, extended along the Γ-X direction. In order to study these features the energy resolution of the system was improved to better than 10 me V.

EXAFS study of yttria stabilized cubic zirconia

EXAFS (Extended X-ray Absorption Fine Structure) studies of cubic (ZrO2)1−x(Y2O3)x are reported for the concentration range 0.09<x<0.4. When the trivalent Y ions are substituted for the tetravalent Zr ions, O2− vacancies are also introduced for charge compensation. However, the distribution of the O2− vacancies in the structure is not known. In this EXAFS study, systematics of disorder and vacancy distribution on the oxygen sublattice are monitored from both the Zr and Y sites as Y, and hence O2− vacancy content, is varied. It is observed that oxygen vacancies occur adjacent to both Y and Zr ions but preferentially in proximity to Zr.

Synthesis of 80 K superconducting YBa2Cu4O8 via a novel route

Abstract The YBa2Cu4O8 (“124”) superconductor with transition temperature at 80 K is synthesized via a solid state reaction for 1 h at 800°C in flowing oxygen with a total pressure of about 2.7 X 102 Pa, followed by cooling and annealing in oxygen at ambient pressure for 20 h at 750°C. This procedure has produced orthorhombic 124 as the main phase with YBa2Cu3Ox (“123”) as a minor impurity phase. Annealing the as-calcined powder in flowing oxygen at 800°C for 24 h under ambient pressure improved the phase purity and nearly phase-pure 124, determined by X-ray diffraction and magnetization measurements, has been obtained.