Electrical conductivity of magnetite at low temperatures

Measurements of the electrical conductivity as a function of temperature down to liquid helium temperature are reported for four single crystal specimens of magnetite, Fe₃O₄, differing in impurity content and in stoichiometry. Such variations have little effect on the low temperature electrical properties.     

Cobalt(II) citrate cubane single-molecule magnet

An investigation of the magnetic properties of the cobalt(II) citrate cubane [C(NH 2) 3] 8{Co 4(cit) 4}.4H 2O reveals that the cluster is a new cobalt(II) single-molecule magnet, with an energy barrier to reorientation of the magnetization, Delta E/ k B = 21 K, and tau 0 = 8 x 10 (-7) s. The compound displays distinct, frequency-dependent peaks in the out-of-phase (chi') component of the ac magnetic susceptibility and magnetization versus field hysteresis loops that are temperature and sweep rate dependent. The hysteresis loops collapse at zero field due to very fast quantum tunneling of the magnetization (QTM).     

Experimental study of coaxial free-electron maser based on two-dimensional distributed feedback

The first experimental study of a coaxial free-electron maser (FEM) based on two-dimensional (2D) distributed feedback is presented. A new type of cavity formed with coaxial 2D surface photonic band gap structures was used. The FEM was driven by a large diameter (7 cm), high-current (500 A), annular electron beam of energy 475 keV. By tuning the amplitude of the undulator or guide magnetic field, modes associated with the different band gaps of the 2D structures were excited. The -band coaxial FEM generated 15 MW of radiation with a 6% conversion efficiency, in excellent agreement with theory.     

Study of the aggregation of human insulin Langmuir monolayer

The human insulin (HI) Langmuir monolayer at the air-water interface was systematically investigated in the presence and absence of Zn(II) ions in the subphase. HI samples were dissolved in acidic (pH 2) and basic (pH 9) aqueous solutions and then spread at the air-water interface. Spectroscopic data of aqueous solutions of HI show a difference in HI conformation at different pH values. Moreover, the dynamics of the insulin protein showed a dependence on the concentration of Zn(II) ions. In the absence of Zn(II) ions in the subphase, the acidic and basic solutions showed similar behavior at the air-water interface. In the presence of Zn(II) ions in the subphase, the surface pressure-area and surface potential-area isotherms suggest that HI may aggregate at the air-water interface. It was observed that increasing the concentration of Zn(II) ions in the acidic (pH 2) aqueous solution of HI led to an increase of the area at a specific surface pressure. It was also seen that the conformation of HI in the basic (pH 9) medium had a reverse effect (decrease in the surface area) with the increase of the concentration of Zn(II) ions in solution. From the compression-decompression cycles we can conclude that the aggregated HI film at air-water interface is not stable and tends to restore a monolayer of monomers. These results were confirmed from UV-vis and fluorescence spectroscopy analysis. Infrared reflection-absorption and circular dichroism spectroscopy techniques were used to determine the secondary structure and orientation changes of HI by zinc ions. Generally, the aggregation process leads to a conformation change from α-helix to β-strand and β-turn, and at the air-water interface, the aggregation process was likewise seen to induce specific orientations for HI in the acidic and basic media. A proposed surface orientation model is presented here as an explanation to the experimental data, shedding light for further research on the behavior of insulin as a Langmuir monolayer.     

Enantioselective Cobalt‐Catalyzed Intermolecular Hydroacylation of 1,6‐Enynes

We report a cobalt‐catalyzed hydroacylation of 1,6‐enynes with exogenous aldehydes in a domino sequence to construct enantioenriched ketones. The products were obtained in good yields with excellent regio‐, diastereo‐, and enantioselectivity. Furthermore, the chiral products served as valuable precursors to access complex spirocyclic scaffolds with three contiguous stereocenters. The asymmetric hydroacylation process exhibited no C?H crossover and no KIE, thus indicating that the C?H bond cleavage was not involved in the turnover‐limiting step.     

Analytical Solutions for Partitioned Diffusion in Laminates: I. Initial Value Problem with Steady Cauchy Conditions

Studies of the transport of contaminants and nutrients in industrial and environmental systems are complicated by the heterogeneous nature of the supporting porous or permeable media, and by the numerical problems associated with high Peclet number advection and sharp interface models. In order to provide independent theoretical checks of numerical transport theories, this set of papers presents analytical solutions to diffusive transport equations in simplified (one-dimensional) laminate systems subject to partitioning interactions. Here, in Part I, a standard separation of variables technique is used to develop analytical eigenfunction expansions of the concentration solution in an N-laminate system subject to steady Cauchy (third-type) nonhomogeneous boundary conditions. Both Cartesian and radial (axisymmetric) coordinate systems are considered. The solutions are developed for two different interface partitioning formulations, allowing the partitioning processes to be described by instantaneous equilibration mechanisms, or in terms of gradual equilibration mediated by mass transfer coefficients. Worked examples are presented and limitations of the approach discussed.