A 57 Fe Mössbauer study on the FeSe and Fe(Se,Te) superconductors: discontinuities in the hyperfine parameters at T c

Polycrystalline samples of FeSe_0.82 and FeSe_0.5Te_0.5 were synthesized using a solid-state reaction route. Bulk superconductivity was confirmed using SQUID magnetometry. The onset of T _c was at 8.0 K for FeSe_0.82 and 12.5 K for FeSe_0.5Te_0.5. Paramagnetic ⁵⁷Fe Mössbauer spectra were recorded at temperatures between 5.4 and 320 K in transmission geometry. All spectra exhibited simple quadrupole splitting. For FeSe_0.5Te_0.5 a small drop in the quadrupole splitting was observed about T _c upon cooling. Additionally, for both samples the isomer shift and the total absorption started to drop around T _c , indicating a softening of the lattice. The drop is estimated to correspond to at least 60 K from the original Debye temperatures.     

Long‐chain‐branched polyethene by the copolymerization of ethene and nonconjugated α,ω‐dienes

Ethene was copolymerized (1) with 1,5‐hexadiene with rac‐ethylenebis(indenyl)zirconium dichloride/methylaluminoxane (MAO) used as a catalyst and (2) with 1,7‐octadiene with bis(n‐butylcyclopentadienyl)zirconium dichloride/MAO and rac‐ethylenebis(indenyl)hafnium dichloride (Et[Ind]₂HfCl₂)/MAO used as catalysts at 80 °C in toluene. The copolymer microstructure and the influence of diene incorporation on the rheological properties were examined. Ethene and 1,5‐hexadiene formed a copolymer in which a major fraction of the 1,5‐hexadiene was incorporated into rings and a small fraction formed 1‐butenyl branches. The copolymerization of ethene with 1,7‐octadiene resulted in a higher selectivity toward branch formation. Some of the branches formed long‐chain‐branching (LCB) structures. The ring formation selectivity increased with decreasing ethene concentration in the polymerization reactor. Melt rheological properties of the diene copolymers resembled those of metallocene‐catalyzed LCB homopolyethenes and depended on the vinyl content, the catalyst, and the polymerization conditions. At high diene contents, all three catalysts produced crosslinked polyethene. This was especially pronounced with Et[Ind]₂HfCl₂, where only 0.2 mol % 1,7‐octadiene in the copolymer was required to achieve significantly modified rheological properties. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3805–3817, 2001     

Numerical valuation of options under Kou's model

Numerical methods are developed for pricing European and American options under Kou's jump-diffusion model which assumes the price of the underlying asset to behave like a geometrical Brownian motion with a drift and jumps whose size is log-double-exponentially distributed. The price of a European option is given by a partial integro-differential equation (PIDE) while American options lead to a linear complementarity problem (LCP) with the same operator. Spatial differential operators are discretized using finite differences on nonuniform grids and time stepping is performed using the implicit Rannacher scheme. For the evaluation of the integral term easy to implement recursion formulas are derived which have optimal computational cost. When pricing European options the resulting dense linear systems are solved using a stationary iteration. Also for pricing American options similar iterations can be employed. A numerical experiment demonstrates that the described method is very efficient as accurate option prices can be computed in a few milliseconds on a PC. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Superconductivity with transition temperature up to 80 K for TbSr2Cu2.7Mo0.3O7+δ

A Tb-123 phase with the composition, TbSr₂Cu_2.7Mo_0.3O_7+δ, has been synthesized in single-phase form by the solid-state reaction route. Its phase purity has been confirmed from neutron powder diffraction studies. The as-synthesized Tb-123 sample does not show superconductivity down to 5 K. On the other hand, an unusually high antiferromagnetic ordering temperature (T_N) of around 7 K is seen for the Tb moments. After 120-atm-O₂ post-annealing, bulk superconductivity is achieved in this compound with a superconducting transition temperature (T_c) of about 30 K, without any significant effect on T_N. To achieve higher oxygen content and higher T_c, the as-synthesized sample was subjected to high-pressure oxygenation, carried out in a closed cell, at 5 GPa and 400 °C in the presence of AgO as an excess-oxygen source. This sample exhibited superconductivity onset at around 80 K with a Meissner fraction larger than 10% at 5 K. Our observation of superconductivity at 80 K is the highest T_c to-date for the Tb-123 type compounds.     

bb̄-fragmentation and Bπ correlations

We show that experimental data are in very good agreement with predictions from the string fragmentation model by Bowler and Morris. We present a physical interpretation and discuss the relation to results obtained from Perturbative QCD and Local Hadron Parton Duality (LPHD). We also present implications for Bar correlations and the possibility to use these as a tag to study CP violation in B decays.     

Oxygen nonstoichiometry and valence of copper in the Cu-1222 superconductor

Essentially single-phase Cu(Ba_0.67Eu_0.33)₂(Ce_0.33Eu_0.67)₂Cu₂O_9±δ samples of the Cu-1222 phase with a fluorite-structured (Ce,Eu)-O₂-(Ce,Eu) block between the superconductive CuO₂ planes were synthesized in O₂ atmosphere to exhibit superconductivity with T_c around 25 K. Wide-range tuning of oxygen content and thereby the overall hole-doping level of the phase was found possible through (i) temperature-controlled oxygen-depletion (TCOD) annealing carried out in a thermobalance in N₂ at various temperatures (for reduction), and (ii) high-pressure oxygenation (HPO) treatments carried out in a cubic-anvil-type high-pressure apparatus in the presence of various amounts of Ag₂O₂ as an excess oxygen source (for oxidation). For the HPO samples a record-high T_c value of 62 K was achieved. On the other hand, deoxygenation to the oxygen content less than 9±δ≈8.9 was found to kill superconductivity. The degree of hole doping (both in overall and for the CuO₂ plane and CuO_1±δ charge reservoir separately) in the samples is discussed on the bases of Cu L-edge and O K-edge XANES data.     

Hole doping into Co-12s2 copper oxides with s fluorite-structured layers between CuO2 planes

In this work, the first three members (s=1, 2, 3) of the Co-12s2 homologous series of multi-layered copper oxides are gradually doped with holes through high-pressure oxygenation (HPO). The phases differ from each other only by thickness of the fluorite-structured layer block, (Ce,Y,Ca)-[O₂-(Ce,Y)]_s−1, between two identical CuO₂ planes. High-resolution transmission-electron microscopy (HRTEM) and electron diffraction (ED) analyses together with both synchrotron X-ray and neutron powder diffraction data, reveal that as a consequence of HPO the charge-reservoir CoO₄-tetrahedra chains get broken and the lattice symmetry of the Co-12s2 phases changes from orthorhombic to tetragonal. Oxygen contents are analyzed for the samples with wet-chemical and thermogravimetric techniques. The valence state of copper in the CuO₂ plane is determined from Cu L-edge X-ray absorption near-edge structure (XANES) spectra to be compared with the values estimated through bond-valence-sum (BVS) calculations from the crystal structure data. The positive charge induced by oxygen loading (or aliovalent Ca^II-for-Y^III substitution in CoSr₂YCu₂O_7+δ) is found not to be completely accommodated in the CuO₂ planes but be rather effectively trapped at the charge-reservoir Co atoms. Superconductivity appears in the Co-1212 (CoSr₂YCu₂O_7+δ) samples with the copper valence of 2.13 or higher, whereas in the Co-1222 (CoSr₂(Ce_0.25Y_0.75)₂Cu₂O_9+δ) and Co-1232 (CoSr₂(Ce_0.67Y_0.33)₃Cu₂O_11+δ) samples Cu valence does not increase high enough to induce superconductivity.     

Factorization of holomorphic mappings on -spaces

We prove a universal mapping theorem for a large class of holomorphic mappings on a -space, stating that can be locally written in the form where and are bounded linear operators on certain Banach spaces consisting of functions on , and the division is taken pointwise.

     

Solid-phase synthesis of 1,2,3-triazoles via 1,3-dipolar cycloaddition

The solid-phase synthesis of 1,2,3-triazoles via 1,3-dipolar cycloaddition of polymer-bound azides to various alkynes is reported. Polymer-bound azides were synthesized from polymer-bound halides and sodium azide and reacted with alkynes to produce polymer-bound 1,2,3-triazoles. Cleavage of the triazoles was performed with trifluoroacetic acid. A traceless synthesis of 1,2,3-triazoles was developed using 2-methoxy-substituted resin (polymer-bound 4-hydroxy-2-methoxybenzyl alcohol). In addition, a synthesis of 4-hydroxybenzyl-substituted 1,2,3-triazoles from the bromo-Wang resin (4-(bromomethyl)phenoxymethyl polystyrene) was achieved.