236 research outputs found

    Quantum Environments: Spin Baths, Oscillator Baths, and applications to Quantum Magnetism

    Full text link
    The low-energy physics of systems coupled to their surroundings is understood by truncating to effective Hamiltonians; these tend to reduce to a few canonical forms, involving coupling to "baths" of oscillators or spins. The method for doing this is demonstrated using examples from magnetism, superconductivity, and measurement theory, as is the way one then solves for the low-energy dynamics. Finally, detailed application is given to the exciting recent Quantum relaxation and tunneling work in naomagnets.Comment: Chapter in "Tunneling in Complex Systems" (World Sci., edited T. Tomsovic); 97 pages. Published in June 199

    Modulating electronic structure and reactivity of Cr nitride complexes via xxidation

    Get PDF
    Metal nitride complexes exhibit interesting structure and bonding properties that are invoked when discussing the reactivity of these systems. The nitride (N3-) moiety can be either nucleophilic or electrophilic based on a variety of factors such as metal identity, oxidation state, and nature of the ancillary ligands. Herein, the electronic tuning of Cr salen nitride complexes is investigated via modulation of phenolate para-R substituents of varying electron donating ability (R = CF3, tBu, NMe2) in order to influence reactivity. Salen ligands can exhibit non-innocent behavior, implying that redox processes can either be metal or ligand-based. This feature allows the ligand to help facilitate difficult substrate transformations uncommon to Earth-abundant first-row metals. Depending on the para-R group, the locus of oxidation in Cr nitride salen complexes (metal vs. ligand) can be influenced. The electronic structure of oxidized compounds is detailed, allowing for rationalization of nitride reactivity based on oxidation locus

    Interacting Ions in Biophysics: Real is not Ideal

    Get PDF
    Ions in water are important in biology, from molecules to organs. Classically, ions in water are treated as ideal noninteracting particles in a perfect gas. Excess free energy of ion was zero. Mathematics was not available to deal consistently with flows, or interactions with ions or boundaries. Non-classical approaches are needed because ions in biological conditions flow and interact. The concentration gradient of one ion can drive the flow of another, even in a bulk solution. A variational multiscale approach is needed to deal with interactions and flow. The recently developed energetic variational approach to dissipative systems allows mathematically consistent treatment of bio-ions Na, K, Ca and Cl as they interact and flow. Interactions produce large excess free energy that dominate the properties of the high concentration of ions in and near protein active sites, channels, and nucleic acids: the number density of ions is often more than 10 M. Ions in such crowded quarters interact strongly with each other as well as with the surrounding protein. Non-ideal behavior has classically been ascribed to allosteric interactions mediated by protein conformation changes. Ion-ion interactions present in crowded solutions--independent of conformation changes of proteins--are likely to change interpretations of allosteric phenomena. Computation of all atoms is a popular alternative to the multiscale approach. Such computations involve formidable challenges. Biological systems exist on very different scales from atomic motion. Biological systems exist in ionic mixtures (extracellular/intracellular solutions), and usually involve flow and trace concentrations of messenger ions (e.g., 10-7 M Ca2+). Energetic variational methods can deal with these characteristic properties of biological systems while we await the maturation and calibration of all atom simulations of ionic mixtures and divalents

    Investigations of Metal/Organic Interfaces and Metalation Reactions of Organic Semiconductors

    Get PDF
    Modern electronic devices are increasingly based on organic semiconductors. The performance of such devices crucially depends on the properties of the interface between the organic semiconductors and the metal contacts. Understanding the influence of the topology of the organic semiconductor’s conjugated pi-electron system on the interface interaction could greatly improve the device’s performance. Furthermore, the knowledge about reactions of heteroatomic organic semiconductors with metal atoms during electrode fabrication may lead to enhanced lifetimes of such devices. This cumulative dissertation comprises several publications and a number of so far unpublished results, addressing metal/organic interface interactions and metalation reactions of heteroatomic organic semiconductors. The properties of the interfaces are tailored by investigating the alternant aromatic molecules naphthalene and pyrene as well as the nonalternant aromatic molecules azulene and azupyrene on different metallic singlecrystal surfaces. Investigations by means of temperature-programmed desorption reveal stronger desorption energies for the non-alternant molecules on both Ag(111) and Cu(111). The biggest difference is observed on Cu(111), on which azulene and azupyrene are chemisorbed, whereas naphthalene and pyrene are physisorbed. The enhanced interface interaction of the non-alternant molecules is associated with the formation of surface dipoles that lead to stronger intermolecular repulsion between the adsorbed molecules. These results are supported by additional surface science methods, such as X-ray photoelectron spectroscopy or near-edge X-ray absorption fine structure spectroscopy, as well as density functional theory calculations conducted by group members and external collaboration partners. Detailed quantitative analysis of temperature-programmed desorption data of benzene on Cu(111) and Ag(111) yields experimental desorption energies that can be used as a benchmark for theoretical adsorption energies derived by density functional theory calculations. The interactions of metal/organic interfaces are compared with organic/inorganic interfaces in the case of pentacene and its fluorinated derivative perfluoropentacene on Au(111) as well as on bulk and two-dimensional MoS2 in a collaboration project. Organic semiconductors often interact weakly with inorganic surfaces, e.g., the thermal desorption of the first molecular layer is indistinguishable from multilayer desorption. No monolayer desorption peaks are observed as is mostly the case on metal surfaces. However, monolayer desorption of pentacene and perfluoropentacene on MoS2 occurs at significantly higher temperatures than the multilayer desorption. Detailed analysis reveals that the monolayers of both molecules are entropically stabilized. Codeposition of both molecules results in strong attractive intermolecular interactions on MoS2, while these interactions are weaker on Au(111). Metalation reactions of organic semiconductors with metal atoms, e.g., Co on tetraphenylporphyrin and Ca on alpha-sexithiophene, during interface preparation were investigated by means of hard X-ray photoelectron spectroscopy and temperature-programmed desorption mass spectrometry. The thickness of the reaction zone is changed by variation of experimental properties during interface formation. It is found that only the sample temperature during metal atom deposition and the metal atom flux in the case of Ca have an impact on the reaction depth, which is usually limited to few nanometers. In contrast to Co and Ca, Li atoms readily diffuse into the organic bulk and react with tetraphenylporphyrin over several tens of nanometers, forming dilithium tetraphenylporphyrin or monolithium monohydrogen tetraphenylporphyrin depending on the deposited Li amount. Furthermore, the transmetalation reaction of lead(II) tetraphenylporphyrin with Cu atoms on the Cu(111) surface was proven by temperature-programmed desorption. In addition, the Ullmann coupling reaction of bromo- and iodobenzene on Cu(111) was examined. While bromobenzene molecules desorb intact from the Cu(111) surface, iodobenzene molecules dissociate into iodine atoms and phenyl radicals. The latter form biphenyl that desorbs in three distinct desorption peaks at different temperatures. In a collaborative project, the oxidation state and electronic structure of Pb atoms in the newly synthesized Pb3F8 were studied by hard X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy giving evidence for the presence of Pb(II) and Pb(IV) species. The experimental results are complemented by constructional work to improve the temperatureprogrammed desorption setup. Moreover, two Igor Pro 8 scripts were written to quickly import data from different experimental setups and speed up the data treatment

    A Century

    Get PDF
    This book was published on the occasion of celebration of 100 years of IACS in 1976. This is an important and enriched document to know the formative period and history of IACS. The coverage of this book had been classified into four distinct periods: 1. The Early Perid 2. The Period of Prof. C V Raman 3. The Period of Prof. K S Krishnan 4. Post Independence PeriodFinancial assistance was given from Govt. of India & Govt. of W.B. to the Steering Committee for the Centenary Celebration of IACS under Prof. D. Basu, Directo

    Laboratory Directed Research and Development Annual Report - Fiscal Year 2000

    Full text link

    The 1991 research and technology report, Goddard Space Flight Center

    Get PDF
    The 1991 Research and Technology Report for Goddard Space Flight Center is presented. Research covered areas such as (1) earth sciences including upper atmosphere, lower atmosphere, oceans, hydrology, and global studies; (2) space sciences including solar studies, planetary studies, Astro-1, gamma ray investigations, and astrophysics; (3) flight projects; (4) engineering including robotics, mechanical engineering, electronics, imaging and optics, thermal and cryogenic studies, and balloons; and (5) ground systems, networks, and communications including data and networks, TDRSS, mission planning and scheduling, and software development and test
    corecore