Properties of Si δ-layers embedded in GaAs

An ultrathin, a few monolayers (MLs) thick Si δ-layer, has been embedded in GaAs. The Si δ-layer properties have been electrically and structurally characterized. A conductivity transition has been observed, when going from free carrier transport in thin (1 ML) ordered δ-layers to the disordered conditions for Si δ-layers thicker than 4 MLs. Two novel emission bands are observed in photoluminescence (PL) for Si δ-layers in the width range 1–4 MLs, but solely at below bandgap excitation. The pronounced 2D properties of these δ-layers have been clearly demonstrated by an observed blue shift of the PL characterization as the thickness of the Si δ-layer is reduced. The so-derived results on the transition energies and the electronic structure are in good agreement with theoretical predictions obtained by a self-consistent approach.     

Measurement of the directional emittance of polycrystalline beryllium oxide

The spectral emittance of polycrystalline beryllium oxide has been measured at four different angles from 0° to 75°, at a temperature of 150°C. From the spectral measurements the average band emittance in the 8–13 μm window has been calculated. The results show that the band emittance decreases from 0.44 at the angle of 0°, down to 0.23 at an angle 75° off the normal. The low band emittance is caused by a strong reststrahlen band in the wavelength region 9.2–14 μm. This reststrahlen band has been modelled by a Lorentz one-oscillator model. Experiment and calculations show that this band of low emittance increases its width towards shorter wavelengths at inclined angles and covers almost the entire upper atmospheric window 8–13 μm at the angle of 75°.     

Baryon production in lepton-nucleon scattering and diquark fragmentation

We study baryon production in deep inelastic scattering using an extended version of the Lund jet model. There are two contributing sources. The first is baryon production in the target fragmentation. In a scheme related to our earlier work on lowp _hu baryon fragmentation we present some details of the fragmentation of a diquark into baryons and mesons. A non-negligible baryon-antibaryon production is observed ine ⁺ e ^? annihilation. In a previous paper we developed a model for this production, and the same mechanism should also give fast baryons in leptoproduction. In this paper we discuss those features of baryon production which can be more easily studied in a leptoproduction experiment.     

Gigahertz bandwidth measurements of photodetectors and fibers using picosecond pulses generated by semiconductor lasers

A picosecond variable pulse width method has been found to be very useful in the bandwidth determination of Gbit/s photodetectors and fibers. Bandwidths up to 5 GHz have been measured with this technique using picosecond pulses generated from a semiconductor laser.     

A semiclassical model for quark jet fragmentation

A semiclassical model is presented for the way the energy of a fast quark is transformed into observable hadrons. It reproduces the features of 1+1 dimensional QED (the Schwinger model) concerning a flat rapidity distribution in the central region. It also reproduces results from phenomenological considerations, which, based upon scaling, predict that meson formation in the fragmentation region can be described by an iterative scheme, implying a set of coupled integral equations. In particular the model predicts that the probability to find a meson containing the leading quark is independent of the Feynman scaling variablez. The iterative structure corresponds to a Brownian motion with relevance both to the confinement problems and to the distribution of mass in the quark jet.     

Scalar Curvature Rigidity for Asymptotically Locally Hyperbolic Manifolds

Rigidity results for asymptotically locally hyperbolic manifolds with lower bounds on scalar curvature are proved using spinor methods related to the Witten proof of the positive mass theorem. The argument is based on a study of the Dirac operator defined with respect to the Killing connection. The existence of asymptotic Killing spinors is related to the spin structure on the end. The expression for the mass is calculated and proven to vanish for conformally compact Einstein manifolds with conformal boundary a spherical space form, giving rigidity. In the four dimensional case, the signature of the manifold is related to the spin structure on the end and explicit formulas for the relevant invariants are given.     

Ferroelectric, pyroelectric and electro-optic properties of a chiral side chain copolymer with high polarization and a broad C* phase

The synthetic route to a chiral side chain copolymer exhibiting broad smectic phases is described. The copolymer shows a smectic C* phase from below room temperature up to 105°C. Ferroelectric properties of this copolymer are reported as well as results showing electroclinic switching. The pyroelectric effect and the dielectric behaviour of the copolymer are also demonstrated.     

Photo-protons from high-energy bremsstrahlung

Recent results on the yields of high-energy photo-protons from both bremsstrahlung and monochromatic photons incident mainly on carbon are compared with calculations using the Gabriel-Alsmiller model of high-energy photo-induced reactions. Good agreement is found for photons of energies up to 600 MeV.     

Superheavy elements and variations in the nuclear skin thickness

Although the interpretation of the data is uncertain, the recently claimed empirical evidence of element Z = 126 and some of its neighbours, provides the impetus for a reexamination of the nuclear one-body potentials, on the basis of which extrapolations have been made up to now. In particular the skin thickness parameters of the neutrons and protons are of interest and the authors presently consider the possibility that the nucleus may adjust its skin thickness to achieve a maximal shell energy.