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Spin Structure (spin + structure)
Selected AbstractsComparison of Isoelectronic Heterometallic and Homometallic Binuclear Cyclopentadienylmetal Carbonyls: The Iron,Nickel vs. the Dicobalt SystemsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2008Jun D. Zhang Abstract The heterometallic binuclear cyclopentadienylironnickel carbonyl compounds Cp2FeNi(CO)n (n = 3, 2, 1; Cp = ,5 -C5H5) have been studied by density functional theory (BP86) for comparison with the isoelectronic homometallic dicobalt derivatives Cp2Co2(CO)n. The FeNi tricarbonyl is shown to be the doubly bridged isomer Cp2Fe(CO)Ni(,-CO)2 with an Fe,Ni distance of 2.455 Å (BP86), in accord with experiment and in contrast to Cp2Co2(CO)3 where singly and triply bridged but not doubly bridged isomers are found. The dicarbonyl compounds Cp2FeNi(,-CO)2 and Cp2Co2(,-CO)2 both have analogous doubly bridged structures with M=M distances around 2.35 Å, suggesting formal M=M double bonds. The monocarbonyl compounds have analogous singly bridged axial structures Cp2FeNi(,-CO) and Cp2Co2(,-CO) with metal,metal distances in the range 2.05 Å (M2 = Co2) to 2.12 Å (M2 = FeNi) consistent with the formal M,M triple bonds required for the favored 18-electron configuration. Open-shell states of Cp2FeNi(,-CO) are found to have even lower energies than the closed-shell structure, which indicates that the ground state of Cp2FeNi(,-CO) might be a high spin structure. However, the global minimum for the monocarbonyl is found to be a singlet "hot dog" perpendicular Cp2NiFe(CO) structure with a terminal CO group bonded to the iron atom. Other higher energy perpendicular structures are also found for Cp2FeNi(CO)n (n = 3, 2, 1) with terminal CO groups and bridging Cp rings. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Conformally invariant powers of the Dirac operator in Clifford analysisMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 13 2010David Eelbode Abstract The paper deals with conformally invariant higher-order operators acting on spinor-valued functions, such that their symbols are given by powers of the Dirac operator. A general classification result proves that these are unique, up to a constant multiple. A general construction for such an invariant operators on manifolds with a given conformal spin structure was described in (Conformally Invariant Powers of the Ambient Dirac Operator. ArXiv math.DG/0112033, preprint), generalizing the case of powers of the Laplace operator from (J. London Math. Soc. 1992; 46:557,565). Although there is no hope to obtain explicit formulae for higher powers of the Laplace or Dirac operator on a general manifold, it is possible to write down an explicit formula on Einstein manifolds in case of the Laplace operator (see Laplacian Operators and Curvature on Conformally Einstein Manifolds. ArXiv: math/0506037, 2006). Here we shall treat the spinor case on the sphere. We shall compute the explicit form of such operators on the sphere, and we shall show that they coincide with operators studied in (J. Four. Anal. Appl. 2002; 8(6):535,563). The methods used are coming from representation theory combined with traditional Clifford analysis techniques. Copyright © 2010 John Wiley & Sons, Ltd. [source] EPR studies on Na-oleate coated Fe3O4 nanoparticlesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2004Y. Köseo Abstract Superparamagnetic iron oxide nanoparticles were prepared by the co-precipitation technique. Then, fine iron oxide nanoparticles were coated by Na-oleate. Magnetic properties of Na-oleate coated and uncoated iron oxide nanoparticles were investigated by Electron Paramagnetic Resonance (EPR) technique. At room temperature, a single, strong and broad EPR signal was observed for both samples with effective g-values of 2,0839 and 2,18838 for coated and uncoated samples, respectively. The intensity, line width and the resonance field for both coated and uncoated samples are strongly temperature dependent. When the sample is coated with Na-oleate, the line width and the resonance field values of the EPR signal increase due to the decrease in the magnetic interaction between the particles. The total effective magnetic moment of such coated particles is found to decrease, which is most likely due to a non-collinear spin structure originated from the pinning of the surface spins and coated surfactant at the interface of nanoparticles. [source] Interfacial spin structure in epitaxial Fe/FeSn2 bilayers with exchange biasPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2004F. Stromberg Abstract Fe/FeSn2 structures with epitaxial FeSn2 layers have been grown by MBE (Molecular Beam Epitaxy). Exchange bias and pinning phenomena were proved by SQUID magnetometry. In order to elucidate the spin structure at the Fe/FeSn2 interface and in some depth of the FeSn2 layer with CEMS (Conversion Electron Mössbauer Spectroscopy), 57FeSn2 tracer layers of approx. 50 Å thickness have been incorpo- rated in the base structure, the only difference being the isotopic enrichment with 57Fe. An ellipsoidal model was applied to represent the spin structure. A strong out-of-plane component of the spin structure at the interface was observed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Construction of a Novel Topological Frustrated System: A Frustrated Metal Cluster in a Helical SpaceCHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2010Ryuta Ishikawa Abstract A novel topologically frustrated pentanuclear cluster helicate [{CuII(,-L)3}2CuII3(,3 -OH)]3+ (L,=3,5-bis(2-pyridyl)pyrazolate) has been synthesized and characterized. This cluster has a helical arrangement of ligands around the central metal core. Dzyaloshinsky,Moriya interactions are essential components to observe a gradual magnetization and forbidden transitions of high-field/multi-frequency (HF/MF)-ESR. The origin of the magnetic anisotropy of this compound is influenced by its helical spin structure, and consequently, the Cu5 -cluster helicate introduces a unique magnetic anisotropy. This observation is a direct evidence of the topological part of the new spin phase in a magnetic system. [source] On the absence of large-order divergences in superstring theoryFORTSCHRITTE DER PHYSIK/PROGRESS OF PHYSICS, Issue 1 2003S. Davis The genus-dependence of multi-loop superstring amplitudes is estimated at large orders in perturbation theory using the super-Schottky group parameterization of supermoduli space. Restriction of the integration region to a subset of supermoduli space and a single fundamental domain of the super-modular group suggests an exponential dependence on the genus. Upper bounds for these estimates are obtained for arbitrary N-point superstring scattering amplitudes and are shown to be consistent with exact results obtained for special type II string amplitudes for orbifold or Calabi-Yau compactifications. The genus-dependence is then obtained by considering the effect of the remaining contribution to the superstring amplitudes after the coefficients of the formally divergent parts of the integrals vanish as a result of a sum over spin structures. The introduction of supersymmetry therefore leads to the elimination of large-order divergences in string perturbation theory, a result which is based only on the supersymmetric generalization of the Polyakov measure and not the gauge group of the string model. [source] | |||||||||||||