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.     

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.     

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.     

Lissajous singularities

Coherent optical Lissajous states are easily created by nonlinear processes such as second-harmonic generation (SHG). Singular properties of such states are discussed and illustrated theoretically with non-phase-matched SHG of an ellipse field containing a C point.     

Atomic structure of antiphase domain boundaries of a thin Al2O3 film on NiAl(110)

Line defects of a thin alumina film on NiAl(110) have been studied on the atomic level with scanning tunneling microscopy at 4 K. While boundaries between two reflection domains do not expose a characteristic structure, antiphase domain boundaries are well ordered. The latter boundaries result from the insertion of a row of O atoms, as atomically resolved images of the topmost oxygen layer show. The insertion occurs only in two of the three characteristic directions of the quasihexagonal O lattice. Depending on the direction, either straight or zigzagged boundaries form. An atomic characterization of line defects on the oxide surface is a first step to correlate their topographic structure and chemical activity.     

Determination of atomic structure of the metal-oxide interface: Pd nanodeposits on an FeO(111) film

Palladium was vapor deposited on a thin FeO(111) film grown on a Pt(111) substrate. Scanning tunneling microscopy study has revealed that Pd wets the FeO substrate and at elevated temperatures forms extended Pd(111) monolayer islands in contrast to other oxide supports previously studied. For the first time, we have imaged the metal-oxide interface structure with atomic resolution and explained the results on the basis of ab initio calculations.     

Exclusive annihilation pp-->gammagamma in a generalized Parton picture

Exclusive proton-antiproton annihilation into two photons at large s ( approximately 10 GeV2) and /t/,/u/ approximately s can be described by a generalized parton picture analogous to the "soft mechanism" in wide-angle real Compton scattering. The two photons are emitted in the annihilation of a single fast quark and antiquark. The matrix element describing the transition of the pp system to a qq pair can be related to the timelike proton elastic form factors as well as to the quark/antiquark distributions measured in inclusive deep-inelastic scattering. The reaction could be studied with the proposed 1.5-15 GeV high-luminosity antiproton storage ring (HESR) at GSI.     

Efficiency enhancement in free-electron lasers by means of concurrent rf acceleration

Efficiency enhancement in free-electron lasers (FELs) using rf beam acceleration in the wiggler is described. Since the beam tube is a waveguide, there are low and high frequency resonances. Injection of low frequency power can act as an inverse-FEL accelerator concurrently with high frequency power extraction. Simulation of a FEL using this technique shows that substantial efficiency enhancements are possible without significant increases in the beam energy spread, which facilitates the use of energy recovery schemes. The technique is applicable to amplifier and oscillator configurations.