Polytypie bei Doppeloxiden: AgCoO2, AgFeO2, AgCrO2

Polytypes of AgFeO₂, AgCrO₂ and AgCoO₂ were found by indexing of high-resolution X-ray powder diagrams and, in the case of AgFeO₂ also by complementary single-crystal X-ray photographs. The abovementioned compounds occur at least in the polytype 6H (RAMSDELL-notation). The hexagonal unit cell dimensions are as follows:      

Infrared reflectivity of MTPA (TCNQ)2 single crystals

Polarized infrared reflectivity of single crystal MTPA (TCNQ)₂ was measured in the 40–4000 cm^?1 region and evaluated to obtain the dielectric function and conductivity. For E ⊥ b polarization, a very strong coupling between TCNQ intramolecular vibrations and electronic motion is observed. The bare electronic absorption is modelled by a sum of two classical oscillators.     

Über die Anwendung algebraischer Methoden in der Deformationstheorie komplexer Räume

Es handelt sich um einen Beweis der folgenden Sätze, die zuerst von Grauert angegeben wurden (Publ. Math. I.H.E.S. No. 5, 1960; vgl. dies Zbl.100, 80 (1963)):Es seif:X Y eine eigentliche holomorphe Abbildung komplexer Räume, sei einef-platte kohärente analytische Garbe überX; es bezeichneX _y die Faser vonf über einem Punkty Y und die analytische Einschränkung von aufX _y . Dann gilt: (I) Die Funktionend _q (y)=dimH _q (X _y , sind halbstetig nach oben. (II) Ist für einq die Funktiond _q (y) konstant undY reduziert, so ist dieq-te direkte Bildgarbe von unterf lokal frei überY. (III) Die Euler-Poincaré-Charakteristikx(y)=(–1) ^q dimH _q (X _y ,) ist lokal konstant überY. — Der Beweis benutzt systematisch den Begriff des Steinschen Kompaktums (= kompakte semianalytische Menge mit Steinscher Umgebungsbasis). Mit Hilfe der von Frisch bewiesenen Tatsache, daß die Algebra der Schnitte in der Strukturgarbe eines komplexen Raumes über einem Steinschen Kompaktum noethersch ist (Invent. Math.4, 118–138 (1967); vgl. dies Zbl.167, 68 (1969)), gelingt es, die Grothendieckschen Methoden im algebraischen Fall (EGA III) auf die analytische Situation zu übertragen.     

Speeding up IP-based algorithms for constrained quadratic 0–1 optimization

In many practical applications, the task is to optimize a non-linear objective function over the vertices of a well-studied polytope as, e.g., the matching polytope or the travelling salesman polytope (TSP). Prominent examples are the quadratic assignment problem and the quadratic knapsack problem; further applications occur in various areas such as production planning or automatic graph drawing. In order to apply branch-and-cut methods for the exact solution of such problems, the objective function has to be linearized. However, the standard linearization usually leads to very weak relaxations. On the other hand, problem-specific polyhedral studies are often time-consuming. Our goal is the design of general separation routines that can replace detailed polyhedral studies of the resulting polytope and that can be used as a black box. As unconstrained binary quadratic optimization is equivalent to the maximum-cut problem, knowledge about cut polytopes can be used in our setting. Other separation routines are inspired by the local cuts that have been developed by Applegate, Bixby, Chvátal and Cook for faster solution of large-scale traveling salesman instances. Finally, we apply quadratic reformulations of the linear constraints as proposed by Helmberg, Rendl and Weismantel for the quadratic knapsack problem. By extensive experiments, we show that a suitable combination of these methods leads to a drastic speedup in the solution of constrained quadratic 0–1 problems. We also discuss possible generalizations of these methods to arbitrary non-linear objective functions.     

Soil δ15N patterns in old-growth forests of southern Chile as integrator for N-cycling

Old-growth forests of southern Chile represent an important reserve of temperate (rain) forests in the world. Wetter and colder forest ecosystems appear to be more efficient in conserving and recycling N such that mostly non-plant available N species are lost, which could be indicated by more depleted δ¹⁵N values of the soil and plants. Hydrological N loss from the old-growth forests in southern Chile occurs mainly via dissolved organic nitrogen and not via dissolved inorganic N. Forest disturbances (e.g. fire, clear-cutting or enhanced N deposition) cause (abrupt) changes in ecosystem N-cycling processes. In this study, we hypothesized that δ¹⁵N signatures of soil profiles under old-growth forests could be used as an integrator for ecosystem N-cycling, and changes of these δ¹⁵N profiles could be valuable to assess ecosystem resilience towards disturbances. Six old-growth forests were selected in the phytogeographical region of the Valdivian rain forest in southern Chile. One of the sites has been partly burned in February 2002. First, we observed that ecosystems with higher mean annual precipitation and lower mean annual temperature were relatively more depleted in ¹⁵N. Secondly, we found that a forest fire caused a 100-fold increase of the nitrate export and induced an enrichment of the soil δ¹⁵N signal in the upper 20 cm.     

Infinitesimale Deformationen von Diedersingularitäten

Denote by T¹ the vector space of infinitesimal deformations of a dihedral singularity of type D_{n, q}. Using Pinkham's method for quotient surface singularities we prove a formula for dim T¹.     

Three-dimensional models of glutamate receptors

Structural models of glutamate receptors have been produced as part of a multidisciplinary study of neuronal function--both ligand/receptor interactions and ion transport--at the atomic level. The models have concentrated on the agonist binding and transmembrane domains of ionotropic (i.e. ligand-gated) glutamate receptors (iGluRs), and have aided our understanding of the molecular determinants of (1) ligand binding and (2) channel activity. The model building process involved a combination of homology modelling, distance geometry, molecular mechanics, protein-ligand and protein-protein docking, electrostatic calculations and manual adjustment, in conjunction with restraints from site-directed mutagenesis, ligand binding and electrophysiological studies. The initial models were used to produce hypotheses which were tested experimentally; these models have been subsequently refined as part of an extremely effective multidisciplinary study using an iterative molecular modelling/experimental verification cycle in which restraints derived from experimental studies are used at all stages, and the findings from one round of modelling are used as restraints in the next. By studying a variety of agonists and antagonists, details have been built up of (1) those residues involved in ligand binding and (2) the role of agonist binding (i.e. agonist-induced conformational change) in channel gating. The models also aid our understanding of the conductance properties of the channels.     

An interface clusters mixture model for the structure of amorphous silicon monoxide (SiO)

The present state of research on the structure of amorphous silicon monoxide (SiO) is reviewed. The black, coal-like modification of bulk SiO is studied by a combination of diffraction, microscopy, spectroscopy, and magnetometry methods. Partial radial distribution functions of SiO are obtained by X-ray, neutron and electron diffraction. Disproportionation of SiO into Si and SiO₂ is verified. High resolution TEM gives an upper limit of less than 2 nm for the typical Si cluster size. The Si K-edge electron energy-loss near-edge structure (ELNES) data of SiO are interpreted in terms of the oxidation states Si⁴⁺ and Si⁰. X-ray photoelectron spectroscopy gives first details about possible stoichiometric inhomogeneities related to internal interfaces. The wipe-out effect in the ²⁹Si MAS NMR signal of SiO is confirmed experimentally. The new estimation of the wipe-out radius is about 1.1 nm. First-time W-band, Q-band, and X-band ESR and SQUID measurements indicate an interfacial defect structure. Frequency distributions of atomic nearest-neighbours are derived. The interface clusters mixture model (ICM model) suggested here describes the SiO structure as a disproportionation in the initial state. The model implies clusters of silicon dioxide and clusters of silicon surrounded by a sub-oxide matrix that is comparable to the well-known thin Si/SiO₂ interface and significant in the volume because of small cluster sizes.