Showing papers by "Vienna University of Technology published in 1988"

First-principles calculation of the electric-field gradient in hcp metals.

Abstract: The electric-field gradient (EFG) for all hcp metals from Be to Cd is obtained from energy-band calculations using the full-potential linearized-augmented-plane-wave (LAPW) method. Our first-principles method, which does not rely on any Sternheimer antishielding factor, yields EFG's in good agreement with experiment and predicts also the sign of the EFG's. The EFG was found to be determined mainly by the nonspherical distribution of the valence-electron density close to the nucleus. In general, contributions to the EFG originating from p states dominate. This is the case even for transition metals, where the d anisotropy is large.

An Elimination Algorithm for the Computation of All Zeros of a System of Multivariate Polynomial Equations

Abstract: A direct numerical method is proposed for the determination of all isolated zeros of a system of multivariate polynomial equations. By “polynomial combination”, the system is reduced to a special form which may be interpreted as a multiplication table for power products modulo the system. The zeros are then formed from an ordinary eigenvalue problem for the matrix of the multiplication table. Degenerate situations may be handled by perturbing them into general form and reaching the zeros of the unperturbed system via a homotopy method.

Evidence for a cytoplasmic pathway of oxalate biosynthesis in Aspergillus niger.

Abstract: Oxalate accumulation of up to 8 g/liter was induced in Aspergillus niger by shifting the pH from 6 to 8. This required the presence of Pi and a nitrogen source and was inhibited by the protein synthesis inhibitor cycloheximide. Exogenously added 14CO2 was not incorporated into oxalate, but was incorporated into acetate and malate, thus indicating the biosynthesis of oxalate by hydrolytic cleavage of oxaloacetate. Inhibition of mitochondrial citrate metabolism by fluorocitrate did not significantly decrease the oxalate yield. The putative enzyme that was responsible for this was oxaloacetate hydrolase (EC 3.7.1.1), which was induced de novo during the pH shift. Subcellular fractionation of oxalic acid-forming mycelia of A. niger showed that this enzyme is located in the cytoplasm of A. niger. The results are consistent with a cytoplasmic pathway of oxalate formation which does not involve the tricarboxylic acid cycle.

Comparison of electron attenuation lengths and escape depths with inelastic mean free paths

Abstract: It is shown that calculations of electron attenuation lengths and escape depths would require modification of definitions proposed in the literature, since these definition are not universally applicable. Exemplary experimental arrangements are indicated for which more general definitions are necessary, and such definitions are proposed. A Monte Carlo program has been developed with which it is possible to estimate attenuation lengths and escape depths. A comparison of these parameters is made for photoelectrons arising from s levels.

Penetrability of White Rot-Degraded Pine Wood by the Lignin Peroxidase of Phanerochaete chrysosporium.

Abstract: The penetration of enzymes into wood cell walls during white rot decay is an open question A postembedding immunoelectron microscopic technique was the method of choice to answer that question Infiltration of pine wood specimens with a concentrated culture filtrate greatly improved the labeling density and, thereby, reproducibility Characterization of the concentrated culture filtrate by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting (immunoblotting) revealed three closely spaced proteins of molecular weights about 42,000 showing immunoreactivity against anti-lignin peroxidase serum It was shown by immunogold labeling that lignin peroxidase of Phanerochaete chrysosporium is located on the surface of the wood cell wall or within areas of heavy attack It did not diffuse into undecayed parts of the cell wall The reasons for preventing lignin peroxidase from penetrating wood cell walls during white rot decay are discussed

Organic acid gas and liquid‐phase measurements in Po Valley fall‐winter conditions in the presence of fog

Abstract: Concurrent gas and liquid phase organic acid measurements during radiation fog episodes are reported. Gas phase concentrations of HCOOH and CH 3 COOH exhibit large variations from the detection limit (5 nmol m -3 0.1 ppb) to 150 nmol m -3 (3.5 ppb). Liquid phase concentration ranges of 11-175 and 10-269 μM were observed for HCOOH and CH 3 COOH, respectively. Large deviations from Henry’s law equilibrium were observed for HCOOH when the pH of fog droplet solutions was above 5. Our hypothesis is that in this range, the bulk sample pH might not be representative of the acidity of individual droplets in equilibrium with the gas phase. The organic acids in the Po Valley fall-winter conditions seem to originate from anthropogenic processes. DOI: 10.1111/j.1600-0889.1988.tb00109.x

Electronic structure of hcp metals.

Abstract: The electronic structure of the hexagonal-closed-packed elements Be, Mg, Sc, Ti, Co, Zn, Y, Zr, Tc, Ru, and Cd is calculated self-consistently by means of the full potential linearized augmented-plane-wave method. Energy-band structures, total and partial densities of states, as well as electron densities are presented and discussed in context with available experimental and theoretical data. Our results confirm that most of the older non-self-consistent calculations obtained realistic energy-band structures, while calculations based on the quantum-defect method generally yield completely different results.

Shock metamorphism of deformed quartz

Abstract: Deformed, synthetic quartz containing a dislocation density of 2.9 ± 1.9 × 108/cm2 and abundant bubbles and small inclusions was shocked to peak pressures of 12 and 24 GPa. The resultant material was inhomogeneously deformed and extremely fractured. The 12 GPa sample contained large regions lacking apparent shock deformation, suggesting that the original microstructure of a quartz target may be distinguished in low-stress shocks with minimal annealing. No change in dislocation density was caused by shock loading except in regions containing shock lamellae, where the dislocation density was lowered. Generally the same types of microstructures were induced by shock of deformed quartz as by shock of relatively defect-free as-grown crystals. Glass-filled veins were abundant, especially at lower stresses, and contained angular fragments of quartz welded together. Microfaults formed on $${{\{ 10\bar 11\} } \mathord{\left/ {\vphantom {{\{ 10\bar 11\} } {\{ 01\bar 11\} }}} \right. \kern- ulldelimiterspace} {\{ 01\bar 11\} }}$$ and $${{\{ 11\bar 22\} } \mathord{\left/ {\vphantom {{\{ 11\bar 22\} } {\{ 1\bar 212\} }}} \right. \kern- ulldelimiterspace} {\{ 1\bar 212\} }}$$ , inclined close to 45° to the shock propagation direction. Curviplanar features occurred in groups, with contrast indicating Moire patterns, twins, and stacking faults or related structures; most were interpreted as fractures, possibly welded together with glass. Regions containing shock lamellae sets were present. Lamellae sets were uncommon at 12 GPa, but distributed every few microns at 24 GPa. Lamellae occurred in a spectrum of habits ranging from 35–1500 A in thickness, from 35 A upward in spacing, and from closely-packed parallel sets to networks of diverse orientations; some lamellae were not parallel-sided, but wedge-shaped with basal and $${{\{ 10\bar 13\} } \mathord{\left/ {\vphantom {{\{ 10\bar 13\} } {\{ 01\bar 13\} }}} \right. \kern- ulldelimiterspace} {\{ 01\bar 13\} }}$$ edges, Thick lamellae were connected to glassy veins, and the wedge-shaped type generally narrowed away from veins; they also subdivided and merged along their length. Lamellae were dominantly basal at 12 GPa, and at 24 GPa on $${{\{ 10\bar 12\} } \mathord{\left/ {\vphantom {{\{ 10\bar 12\} } {\{ 01\bar 12\} }}} \right. \kern- ulldelimiterspace} {\{ 01\bar 12\} }}$$ , with poles normal to the shock direction. We propose that they are not shear features, but rather glass-filled tensile fractures. Vitrification was widespread, especially at 24 GPa, apparently more so than in shock of as-grown material. This suggests that index of refraction is not an appropriate shock paleopiezometer, as it depends on the defect structure of the starting material. Neither lamella width nor spacing was correlated with shock stress; however, the criterion of multiple glass lamellae sets as indications of shock deformation and its intensity are consistent with our measurements. Dislocation density was lowered in lamellae-containing and glassy areas, possibly removed by nucleation of and absorption by lamellae. No high-pressure phases were observed. Based on the complete set of observed features, it appears that shock deformation in quartz is primarily brittle-melt deformation, with an important role played by hot, fluid silica.

Volume approximation of convex bodies by inscribed polytopes

Abstract: A convex body C in d-dimensional Euclidean space E ~ is a compact convex subset of lE a with non-empty interior. We say that C is of differentiability class ~k if its boundary considered as a manifold is of class ~k. The symmetric difference metric 6 s, also called Nikodym metric, on the space of convex bodies is defined for any two convex bodies as the volume of their symmetric difference. Given a convex body C, let ~ = ~ ( C ) (n = d + l, d + 2,...) denote the family of convex polytopes P having at most n vertices and being inscribed into C, that is, all vertices are on the boundary bdC of C. It is well known that for each n there is a polytope P, ~ ~ such that ~s(C, P.) = ~s(C, ~,~)( = inf {~s(C, P): P ~ ~.~}).

Atomic and electronic structure of crystalline and amorphous alloys. I. Calcium-magnesium compounds

Abstract: Calculations of the atomic and electronic structure of Ca-Mg glasses and of the crystalline intermetallic compound ${\mathrm{CaMg}}_{2}$ are presented. For the glasses, the calculations are based on realistic models of the atomic structure constructed by a molecular-dynamics computer simulation linked to a steepest-descent potential-energy mapping. Interatomic forces are described in terms of pseudopotential-derived effective pair potentials. Our results for the atomic structure are in very good agreement with x-ray-diffraction data. They show that the topological short-range order in the glass is similar to the atomic arrangement in the crystalline Laves phase and best described as a highly defective tetrahedral close packing. The analogy between the glassy and the crystalline phases extends to the electronic structure. In both the crystalline and the amorphous alloys we find an essentially free-electron-like density of states (DOS), but close to the Fermi level the DOS is enhanced due to the onset of Ca 3d states. We find no evidence for the existence of a structure-induced minimum in the DOS near ${E}_{F}$. Our results are in good agreement with photoemission measurements and electronic specific-heat data.

Differential regulation of synthesis of multiple forms of specific endoglucanases by Trichoderma reesei QM9414.

Abstract: A method consisting of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and subsequent detection of endoglucanases by blotting with a polyclonal antibody against endoglucanase I was used to investigate the effect of induction and carbon catabolite derepression on the synthesis of multiple forms of endoglucanase I by Trichoderma reesei. Five forms appeared upon growth on cellulose, whereas four and only two appeared upon growth on lactose (carbon catabolite derepression) and induction by sophorose in a resting cell system, respectively. All endoglucanases detected resembled endoglucanase I in their specificity, since they exhibited no activity toward xylan or paranitrophenyl-beta-D-lactobioside. A small (25-kilodalton) endoglucanase only appeared during growth on cellulose. None of the multiple forms arose by postsecretional modification. The results indicate that sophorose may not be the only compound mediating cellulose induction of the specific endoglucanases in T. reesei.

The atomic and electronic structure of metallic glasses: search for a structure-induced minimum in the density of states

Abstract: The atomic and electronic structures of amorphous and crystalline Mg-Zn alloys are studied by computer simulation, electronic band-structure calculations and photoemission measurements. The spectra for the metallic glasses and for pure crystalline zinc show a narrow band of Zn 3d states centred at a binding energy EB of about -9.7 eV, overlapping the bottom of a broad sp band. There are indications of a minimum in the electronic density of states at the Fermi level for the glasses and for the pure metals. Molecular dynamics and potential-energy mapping calculations based on pseudopotential-derived interatomic forces are used to construct models for the atomic structure, with no other input than the composition and the atomic numbers and atomic weights of the components. The analysis of these models-which are in reasonable agreement with X-ray and neutron diffraction data-shows that the local topology of the glassy structure is very similar to that of the stable crystalline intermetallic compounds. The glassy structure is best described as a disordered tetrahedral close packing with a weak tendency to chemical short-range order whose precise degree remains to be detailed. The linearised muffin-tin orbital method in the atomic sphere approximation is used to perform self-consistent calculations of the electronic DOS of crystalline Mg and Zn, of the hexagonal Laves phase MgZn2 and of 'amorphous' supercells (each containing 60 atoms) representing glassy MgZn2 and Mg7Zn3 alloys. In each case the authors find a minimum in the DOS at EF, and d bands centred at EB approximately=-7.5 eV. A transition-state calculation shows that the d-band position in the photoemission spectra is shifted relative to the electronic eigenvalue due to self-energy corrections. Photoemission and X-ray emission intensities are calculated from the partial local DOS and the self-consistent potentials in a single-scatterer final-state approximation. The comparison with experimental confirms the validity of the electronic structure calculations. The work represents one of the first ab initio calculations of the atomic and the electronic structure of a metallic glass, and the first confirmation of the existence of a minimum in the electronic DOS at EF. The relevance of the DOS minimum to the structure-potential relationship and to the stability of the glassy phase is discussed.

Ultrastructural localization of ligninase ofPhanerochaete chrysosporium by immunogold labeling

Abstract: Ligninase was localized in the ultrastructure of hyphae ofPhanerochaete chrysosporium cultivated in liquid culture and wood, respectively, after immunolabeling The application of different techniques of fixation is discussed Ligninase was localized mainly in the hyphal cytoplasm close to the plasmalemma and was bound to the latter No immunolabeling was observed in or on the hyphal wall or extracellular structures Ligninase was detectable extracellularly only after fixation with picric acid As ligninase was localized in the wood cell wall only in isolated cases, the question of penetration into the wood cell wall still remains unclear

Deviations from exponential decay in the case of spontaneous emission from a two-level atom.

Abstract: A mathematically rigorous treatment of the Weisskopf-Wigner model of atomic spontaneous emission is presented. For the first time to the authors' knowledge, it is shown that in the correct asymptotic treatment with a cutoff frequency at which the dipole approximation breaks down, the main contribution to the long-time deviation from the exponential decay is of the order of 1/(t ${\mathrm{ln}}^{2}$t). This contradicts the results of previous authors who have obtained a long-time behavior of the order of 1/${t}^{2}$ by nonrigorous mathematical treatment of the same model in the dipole approximation. However, we will show that the result 1/${t}^{2}$ can still be obtained if the retardation effects are taken into account, i.e., if no dipole approximation is made.

Minimal ellipsoids and their duals

Abstract: It is proved that “most” convex bodies in IE d touch the boundaries of their minimal circumscribed and their maximal inscribed ellipsoids in preciselyd(d+3)/2 points. A version of the former result shows that for “most” compact sets in IE d the corresponding optimal designs, i.e. probability measures with a certain extremal property, are concentrated ond(d+1)/2 points.

VIP-MDBS: a logic multidatabase system

Abstract: We present a multidatabase management system built in Vienna Integrated Prolog (VIP) for cooperative management of autonomous databases. Data in different databases may differ with respect to naming, structures and value types. VIP-MDBS (VIP MultiDataBase System) allows the ability to manipulate them jointly and in a non-procedural way. Its features are similar to those of the relational multidatabase language MSQL, but adapted to logic programming. We introduce the concept of so-called semantic relations, a concept which stems from the extension of global views by deductiveness. VIP-MDBS allows for representation of intentional data and formulation of recursive multiple queries.

Dynamics of Linear Elastic Structures with Selfstress: A Unified Treatment for Linear and Nonlinear Problems

Abstract: A semi-numerical method for the solution of dynamic problems of linear and nonlinear structures is presented under the unifying aspect of sources of self-stresses. Conceptually applications are possible to the Finite Element and the Boundary Element Method in time as well as in the frequency domain. In addition to physical nonlinearities of viscoplasticity also geometrically nonlinear problems are considered. Effects of refined linear theories are discussed. Unter dem einheitlichen Aspekt der Eigenspannungsquellbelastung wird ein seminumerisches Verfahren zur Berechnung linearer und nichtlinearer Konstruktionen vorgestellt. Die numerische Realisierung erscheint sowohl mittels der Finiten Elemente- als auch der Randelement-Methode im Zeit- wie im Frequenzbereich moglich, und die Zusammenhange werden hergestellt. Zusatzlich zu den Materialnichtlinearitaten viskoplastischer Korper werden auch geometrisch nichtlineare Probleme behandelt. Der Unterschied zwischen den klassischen und den verfeinerten Theorien linear elastischer Konstruktionen wird ebenfalls unter dem Gesichtspunkt einer Belastung durch Eigenspannungsquellen betrachtet.

Calculated photoemission and x-ray emission spectra of Bi2Sr2CaCu2O8.

Abstract: Angle-integrated photoemission and inverse photoemission (IP) spectra of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/ were calculated using the single-scatterer final-state approximation and densities of states obtained from a highly precise local-density band calculation. The photoemission spectra of the valence band show mainly Cu d states for high photon energies, and an increasing O p contribution with decreasing photon energies, which brings about qualitative changes in the spectra. The Bi contribution is small in all valence-band spectra, but the unoccupied Bi p states give rise to a peak in the IP spectrum. The calculated O-K..cap alpha.. x-ray emission spectrum (XES) is found to be narrower than might be expected from the O p density of state (DOS) alone due to the different core-level positions for the inequivalent oxygen atoms. By contrast, the calculated Cu L/sub III/ XES closely resembles the Cu d contribution to the DOS.

Deviations from uniformity in random strings

Abstract: We show that almost all binary strings of length n contain all blocks of size (1-e)log2 n a close to uniform number of times. From this, we derive tight bounds on the discrepancy of random infinite strings. Our results are obtained through explicit generating function expressions and contour integration estimates.

Atomic and electronic structure of crystalline and amorphous alloys. II. Strong electronic bonding effects in Ca-Al compounds.

Abstract: Calculations of the atomic and electronic structure of ${\mathrm{Ca}}_{\mathrm{x}}$${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ glasses (with x=0.70, 0.60, 0.50, and 0.33) and of the crystalline intermetallic compound ${\mathrm{CaAl}}_{2}$ are presented. For the amorphous alloys the calculations are based on realistic models for the atomic structure constructed by a molecular-dynamics simulation linked to a steepest-descent potential-energy mapping. The effective interatomic forces are calculated using pseudopotential theory. We find that both the atomic and the electronic structures are dominated by strong electronic bonding effects: (a) The strong (s,p,d) hybridization of the free-electron-like conduction band of pure Al is broken up on alloying with Ca, and we find nearly separate Al 3s and Al 3p bands which are much narrower than the s and p bands in pure Al; (b) only the Al 3p states interact substantially with the Ca states. The interatomic electron transfer is small, but we find a substantial intra-atomic d-to-s transfer on the Al sites and an s-to-d transfer on the Ca sites. In the atomic structure the s-d promotion leads to a strong contraction of the Ca-Ca bonds in both the crystalline and the amorphous alloys compared to pure Ca, and a preferential Ca-Al bonding in the Al-rich but not in the Ca-rich alloys. The calculated electronic structure is well confirmed in all its details by heat-capacity, photoemission, and soft-x-ray emission spectra. The x-ray-diffraction data for the atomic structure corroborate the predicted compression of the Ca-Ca distances and the overall form of the correlation functions (which points to a local topology best described as a disordered tetrahedral close packing) but show distinctly lower and broader peaks. We argue that this is due to the short mean free path of the electrons, which will lead to a damping of the oscillations in the interatomic interactions.

Polarized neutron interferometry: A survey

Abstract: A brief summarizing review is given of all those neutron interferometric experiments performed hitherto which explicitly use the spin- 1 2 particle properties of the neutron. It covers topics as the verification of the 4π-periodicity of spinors, the cooperative action of nuclear phase shift and spin-rotation on the neutron wave function, the demonstration of the quantum mechanical principles of fermion spin-state superposition and the more recent double resonance experiments where the two interfering beams propagate through spatially separated oscillatory magnetic fields. Finally a proposal will be presented also for a so-called “late-choice” experiment with polarized neutrons.

FTIR-ATR-spectroscopic analysis of Bis-Crown ether based PVC-membranes - III. In situ spectroscopic and electrochemical methods to study surface and bulk membrane processes

Abstract: Descriptions et representations schematiques: 1) de la cellule reflexion totale attenuee-IR pour mesures in situ (avec des films minces, avec des films epais); 2) de dispositifs permettant de determiner les caracteristiques de reponse dynamique des membranes. Avantages de la methode proposee pour suivre la diffusion de constituants, ayant un interet, a travers la membrane dans des conditions presque reelles. Resultats obtenus dans differentes conditions (en particulier avec la membrane PVC/DOS/BME-44)

Collisional and electronic processes under ion, electron and photon bombardment of alkali and alkaline-earth halides

Abstract: The interaction of energetic ions or electrons with alkali halides and alkaline-earth halides results in elastic and inelastic energy transfer to the near surface region. The emission of secondary sputtered and desorbed particles has been studied to identify these processes. In particular, we have compared the emission of neutral ground state and excited alkali and alkaline-earth atoms from NaCl, LiF and CaF2. The intensity of the sputtered and desorbed particles and their velocity distributions have been measured as a function of the target temperature between room temperature and 400 ° C. For NaCl under ion bombardment neutral ground state Na atoms desorb predominantly (ion-beam-induced) thermally, even at room temperature, while Ca atoms from CaF2 desorb mainly thermally above 300 °C and purely collisional below 250 °C. For 400 eV electron bombardment desorption of thermal neutral ground state atoms is observed whenever a thermal contribution is present under ion bombardment. Excited Na and Ca atoms with energies in the eV range have been observed for ion bombardment. For electron bombardment excited Na has been identified with thermal velocities at all target temperatures. Contrary, excited Ca was not found even at target temperatures where substantial ground state Ca is desorbed. For NaCl as well as CaF2 and electron as well as ion bombardment inelastic processes play an important role for particle ejection above a certain target temperature.