Theoretical studies of superconductivity in doped BaCoSO

We investigate superconductivity that may exist in the doped BaCoSO, a multi-orbital Mott insulator with a strong antiferromagnetic ground state. The superconductivity is studied in both t-J type and Hubbard type multi-orbital models by mean field approach and random phase approximation (RPA) analysis. Even if there is no C₄ rotational symmetry, it is found that the system still carries a d-wave like pairing symmetry state with gapless nodes and sign changed superconducting order parameters on Fermi surfaces. The results are largely doping insensitive. In this superconducting state, the three \({t_{{2_g}}}\) orbitals have very different superconducting form factors in momentum space. In particular, the intra-orbital pairing of the \({d_{{x^2} - {y^2}}}\) orbital has an s-wave like pairing form factor. The two methods also predict very different pairing strength on different parts of Fermi surfaces. These results suggest that BaCoSO and related materials can be a new ground to test and establish fundamental principles for unconventional high temperature superconductivity.     

Mechanistic study on Mo(CO)6‐catalyzed intramolecular [2 + 2] or [2 + 2 + 1] cycloaddition reaction of 5‐allenyl‐1‐ynes

By means of density functional theory, the Mo(CO)₆‐catalyzed intramolecular [2 + 2] or [2 + 2 + 1] cycloaddition reaction of 5‐allenyl‐1‐ynes was investigated. All the intermediates and transition states were optimized completely at B3LYP/6‐311++G(d,p) level (LANL2DZ(f) for Mo). Calculations indicate that the complexation of 5‐allenyl‐1‐ynes with Mo(CO)₆ occurred preferentially at the triple bond to give the complex M1 and then the complexation with the distal double bond of the allenes generates the complex M5 . In this reaction, Mo(CO)₆‐catalyzed intramolecular [2 + 2] cycloaddition is more favorable than [2 + 2 + 1] cycloaddition. The reaction pathway Mo(CO)₆ + R → M5 → T7 → M12 → M13 → T11 → M18 → P4 is the most favorable one, and the most dominant product predicted theoretically is P4 . The solvation effect is remarkable, and it decreases the reaction energy barriers. Copyright © 2011 John Wiley & Sons, Ltd.     

Possible higher temperature superconductivity in the modulation-doped superlattice structure of cuprate superconductors

We design a new structure for a cuprate superconductor indicating the possibility of higher temperature superconductivity using our recently proposed composite fermions theory. It is constructed with modulation-doped superlattice structures, which are often used in the design of semiconductor superlattice devices. The superconductive critical temperature ($T_c$) was calculated in the superlattice structures of the superconductor in which the optimal doped CuO₂ layer was sandwiched between two less-doped CuO₂ layers. We find that if these structures could be realized in a cuprate superconductor such as Bi₂Sr₂Ca₂Cu₃O₁₀ or HgBa₂Ca₂Cu₃O₉, the highest $T_c$ could attain the level of 300 K at atmospheric pressure.     

The ν3 − ν4 difference band contribution to the CCl4 symmetric stretch (ν1) mode

The Raman spectrum of the symmetric stretching vibration (ν₁) of liquid carbon tetrachloride observed at 295 K and reported repeatedly over the last 80 years clearly shows four of the five more abundant isotopomers at 440–470 cm⁻¹. At the lower energy end of this spectrum, additional intensity due to isotopomeric contributions from the symmetric stretch for v = 1 → 2 (hotbands) partially overlaps the prominent v = 0 → 1 features, and accounts for about 18% of the integrated intensity at 295 K in agreement with theory. When these two patterns are modeled and subtracted from the experimental spectrum, a feature underlying almost exactly the C³⁵Cl₄ (v = 0 → 1) band at 462.5 cm⁻¹ becomes apparent. We propose that this feature is the ν₃ − ν₄ difference band. Observations at lower temperatures, and of the combination bands, and the polarized Raman spectra are consistent with this hypothesis. Copyright © 2014 John Wiley & Sons, Ltd.     

Superconductivity under uniaxial compression in β-(BDA-TTP) salts

In order to clarify the mechanism of organic superconductor β-(BDA-TTP) salts. We study the superconductivity under uniaxial compression with non-dimerized two-band Hubbard model. We have calculated the uniaxial compression dependence of T_c by solving the Eliashberg’s equation using the fluctuation exchange (FLEX) approximation. The transfer integral under the uniaxial compression was estimated by the extended Huckel method.We have found that non-monotonic behaviors of T_c in experimental results under uniaxial compression are understood taking the spin frustration and spin fluctuation into account.     

Structure and superconductivity of HgBa2CuO4+δ

We have used neutron powder diffraction to investigate the defect structure of HgBa₂CuO_4+δ. An interstitial oxygen defect in the Hg plane is the primary doping mechanism. A superconducting transition temperature, T_{c onset}, of 95 K is achieved when ≈0.06 oxygen atoms per formula unit are incorporated at this site by annealing the sample at 500°C in pure oxygen. Annealing in argon at 500°C lowers the oxygen content in this site to ≈0.01 and results in a T_c of 59 K. The neutron powder diffraction data give evidence for a second defect in the Hg plane which we conclude involves the substitution of copper for about 8% of the mercury and the incorporation of additional oxygen (≈0.1 atoms per formula unit), presumably bonded to the copper defects. In the present samples, the concentration of this defect does not vary with synthesis conditions and its contribution to doping is, therefore, unclear. The structure of the compound is the same at room temperature and superconducting temperatures.     

Bulk superconductivity in single‐phase Bi3O2S3

We report the synthesis of single‐phase Bi₃O₂S₃ sample and confirm the occurrence of bulk superconductivity with transition temperature at 5.8 K. The Bi₃O₂S₃ superconductor is categorized as typical type‐II superconductor based on the results of both temperature and magnetic field dependences of magnetization. Hall coefficient measurements give evidence of a multiband character, with a dominant conduction mainly by electron‐like charge carriers. The charge carrier density is about 1.45 × 10¹⁹ cm^–3, suggesting that the system has very low charge carrier density.

     

A new method for functionalizing α,γ‐dinitro compounds at the β‐position: application to the cyclization of β‐alkoxy‐α,γ‐dinitro compounds

Treatment of 2,4‐dinitropentane with bromine and sodium methoxide in methanol, affords formation of an ether product, 2,4‐dibromo‐3‐methoxy‐2,4‐dinitropentane, in 59% yield as a mixture of three diastereomers. This observation has led to a general synthesis of 3‐alkoxy‐2,4‐dibromo‐2,4‐dinitropentanes, obtained in 75‐86% yield from 2,4‐dibromo‐2,4‐dinitropentane as the preferred reactant. 4‐Bromo‐2,4‐dinitro‐2‐pentene has been identified as an intermediate in these reactions. The nitroalkene has been isolated and undergoes conjugate addition with alkoxides to afford the same ether products after brominative work‐up. The nitroalkene undergoes conjugate addition with sodium azide to give 3‐azido‐2,4‐dibromo‐2,4‐dinitropentane in 38% yield as a mixture of two isomers in which the (R*,R*) isomer predominates. Sequential treatment of 2,4‐dibromo‐2,4‐dinitropentane with sodium methoxide followed by sodium iodide and acetic acid gives 3‐methoxy‐2,4‐dinitropentane in 63% yield, the overall product of simple methoxylation of 2,4‐dinitropentane. However, attempted complete debromination of 2,4‐dibromo‐3‐methoxy‐2,4‐dinitropentane with excess sodium iodide and acetic acid results only in monodebromination to give 2‐bromo‐3‐methoxy‐2,4‐dinitropentane in 86% yield. Likewise, 2‐bromo‐3‐ethoxy‐2,4‐dinitropentane is formed in 93% yield from the ethoxy analog. A mechanistic rationale is offered for condition‐specific removal of the second Br atom in these reactions. Treatment of 3‐methoxy‐2,4‐dinitropentane with potassium acetate/iodine in dimethyl sulfoxide affords formation of 4,5‐dihydro‐3,4‐dimethyl‐3‐methoxy‐4‐nitroisoxazole 2‐oxide in 30% yield as a single diastereomer. Conversion of 2‐bromo‐3‐methoxy‐2,4‐dinitropentane in 15% yield to 4,5‐dihydro‐3,4‐dimethyl‐3‐methoxy‐4‐nitroisoxazole 2‐oxide is also possible by using potassium acetate in dimethyl sulfoxide. The mechanistic pathways for formation of 4,5‐dihydro‐3,4‐dimethyl‐3‐methoxy‐4‐nitroisoxazole 2‐oxide apparently involve unstable 3‐methoxy‐1,2‐dimethyl‐1,2‐dinitrocyclopropane as the common intermediate. Similarly, 2‐bromo‐3‐ethoxy‐2,4‐dinitropentane affords 4,5‐dihydro‐3‐ethoxy‐3,4‐dimethyl‐4‐nitroisoxazole 2‐oxide in 13% yield. Copyright © 2013 John Wiley & Sons, Ltd.     

Relative stabilities and molecular structures of the isomeric enol ethers and carboxylic esters derived from α‐acetyl‐γ‐butyrolactone and α‐acetyl‐δ‐valerolactone

Recently recorded ¹⁷O NMR spectra of compounds studied in a previous work (Taskinen E. Acta Chem. Scand. 1985; B39 : 489–494) dealing with the thermodynamics of isomerization of the enol ethers of α‐acetyl‐γ‐butyrolactone reveal an error in compound identification, caused by an unexpected isomerization reaction during the synthetic procedure. Thus, acid‐catalyzed treatment of the lactone with HC(OR)₃ in the respective alcohol ROH is shown to lead initially to the desired enol ethers which, however, are gradually isomerized to a mixture of the enol ethers and an ester of 2‐methyl‐4,5‐dihydrofuran‐3‐carboxylic acid. As a result, only one of the two isomeric compounds detected in the previous equilibration study was the expected enol ether (the thermodynamically more stable E isomer) of α‐acetyl‐γ‐butyrolactone, while the other, dominating species was the respective carboxylic ester. In the present work, the evidence provided by the ¹⁷O NMR spectra is presented, and the relative stabilities of the isomeric compounds are discussed on the basis of computational enthalpy data. The treatment is also extended to the respective isomeric compounds derived from α‐acetyl‐δ‐valerolactone. Copyright © 2007 John Wiley & Sons, Ltd.     

Mechanistic investigations in α‐hydroxycarbonyls reduction by BH4‐

BH₄^‐, a well‐known and widely used reducing agent for carbonyl compounds, has been reported to have the ability to participate in dihydrogen bonding, an interaction with applications in catalysis, stereoselectivity and crystal engineering. Specifically, α‐hydroxycarbonyls are activated for reduction by dihydrogen bonding that occurs between BH₄^‐ and hydroxyl group. We explored the effect of the interaction on the mechanism of these reactions by examining their activation parameters. We found that dihydrogen bonding activates α‐hydroxycyclopentanone for reduction with NBu₄BH₄ by lowering the activation enthalpy by 6.6 kcal/mol. While the activation entropy is a significant component of the barrier, the changes resulting from the occurrence of dihydrogen bonding are manifested predominantly in the enthalpy term. Computational studies suggest that, while internal hydrogen bonding is allowed by the flexibility of the carbon backbone, that interaction is outweighed by dihydrogen bonding once BH₄^‐ is present in the system. Experimentally, a red shift of the hydroxyl frequency is observed upon addition of BH₄^‐ to the reaction mixture, suggesting a dihydrogen bonding interaction. The flexibility of the substrate's skeleton or the selectivity of the hydride sites in BH₄^‐ does not account for the lack of directing effect of the dihydrogen bonding. When a substrate with a rigid naphthalene backbone moiety, 2‐hydroxyacenaphthylen‐1(2H)‐one, is reduced, the stereochemical outcome is very similar to the one corresponding to the α‐hydroxycyclopentanone. Copyright © 2012 John Wiley & Sons, Ltd.     

Giant dielectric constant observed in Ba0.96Sr0.04Zrx Y0.005Ti0.995–xO3–δ ceramics

Ba_0.96Sr_0.04Zr_x Y_0.005Ti_0.995–xO_3–δ ceramics were prepared by conventional sintering. The dielectric properties and the Curie temperature of the ceramics were studied. The Ba_0.96Sr_0.04Zr_x Y_0.005Ti_0.995–xO_3–δ ceramics (x = 0.04) exhibit a giant dielectric constant (ε_r ～ 10⁵). The peak value of dielectric constant is ～250,000 at 100 Hz and ～70,000 at 1 MHz for Ba_0.96Sr_0.04Zr_x Y_0.005Ti_0.995–xO_3–δ ceramics (x = 0.04). The results show that Ba_0.96Sr_0.04Zr_x Y_0.005Ti_0.995–xO_3–δ (x = 0.04) ceramics are a promising candidate material for microelectronic applications. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Excess molar volumes of ternary mixtures {n-butylacetae + 1-butanol + 1, 2-butanediol} and {n-butylacetate + 1-butanol + 1, 3-butanediol} at T = 303.15 K

Excess molar volumes of the ternary systems formed by {n-butylacetate + 1-butanol + 1,2-butanediol } and {n-butylacetate + 1-butanol + 1,3-butanediol} were measured at 303.15 K for the whole composition range. The excess molar volumes, V_m^E, for binary mixtures of {n-butylacetate + 1-butanol, + 1,2-butanediol and + 1,3-butanediol} are positive and for the binary mixtures of {1-butanol + 1,2-butanediol and + 1,3-butanediol} are negative. Several empirical expressions are used to predict and correlate the ternary excess molar volumes from experimental results on the constituted binaries and analyzed to gain insight about liquid mixture interactions.     

Electronic transitions in α, θ and γ polymorphs of ultraporous monolithic alumina

Spectroscopy of α, θ, and γ phases of high‐purity ultraporous alumina has been studied at cryogenic temperatures of 7 K in the near‐IR–VUV range of spectra with synchrotron radiation excitation. The UV photoluminescence (PL) spectra are dominated by optical transitions of self‐trapped excitons, while the PL excitation spectra are assigned to free excitons and interband transitions. The analysis of PL excitation spectra indicates a tendency to fundamental bandgap narrowing in order of 9.36 eV (α) to 7.60 eV (θ) and 6.85 eV (γ). Structural defects related to oxygen vacancies are responsible for the visible F⁺/F transitions decrease in order γ > θ > α. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ca‐doped Ba2Cu3O7–δ bicrystal junctions fabricated on asymmetric SrTiO3 substrates

Bicrystal grain–boundary Josephson junctions of Ca–doped YBa₂Cu₃O_7–δ that is Y_0.7Ca_0.3Ba₂Cu₃O_7–δ were fabricated on three bicrystal SrTiO₃ (001) substrates with asymmetric 30°, 40° and 45° orientations. An enhancement of the critical current density in these Ca–doped junctions was observed when compared with normal YBCO grain–boundary junctions with similar angular orientations. The observed increase in the critical current density is large for the junctions fabricated on the asymmetric 30° bicrystal substrate and small or negligible, for those on the asymmetric 45° bicrystal substrate. The critical current was modulated by a magnetic field applied in the plane of the junctions. However, the Fraunhofer pattern observed due to the applied magnetic field deviates from the ideal one.     

Photoluminescence in γ-irradiated α-quartz investigated by synchrotron radiation

We report an experimental investigation of the photoluminescence, under excitation by synchrotron radiation within the absorption band at 7.6 eV, induced in γ-irradiated α-quartz. Two emissions centered at 4.9 and 2.7 eV are observed at low temperature: the former decreases above 40 K, whereas the second band exhibits an initial slight increase and its quenching is effective above 100 K. Furthermore, the decay kinetics of both emissions occur in a time scale of nanoseconds: at T=17.5 K we measured a lifetime τ1.0 ns for the photoluminescence at 4.9 eV and τ3.6 ns for that at 2.7 eV. These results give new insight on the optical properties associated with defects peculiar of crystalline matrix, also on the basis of their comparison with previous studies on silica.     

Spectroscopic and structural investigations of α‐, β‐, and γ‐AlH3 phases

With its reputation as a high‐energy density fuel, aluminum hydride (AlH₃) has received renewed attention as a material that is particularly suitable, not only for hydrogen storage but also for rocket propulsion. While the various phases of AlH₃ have been investigated theoretically, there is a shortage of experimental studies corroborating the theoretical findings. In response to this, we present here an investigation of these compounds based primarily on two research areas in which there is the greatest scarcity of information in the literature, namely Raman and infrared (IR) absorption analysis. To the authors' knowledge, this is the first report of experimental far‐IR absorption results on these compounds. Two different samples prepared by broadly similar ethereal reactions of AlCl₃ with LiAlH₄ were analyzed. Both Raman and IR absorption measurements indicate that one sample is purely γ‐AlH₃ and that the other is a mixture of α‐, β‐, and γ‐AlH₃ phases. X‐ray diffraction confirms the spectroscopic findings, most notably for the β‐AlH₃ phase, for which optical spectroscopic data are reported here for the first time. Copyright © 2010 John Wiley & Sons, Ltd.     

Peltier-effect in the mixed state of (Bi,Pb)2Sr2Ca2Cu3Oδ

We present measurements of the Peltier-effect in the mixed state of Bi_1.76Pb_0.24Sr₂Ca₂Cu₃O_δ. The Peltier-coefficient broadens in a magnetic field quite similar to the resistivity and the thermopower. Comparison with the thermopower shows that Onsagers relation holds well. The occurrence of the Peltier-heat in the mixed state well below T_c implies that the electric current is accompanied by a large heat current. We show that the vortex contribution to the Peltier-heat is negligibly small. Therefore the heat current has to be attributed to normal quasiparticle excitations. Our results indicate that this quasiparticle contribution to the heat current remains large even at temperatures far below T_c.     

Stereochemical control in the structures of linear δ,α‐hybrid tripeptides containing tetrahydrofuran amino acids

Structural and electronic properties of diastereomers of tetrahydrofuran amino acids (TAA) derived tripeptide, Boc‐TAA‐Leu‐Val‐OMe, are studied using density functional theory. Predicted secondary folding patterns with hydrogen bonded pseudocycles of different sizes in peptides containing (2R,5S)‐cis‐TAA and (2S,5R)‐cis‐TAA are confirmed by detailed NMR studies of both, and single crystal X‐ray analysis of the former. A novel unusual folding pattern emanating from three‐centered hydrogen bond is found in peptide with (2R,5S)‐cis relationship. Stereochemical control on the orientation of interacting sites is substantiated by structural analysis of the peptides. Using natural bonding orbital and atoms in molecules analyses, charge transfer interactions are analyzed. Copyright © 2010 John Wiley & Sons, Ltd.