Processing Techniques for the 93 K Superconductor Ba2YCu3O7

Superconductivity above the temperature of liquid nitrogen in copper oxide-based systems has led to optimism that superconductors may at last find wide application. The critical temperature, however, is just one of the required parameters. The materials must be made into usable forms such as wire, thick films, thin films, and bulk ceramics. In addition, the critical current in these various forms is a crucial test of their utility. This article reviews the processing techniques used to fabricate potentially useful forms and assesses remaining problems. Considerable improvement in critical current density is necessary in bulk materials, and thin films need to be made compatible with other thin-film technology.     

Bulk Superconductivity at 122 K in T1(Ba,Ca)2Ca3Cu4O10.5+8 with Four Consecutive Copper Layers

The observed increase of superconducting transition temperature (T(c)) with the number of copper oxide planes continues in the four-[CuO(2)](-2) layer (single TI layer) oxide superconductor, which has been prepared with > 80% purity and was magnetically aligned for crystallographic identification. A master scaling curve is proposed, which ties together the T(c)'s of virtually all known Bi and Tl oxide superconductors, and shows that the Tl(Bi) layers play an essential role in the superconductivity. publication 350 of the Barnett Institute.     

A New High-Temperature Superconductor: Bi2Sr3-x Cax Cu2O8+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.     

A-Axis--Oriented YBa2Cu3O7/PrBa2Cu3O7 Superlattices

A modulated structure has been fabricated from high transition temperature superconductors where the individual CuO(2) planes are composed of alternating superconducting and insulating strips. This structure is made by growing a-axis-oriented YBa(2)Cu(3)O(7)/PrBa(2)Cu(3)O(7) superlattices by 90 degrees off-axis sputtering on (100)SrTiO(3) and (100)LaAlO(3) substrates. Superlattice modulation is observed to a modulation wavelength of 24 angstroms (12 angstroms-YBa(2)Cu(3)O(7)/12 angstroms-PrBa(2)Cu(3)O(7)), both by x-ray diffraction and by cross-sectional transmission electron microscopy. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield of 3 percent. Quasi-one-dimensional conductivity should be obtainable in these structures.     

Long-range incommensurate charge fluctuations in (Y,Nd)Ba2Cu3O(6+x)

The concept that superconductivity competes with other orders in cuprate superconductors has become increasingly apparent, but obtaining direct evidence with bulk-sensitive probes is challenging. We have used resonant soft x-ray scattering to identify two-dimensional charge fluctuations with an incommensurate periodicity of ~3.2 lattice units in the copper-oxide planes of the superconductors (Y,Nd)Ba(2)Cu(3)O(6+)(x), with hole concentrations of 0.09 to 0.13 per planar Cu ion. The intensity and correlation length of the fluctuation signal increase strongly upon cooling down to the superconducting transition temperature (T(c)); further cooling below T(c) abruptly reverses the divergence of the charge correlations. In combination with earlier observations of a large gap in the spin excitation spectrum, these data indicate an incipient charge density wave instability that competes with superconductivity.     

Inclined-Field Structure, Morphology, and Pinning of the Vortex Lattice in Microtwinned YBa2Cu3O7

A detailed small-angle neutron scattering study of the vortex lattice in a single crystal of YBa(2)Cu(3)O(7) was made for a field of 0.5 tesla inclined at angles between 0 and 80 degrees to the crystalline c axis. The vortex lattice is triangular for all angles, and for angles less than or equal to 70 degrees its orientation adjusts itself to maximize the pinning energy to densely and highly regularly spaced twin planes. These observations have important implications for the microscopic flux-pinning mechanism, and hence for the critical current achievable in YBa(2)Cu(3)O(7). For large angles (about 80 degrees) the vortex lattice consists of independent chains in the orientation predicted by anisotropic London theory.     

Cu、Zn对烤烟幼苗吸收K、Ca、Fe的影响

研究了溶液培养条件下Cu．Zn对烟草幼苗吸收K．Ca、Fe的影响。结果表明，烟草幼苗对Cu．Zn的吸收随溶液中Cu^2 、Zn^2 浓度的升高而增加，Zn对Cu的吸收起抑制作用。Ca吸收随溶液Cu^2 浓度的升高而呈下降趋势，而随溶液Zn^2 浓度的升高而有增加趋势；K吸收随溶液中Cu^2 、Zn^2 浓度的增大在茎叶中有升高趋势，而在根部有下降趋势；幼苗对于Fe吸收随溶液中Cu^2 、Zn^2 浓度的增大而略有增加，但差异不显著。     

Crystal Structure of Kernite, Na2B4O6(OH)2 {middle dot} 3H2

Kernite, Na(2)B(4)O(6)(OH)(2). .3H(2)0, contains parallel infinite chains of the borate polyanion[B(4)O(6)(OH)(2)](n)(2n-). The chains are composed of six-membered rings containing one boron-oxygen triangle and two boron-oxygen tetrahedra. The rings are linked through commonly shared boron-oxygen tetrahedra.     

An Inorganic Double Helix: Hydrothermal Synthesis, Structure, and Magnetism of Chiral [(CH3)2NH2]K4[V10O10(H2O)2(OH)4(PO4)7]{middle dot}4H2O

Very complicated inorganic solids can be self-assembled from structurally simple precursors as illustrated by the hydrothermal synthesis of the vanadium phosphate, [(CH(3))(2)NH(2)]K(4)[V(10)O(10)(H(2)O)(2)(OH)(4)(PO(4))(7)].4H(2)O, 1, which contains chiral double helices formed from interpenetrating spirals of vanadium oxo pentamers bonded together by P(5+). These double helices are in turn intertwined with each other in a manner that generates unusual tunnels and cavities that are filled with (CH(3))(2)NH(2)(+) and K(+) cations, respectively. The unit cell contents of dark blue phosphate 1, which crystallizes in the enantiomorphic space group P4(3) with lattice constants a = 12.130 and c = 30.555 angstroms, are chiral; only one enantiomorph is present in a given crystal. Magnetization measurements show that 1 is paramagnetic with ten unpaired electrons per formula unit at higher temperatures and that antiferromagnetic interactions develop at lower temperatures.     

Possible Evidence for Superconducting Layers in Single Crystal YBa2Cu3O7-x by Field Ion Microscopy

The high-transition-temperature superconducting ceramic material YBa(2)Cu(3)O(7-x) (0< x < 0.5) has been examined by field ion microscopy. Specimens from nominally superconducting and nonsuperconducting samples(determined by magnetic susceptibility measurements) were studied by field ion microscopy and significant differences were found. Preferential imaging of atomic or molecular layers, due to preferential field evaporation, field ionization, or both, was found in the superconducting phase below the transition temperature and is interpreted as possible evidence for the occurrence of relatively highly conducting layers in the YBa(2)Cu(3)O(7-x) unit cell perpendicular to the orthorhombic c-axis. Similar results were obtained for YbBa(2)Cu(3)(7-x).     

Confirmation of the Crystal Structure of Tetrahedrite, Cu12Sb4S13

The crystal structure of tetrahedrite, proposed in 1934 by Pauling and Neuman, has been confirmed. Least-squares refinement reduced the initial disagreement factor from 27.9 to 4.9 percent. Antimony displays three short orthogonal bonds to sulfur atoms. Two types of copper are tetrahedrally and triangularly coordinated by sulfur, respectively.     

Djurleite (Cu1.94S) and Low Chalcocite (Cu2S): New Crystal Structure Studies

Additional x-ray structure studies on low chalcocite generally confirm the previously reported structure but show that either disorder is present or the true space group is not P2(1)/c but Pc, four of the 96 copper atoms in the monoclinic unit cell taking on twofold (linear) coordination. The crystal structure of djurleite has been solved in space group P2(1)/n, the monoclinic cell having parameters a = 26.897, b = 15.745, and c = 13.565 angstroms; beta = 90.13 degrees ; and a content of 248 copper and 128 sulfur atoms. Of the 62 different copper atoms in the structure, 52 are in threefold, triangular coordination with sulfur, nine in tetrahedral, and one in linear coordination.     

Krinovite, NaMg2CrSi3O10: A New Meteorite Mineral

An unusual new silicate, krinovite, has been discovered within graphite nodules in three iron meteorites. Its ratio of silicon to oxygen of 3 : 10 suggests a rare kind of silicate polymerization. The meteorite nodules in which it occurs exhibit a chemical fractionation that differs from that of both stone meteorites and terrestrial basalt.     

Crystal structure of the human K2P TRAAK, a lipid- and mechano-sensitive K+ ion channel

TRAAK channels, members of the two-pore domain K(+) (potassium ion) channel family K2P, are expressed almost exclusively in the nervous system and control the resting membrane potential. Their gating is sensitive to polyunsaturated fatty acids, mechanical deformation of the membrane, and temperature changes. Physiologically, these channels appear to control the noxious input threshold for temperature and pressure sensitivity in dorsal root ganglia neurons. We present the crystal structure of human TRAAK at a resolution of 3.8 angstroms. The channel comprises two protomers, each containing two distinct pore domains, which create a two-fold symmetric K(+) channel. The extracellular surface features a helical cap, 35 angstroms tall, that creates a bifurcated pore entryway and accounts for the insensitivity of two-pore domain K(+) channels to inhibitory toxins. Two diagonally opposed gate-forming inner helices form membrane-interacting structures that may underlie this channel's sensitivity to chemical and mechanical properties of the cell membrane.     

Epitaxial and Smooth Films of a-Axis YBa2Cu3O7

YBa(2)Cu(3)O(7) films have been grown epitaxially on SrTiO(3) (100) and LaAlO(3) (100) substrates with nearly pure a-axis orientation and with transition temperature T(c) (R = 0) of 85 K. A unique feature of these films is their smooth surface. These smooth surfaces enable the growth of short-period superlattices with well-defined modulations. The films are untwinned and the grains grow with their c-axis along one of two perpendicular directions on the substrate ([100] or [010]). The fabrication of sandwich-type Josephson junctions with good characteristics may now be possible because unlike c-axis-oriented films, the superconducting coherence length of these smooth films is appreciably large perpendicular to their surfaces.     

Dislocations and Flux Pinning in YBa2Cu3O7-delta

Bulk YBa(2)Cu(3)O(7-delta) superconductors, under certain processing conditions such as melt texturing, exhibit a very high dislocation density of 10(9) to 10(10) per square centimeter. In addition, the density of low-angle grain boundaries in such samples can be significantly increased (to less than 700-nanometer spacing) through a dispersion of submicrometer-sized Y(2)BaCuO(5) inclusions. These defect densities are comparable to those in high critical current thin films as revealed through scanning tunneling microscopy, and yet the critical current densities in the bulk materials (at 77 kelvin and a field of 1 tesla for example) remain at a 10(4) amperes per square centimeter level, about two orders of magnitude lower than in thin films. The results imply that these defect density levels are not significant enough to explain the difference in flux pinning strength between the thin film and bulk materials. The observation of spiral-like growth of the superconductor phase in bulk Y-Ba-Cu-O is also reported.     

Nuclear Resonance Properties of YBa2Cu3O6+x Superconductors

Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) results for copper-63, oxygen-17, and yttrium-89 nuclei in the superconducting composition range of YBa(2)Cu(3)O(6+x) (0.4 相似文献