Adding Remnant Magnetization and Anisotropic Exchange to Propeller‐like Single‐Molecule Magnets through Chemical Design

The selective replacement of the central iron(III) ion with vanadium(III) in a tetrairon(III) propeller‐shaped single‐molecule magnet has allowed us to increase the ground spin state from S=5 to S=13/2. As a consequence of the pronounced anisotropy of vanadium(III), the blocking temperature for the magnetization has doubled. Moreover, a significant remnant magnetization, practically absent in the parent homometallic molecule, has been achieved owing to the suppression of zero‐field tunneling of the magnetization for the half‐integer molecular spin. Interestingly, the contribution of vanadium(III) to the magnetic anisotropy barrier occurs through the anisotropic exchange interaction with iron(III) spins and not through single ion anisotropy as in most single‐molecule magnets.     

Design and Critical Evaluation of Improved Electrothermal Vaporization Flame Atomic Absorption/Emission Spectrometry for Direct Determination of Trace Metals in Microliter Samples

Improved tantalum-filament electrothermal vaporization flame atomic absorption spectrometry (ETV-FAAS) was developed and used for the direct determination of trace metals in microliter samples. Studies have been made for optimizing the experimental parameters that affect the performance of sample introduction to the flame. Linear calibration graphs are shown for the elements Mn (10–200 ng), Pb (5–200 ng), Cu (5–100 ng), Cd (5–50 ng), Li (1–20 ng), Na (10–80 ng), and K (10–80 ng), using only 10 μl of standard solutions. The detection limits of the elements by ETV-FAAS were much lower than those of conventional FAAS. Absolute detection limits for all elements studied were less than 0.1 ng. The relative standard deviation values for the elements were <10%. The developed method was also applied to the determination of lead concentration in blood samples.     

The flash photolysis of aqueous solutions of rhodizonic and croconic acids

The flash photolysis of aqueous solutions of rhodizonic and croconic acids has been studied in the presence and absence of electron acceptors. No transient absorption which could be identified with an excited state was observed with either anion. The rate of recovery of the ground state in the absence of additives was a first-order process with both acids and gave rate constants for deactivation of the excited state, k_D, of 2.4 × 10⁵ s^?1 for rhodizonate and 2.8 × 10⁵ s^?1 for croconate. With croconate dianion in the presence of three acceptors, 4-nitrobenzylbromide, methylviologen, and biacetyl, a transient absorption was detected, with a maximum absorbance at 500 nm, and was tentatively identified with the monoanion radical, formed following electron transfer to the acceptor. From the rate of growth of the transient, rate constants for the rate of electron transfer to the acceptor were measured as follows: 4-nitrobenzylbromide: 2.8 × 10⁹ M^?1 s^?1; methyl viologen: 3.7 × 10¹⁰ M^?1 s^?1; and biacetyl: 2.0 × 10⁸ M^?1 s^?1. The significance of the measurements is discussed in relation to the mechanism proposed for the photochemical reactions of these dianions. © 1994 John Wiley & Sons, Inc.     

Proximity induced enhancement of the Curie temperature in hybrid spin injection devices

We investigate the increase of the Curie temperature T(C) in a lateral spin injection geometry where the ferromagnetic (Ga,Mn)As injector and detector contacts are capped by a thin iron film. Because of interlayer coupling between Fe and (Ga,Mn)As T(C) gets enhanced by nearly 100% for the thinnest (Ga,Mn)As films. The use of the proximity effect might pave the way for practical implementation of spintronic devices.     

Identification of different electron screening behavior between the bulk and surface of (Ga,Mn)As

We report x-ray photoemission spectroscopy results on (Ga,Mn)As films as a function of both temperature and Mn doping. Analysis of Mn 2p core level spectra reveals the presence of a distinct electronic screening channel in the bulk, hitherto undetected in more surface sensitive analysis. Comparison with model calculations identifies the character of the Mn 3d electronic states and clarifies the role, and the difference between surface and bulk, of hybridization in mediating the ferromagnetic coupling in (Ga,Mn)As.     

Fusion of 60Ni + 100Mo near and below the Coulomb barrier

Various sub-barrier capture reactions with ^16,18O and ^40,48Ca are treated within the quantum diffusion approach. The role of neutron transfer in these capture reactions is discussed. The quasielastic and capture barrier distributions are analyzed and compared with the recent experimental data.     

The Unexpected Role of SeVI Species in Epoxidations with Benzeneseleninic Acid and Hydrogen Peroxide

Benzeneperoxyseleninic acid has been proposed as the key intermediate in the widely used epoxidation of alkenes with benzeneseleninic acid and hydrogen peroxide. However, it reacts sluggishly with cyclooctene and instead rapidly decomposes in solution to a mixed selenonium–selenonate salt that was identified by X‐ray absorption and ⁷⁷Se NMR spectroscopy, as well as by single crystal X‐ray diffraction. This process includes a selenoxide elimination of the peroxyseleninic acid with liberation of oxygen and additional redox steps. The salt is relatively stable in the solid state, but generates the corresponding selenonic acid in the presence of hydrogen peroxide. The selenonic acid is inert towards cyclooctene on its own; however, rapid epoxidation occurs when hydrogen peroxide is added. This shows that the selenonic acid must first be activated through further oxidation, presumably to the heretofore unknown benzeneperoxyselenonic acid. The latter is the principal oxidant in this epoxidation.     

Growth mechanism, microstructure and transport properties of Sr1−x La x CuO2 (x=0.10−0.15) thin films

Sr_1−x La _x CuO₂ (x=0.10−0.15) thin films with an infinite-layer type structure were grown on BaTiO₃ buffered (001) SrTiO₃ substrates by pulsed laser deposition (PLD). The evolution of the growth front was monitored, in-situ, by high-pressure reflection high-energy electron diffraction (RHEED), while the surface morphology was analyzed by means of atomic force microscopy (AFM), ex-situ. X-ray diffraction (XRD) was used to determine the evolution of the film structure with deposition and cooling parameters, as well as to study the type and level of epitaxial strain in the Sr_1−x La _x CuO₂ films. The RHEED data showed that the Sr_1−x La _x CuO₂ films grow on BaTiO₃/SrTiO₃ following a 2D or Stranski-Krastanov mechanism, depending on the La doping level. The transition point (critical thickness d _c) from layer-by-layer like (2D) to island (3D) growth depends on the film stoichiometry: decreasing the La doping concentration x from 0.15 to 0.10, the critical thickness d _c increases from ∼45 nm to ∼75 nm. In order to induce superconductivity, the Sr_1−x La _x CuO₂ films were cooled down under reduction conditions. The as-deposited films showed semiconducting or metallic behavior, the resistivity decreasing with increasing La concentration. Post-deposition vacuum annealing resulted in a superconducting transition onset (but no zero resistance down to 4.2 K) only for some of the x=0.15 Sr_1−x La _x CuO₂ films.     

Monoenergetic-proton-radiography measurements of implosion dynamics in direct-drive inertial-confinement fusion

Time-gated, monoenergetic radiography with 15-MeV protons provides unique measurements of implosion dynamics in direct-drive inertial-confinement fusion. Images obtained during acceleration, coasting, deceleration, and stagnation display a comprehensive picture of spherical implosions. Critical information inferred from such images, hitherto unavailable, characterizes the spatial structure and temporal evolution of self-generated fields and plasma areal density. Results include the first observation of a radial electric field inside the imploding capsule. It is initially directed inward (at approximately 10(9) V/m), eventually reverses direction ( approximately 10(8) V/m), and is the probable consequence of the evolution of the electron pressure gradient.