Critical dimensions for strings,bags,...

Consider extendedd-dimensional objects such as strings (d=1), membranes (d=2), ... It is argued that their quantum theories cannot be consistent in space-times of dimension exceeding the critical valueD=2+24/d (orD=2+8/d in the supersymmetric case). This rules out consistent theories for objects extended in more than 5 (3 in the supersymmetric case) space-like dimensions.     

Evidence for field induced trapping of a defect dipole in MgO-crystals

A thermally stimulated depolarisation peak from a MgO single crystal shows a peculiar response to sequences of poling conditions. A field induced mechanical relaxation which fixes (or frees) an anisotropic defect dipole is suggested.     

ESCA studies of metal-oxygen surfaces using argon and oxygen ion-bombardment

ESCA has been used to monitor alterations of catalytically and electrochemically important metal-oxygen surfaces following exposure to Ar⁺ and O₂⁺ ion bombardment. This treatment resulted not only in sputtering, but also, in many cases, in reduction to the corresponding metal or lower oxide. A model based on bulk thermodynamic free energy considerations Is proposed to explain this phenomenon. We have also exploited this approach to obtain an in-depth concentration profile of various oxidation states of an element, to selectively prepare desired surface oxide compositio and to aid in interpreting complex O ls spectra. Results obtained from metal-oxygen surfaces for Ni, Ru and Mo are presented. Ni₂O₃ and RuO₃, which are gross defect structures of the bulk species, are present on NiO and RuO₂ respectively, with the former being confined to the surface layers. The MoO₂, on the other hand, is covered with a surface layer of MoO₃ present as a regular crystal structure.     

Ab initio bandstructure of a layer of nitrogen molecules oriented in a herringbone structure

Dispersion curves for nitrogen molecules physisorbed on graphite in the herringbone (2√3 x √3) structure are calculated and compared to recent angular resolved photoemission spectra. The calculated splitting at Г̄ of the 3σ_g derived molecular state and its disappearance at the Brillouin zone boundary is in good agreement with the observed spectrum. It is shown that the photoemission data for CO/Ag(1 1 1) can be also interpreted in terms of a herringbone structure.     

Mechanistic divergence of two closely related aldol-like enzyme-catalysed reactions

The analysis of the interaction of threose 4-phosphate and 2-deoxyerythrose 4-phosphate with 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAH7PS) reveals previously unrecognised mechanistic differences between the DAH7PS-catalysed reaction and that catalysed by the closely related enzyme, 3-deoxy-D-manno-octulosonate 8-phosphate synthase (KDO8PS).     

Self-organization of gold atoms on a polar FeO(111) surface

The spatial distribution of single Au atoms on a thin FeO film has been investigated by low-temperature scanning tunneling microscopy and spectroscopy. The adatoms preferentially adsorb on distinct sites of the Moiré cell formed by the oxide layer and the Pt(111) support and arrange into a well-ordered hexagonal superlattice with 25 angstroms lattice constant. The self-organization is the consequence of an inhomogeneous surface potential within the FeO Moiré cell and substantial electrostatic repulsion between the adatoms.     

Omega- and Omega+ production in central Pb + Pb collisions at 40 and 158A GeV

Results are presented on Omega production in central Pb+Pb collisions at 40 and 158A GeV beam energy. For the first time in heavy ion reactions, rapidity distributions and total yields were measured for the sum Omega(-) + Omega(+) at 40A GeV and for Omega(-) and Omega(+) separately at 158A GeV. The yields are strongly underpredicted by the string-hadronic UrQMD model but agree better with predictions from hadron gas models.     

Recognition of filtered words in noise at higher-than-normal levels: decreases in scores with and without increases in masking

To examine spectral effects on declines in speech recognition in noise at high levels, word recognition for 18 young adults with normal hearing was assessed for low-pass-filtered speech and speech-shaped maskers or high-pass-filtered speech and speech-shaped maskers at three speech levels (70, 77, and 84 dB SPL) for each of three signal-to-noise ratios (+8, +3, and -2 dB). An additional low-level noise produced equivalent masked thresholds for all subjects. Pure-tone thresholds were measured in quiet and in all maskers. If word recognition was determined entirely by signal-to-noise ratio, and was independent of signal levels and the spectral content of speech and maskers, scores should remain constant with increasing level for both low- and high-frequency speech and maskers. Recognition of low-frequency speech in low-frequency maskers and high-frequency speech in high-frequency maskers decreased significantly with increasing speech level when signal-to-noise ratio was held constant. For low-frequency speech and speech-shaped maskers, the decline was attributed to nonlinear growth of masking which reduced the "effective" signal-to-noise ratio at high levels, similar to previous results for broadband speech and speech-shaped maskers. Masking growth and reduced "effective" signal-to-noise ratio accounted for some but not all the decline in recognition of high-frequency speech in high-frequency maskers.     

The preference for anti over Gauche migration in the Baeyer-Villiger reaction

Explanations for stereoselectivity in the Baeyer-Villiger reaction have relied on the assumption that the antiperiplanar (app) group migrates. However, the magnitude of the preference for app-migration over gauche migration is unknown. To investigate this, the energy differences between the two were estimated from ab initio calculations. App-migration was found to be the preferred pathway since no transition structure could be located for gauche migration. Barriers for gauche migration were estimated by performing constrained optimizations. App-migration was found to be strongly favored with a barrier that is at least 3.8 kcal/mol and as much as 58.0 kcal/mol lower in energy than the gauche migration barrier.