Field-Dependent Microwave Absorption in High-Tc Superconductors

Substantial field-dependent microwave absorption is found in superconducting materials of the YBa2Cu3O7 class. Abrupt changes occur over a 20-G range at zero field which are qualitatively different for polycrystalline and single-crystal materials. A strong hysteresis in polycrystalline samples is quenched by modulation fields of 0.2 Gpp. Formally the dissipative processes observed may be described in terms of flux relaxation, but the underlying physical mechanisms remain unresolved. As measurements on 100 g specimens can be performed in a few seconds, this property may prove useful for material characterization

Microwave Loss and Oxygen Annealing in Yba2cu3ox Single Crystals

Reversible thermal annealing of YBa2Cu3Ox single crystals in air has been studied by microwave loss measurements. Among the advantages of this contactless technique are the following: Crystals are kept in sealed capillaries throughout cycles of annealing and low-temperature measurement; features are resolved in loss versus temperature measurements that are not revealed by other techniques. Microwave loss is interpreted in terms of the Gittleman-Rosenblum model of fluxon motion in a viscous medium. Narrow temperature ranges are found for the formation of the 60- and 90-K phases. Optimal microwave response is obtained after but a few hours of annealing, indicating the importance of c-axis diffusion

Bulk Superconductivity in Single Cuo Layer Bi-Sr-Cu-O Ceramics

Despite early recognition that a relatively low-temperature superconductor occurs in samples of approximate composition Bi2Sr 2CuO6, studies of this material have been plagued by difficulties in the preparation of single phase material with a large superconducting fraction. We report the first preparation of ceramic samples in the Bi-Sr-Cu-O system that show bulk superconductivity at 4-15 K. Structural, chemical, and electronic characterizations of superconducting and nonsuperconducting ceramics of composition Bi2.05Sr 1.95CuO6 are described

Preparation and Solid State Characterization of 1,2,3,5-Diselenadiazolyl [HCN2Se2]·

The reaction of N,N,N'-tris(trimethylsilyl)formamidine with SeCl2 affords 1,2,3,5-diselenadiazolium chloride, which can be reduced with triphenylantimony to the corresponding diselenadiazolyl radical [HCN2Se2]·; the solid state structure and conductivity of the dimer [HCN2Se2]2 are reported.

Growth of Superconducting Single Crystals in the Bi-Sr-Ca-Cu-O: System from Alkali Chloride Fluxes

The discovery of superconductivity at temperatures above the boiling point of nitrogen has led to intense interest in the physics and chemistry of materials exhibiting this behaviour. Although some experiments can be performed on sintered polycrystalline samples, large single crystals are important for many measurements of physical properties, including investigations of the relationship between structural parameters and superconducting properties. Like its predecessors YB2Cu3O7 (refs 1-4) and La2-xSrxCuO4 (ref. 5), the recently discovered 84-K bismuth-strontium-calcium cuprate superconductor6-9 melts incon-gruently, and crystals have been grown from eutectic melts10. This technique yields crystals large enough for some experiments, but is generally difficult to control, placing limits on the size of crystals that can be grown. In addition, the crystals must be mechanically separated from the melt. Here we report the growth of single crystals of Bi2.2Sr 2Ca0.8Cu2O8+δ from alkali chloride fluxes. The crystals are superconductors, showing large Meissner effects and zero-resistance transitions above 80 K. Flux growth is a standard high-temperature solution growth technique, and should allow crystal size and quality to be controlled by parameters such as equilibration temperature, cooling rate and melt composition. The fluxes reported here are easily washed from the crystals

Stoichiometry and Superconductivity in Single Layer Bi2+Xsr2-YCuO6+-Δ

An insulating polymorph with a composition close to Bi2Sr2CuO6 complicates the synthesis of the single copper layer superconductor Bi2+xSr2-yCuO6±δ. We concentrate on the region where the material is single phase in air synthesis (2.18 \u3c x \u3c 2.29, 1.44 \u3c y \u3c 1.7). The physical properties from insulating to marginally conducting as the composition moves toward x = y = 0. Like to two-layer compound, the material has a superlattice due to a buckling of the planar structure, however the superlattice coherence is degraded by defects at finite values of x and y. Density measurements indicate that the nonstoichiometry of Bi2+xSr2-y CuO6±δ is not accompanied by the formation of a large number of vacancies. The insulating polymorph, Bi2Sr2CuO6, can be suppressed by synthesis in modest oxygen pressures (above 5-10 atm). The result is a bulk superconductor (Tc ≈ 10 K)

Crystal Growth and Substitutional Chemistry of Pb2Sr2MCu3O8

We report the crystal growth and ceramic preparation of Pb2Sr2MCu3O8 phases containing a variety of lanthanoids and lanthanoid/alkaline-earth metal solid solutions. These recently discovered high-temperature superconductors add to our understanding of the chemical and structural features key to superconductivity in cuprates. The growth of millimeter-sized crystals from PbO-rich fluxes is described. Ceramic samples of the Pb2Sr2MCu3O8 phase are formed under low oxygen partial pressures for the majority of the lanthanoids. Systematic structural changes are observed as a function of lanthanoid size. The substitution of alkaline-earth metals on the lanthanoid site and the substitution of barium for strontium to form Pb2Ba2YCu3O8 are also described

Superconductivity Near 70 K in a New Family of Layered Copper Oxides

A new family of high-temperature superconductors is described, with the general formula Pb 2 Sr 2 ACu 3 O 8+δ . Although they have the planes of CuO 5 square pyramids characteristic of the other copper-oxide superconductors, the new compounds belong to a distinct structural series, with wide scope for elemental substitution. Their unusual electronic configuration also gives new insight into the role of charge distribution among the structural building blocks in controlling superconductivity

Experimental and Theoretical Determination of the Magnetic Susceptibility of C60 and C70

THE magnetic susceptibility of C60 and the possibility of magnetic-field-induced π-electron ring currents in this carbon spheroid have been of interest since the initial experiments on carbon clusters1. If the molecule is regarded as a sphere with a radius of 3.5 Å, on which 60 electrons are free to move, the Pauling ring-current model predicts a ring-current diamagnetic susceptibility 41 times the π-electron ring-current magnetic susceptibility of benzene with the field normal to the plane of the six-membered ring2,3. London theory predicts, however, that the π-electron ring currents in C60 should be weakly paramagnetic or diamagnetic, depending on the relative bond strengths used in the calculation2,3. With the availability of macroscopic quantities of C60 (ref. 4), it is now possible to study experimentally the magnetic properties of the molecule. Here we report on such measurements. We find that the diamagnetism of C60 is small, a result that we attribute to excited-state paramagnetic contributions to the π-electron ring-current magnetic susceptibility. Thus C60 seems to be an aromatic molecule with a vanishingly small π-electron ring-current magnetic susceptibility. We have performed similar measurements on C70, which indicate an appreciable π-electron diamagnetism, consistent with theoretical calculations. We attribute the differences in magnetic properties of these two molecules to their different fractions of five-membered ring structures. The fullerenes may thus constitute a class of compounds of \u27ambiguous\u27 aromatic character, traditional measures of which will not provide an adequate classification

Structure and Physical Properties of Single Crystals of the 84-K Superconductor Bi2.2Sr2Ca0.8Cu2O8+

Single crystal sof the 84-K superconductor Bi2.2Sr2Ca0.8Cu2O8+ were characterized by x-ray diffraction, dc magnetic susceptibility, electrical resistivity, and microwave absorption. The structure has [CuO2] planes separated by calcium atoms, edge-shared bismuth oxide double layers, and an incommensurate superlattice along b with a period of 4.76. The in-plane resistivity above Tc is linear in T, with RT=130 cm. Initial results on Pb substitution yielding Tc\u27s of 107 K are reported