Copper-oxygen Chains Research Articles

Structural anomalies of YBa 2Cu 3O 6.9 at the superconducting transition temperature

A single phase specimen of superconducting YBa 2Cu 3O 6.9 has been investigated by high resolution neutron powder diffraction in the temperature range from 16 to 300K. Complete diffraction patterns taken at 16, 82, 92, 101, 183 and 300K have been analysed by profile fitting methods for variations of structural parameters like lattice constants, atomic positions, bond lengths and isotropic thermal vibrations. Structural anomalies have been observed near the superconducting transition temperature at 92K: a discontinuity in the thermal lattice contraction, a discontinuous change of distinct copper-oxygen bonds (e.g. Cul-O4) and characteristic differences in the thermal vibrations of the atomic constituents in the copper-oxygen chains and layers, which represent the essential configurational components in the 1-2-3-structure. There is a correlation between the superconducting state and structural anomalies of both, chains and layers.

SUPERCONDUCTIVITY IN CHAIN-LIKE STRUCTURES

The copper-oxygen chains which can be responsible for superconductivity in high T c oxides of the type YBa 2 Cu 3 O 7−x are idealized by the Hubbard model in the strong coupling limit. A unitary transformation is found by which the large interaction term is exactly diagonalized. It is shown that a system which is open to fermion number fluctuations and described by the Hubbard model in the strong coupling limit behaves always as a superconductor.

New oxide superconductors

Crystals of the new high temperature superconductor Bi 2Sr 3−xCa xCu 2O 8+y have been grown and studied. Single crystal x-ray diffraction data were used to determine a pseudo tetragonal structure based on an A-centered orthorhombic cell with a = 5.399Å,b= 5.414Å,and c= 30.904Å. The structure contains the copper-oxygen sheets as in La 2CuO 4 and YBa 2Cu 3O 7, but the copper-oxygen chains present in YBa 2Cu 3O 7 do not occur in Bi 2Sr 3−xCa xCu 2O 8+y. Electrical measurements on our single crystals show a resistivity drop at about 116 K and apparent zero resistivity at 91 K. We find superconductivity at 120 K in the Tℓ/Ca/Ba/Cu/O system. Also, we have for the first time prepared phases of the type La 2−xNa xCuO 4. These materials are superconducting with T c's as high as 40 K, and they have some unusual magnetic behavior starting at about 10 K.

High sensitivity of positrons to oxygen vacancies and to copper-oxygen chain disorder in YBa2Cu

Temperature-dependent positron-electron momentum densities have been studied by two-dimensional angular correlation of annihilation radiation from 10 to 320 K in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ samples. The positron ground-state charge density, computed by the linearized augmented-plane-wave method, indicates that in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ delocalized positrons sample preferentially the linear copper-oxygen chains. Positron localization due to disorder in these chains is invoked to explain the striking differences observed between superconducting ($x\ensuremath{\simeq}0.02$) and nonsuperconducting ($x\ensuremath{\simeq}0.70$) samples.

New Superconductors Answer Some Questions

The discussion of the new triple-digit superconductors at the 1988 meeting of the American Physical Society in New Orleans emphasized several points about these materials. These thallium-based materials have taken the critical temperature for superconductivity up to 125 K. There seems to be a direct correlation between the copper-oxygen layers and the materials' superconductivity, i.e. the higher the number of layers, the higher the critical temperature. The information presented here seems to establish that only copper-oxygen layers and not necessarily copper-oxygen chains are responsible for the superconductivity. Prediction is made of a four-layer thallium compound that would be superconducting at 150 to 160 K.

A New High-Temperature Superconductor: Bi 2 Sr 3-x Ca x Cu 2 O 8+y

A new superconductor that displays onset behavior near 120 K has been identified as Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y), with x ranging from about 0.4 to 0.9. Single crystal x-ray diffraction data were used to determine a pseudo-tetragonal structure based on an A-centered orthorhombic subcell with a = 5.399 A, b= 5.414A, and c = 30.904 A. The structure contains copper-oxygen sheets as in La(2)CuO(4) and YBa(2)Cu(3)O(7), but the copper-oxygen chains present in YBa(2)Cu(3)O(7) do not occur in Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y). The structure is made up of alternating double copper-oxygen sheets and double bismuth-oxygen sheets. There are Ca(2+) and Sr(2+) cations between the adjacent Cu-O sheets; Sr(2+) cations are also found between the Cu-O and Bi-O sheets. Electron microscopy studies show an incommensurate superstructure along the a axis that can be approximated by an increase of a factor of 5 over the subcell dimension. This superstructure is also observed by x-ray diffraction on single crystals, but twinning can make it appear that the superstructure is along both a and b axes. Flux exclusion begins in our samples at about 116 K and is very strong by 95 K. Electrical measurements on a single crystal of Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y) show a resistivity drop at about 116 K and apparent zero resistivity at 91 K.

NEW SUPERCONDUCTOR: Groups race to decipher structure

A new round of frenetic competition has begun as researchers around the world scramble to decipher the structure and determine the properties of a new class of high-temperature copper oxide superconductors that was announced about three weeks ago by groups in Japan and at the University of Houston (C&EN, Feb. 1, page 5). Du Pont joined the fray last week when it announced that the new type of superconductor, which consists of bismuth, calcium, strontium, copper, and oxygen, lacks a key structural feature that many scientists had thought to be essential for superconductivity near 100 K. That feature is linear chains of alternating copper and oxygen atoms. Such chains are present in the wellknown 1-2-3 class of superconductors typified by yttrium-bariumcopper oxide (YBa 2 Cu 3 0 7 ) According to Du Pont scientists, the absence of these copper-oxygen chains in the new bismuth-containing material may force a revision of current theories that try to explain why these oxides lose all resistance to electricity...

Diffraction evidence for oxygen-vacancy ordering in annealed Ba2YCu3O7- delta (0.3

Diffuse streaks centered at q=(0,\ifmmode\pm\else\textpm\fi{}(2/5,0) and (0,\ifmmode\pm\else\textpm\fi{}(1/2,0) have been observed in annealed ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ with 0.3\ensuremath{\lesssim}\ensuremath{\delta}\ensuremath{\lesssim}0.4, which has a ${T}_{c}$ of 60 K and a Meissner effect equivalent to the 90 K phase. We interpret the presence of the diffuse streaking as a result of short-range oxygen vacancy ordering. The vacancy ordering has a short correlation length along the copper-oxygen chains (b direction) and a long correlation length perpendicular to the chains along the a direction. The ordered structure is completely uncorrelated parallel to the c axis.