Theoretical birefringence of ultraoriented polyethylene from model structure calculations

The birefringence of model structures of ultraoriented polyethylene has been calculated by using the point-dipole concept and the polarizability tensor of the methylene group derived recently by Pietralla. For well-oriented chains the birefringence is determined by the combination of a negative anisotropy of the polarizability tensor of the methylene group and a positive anisotropy of the Lorentz tensor, both with respect to the chain direction. As a consequence it decreases with increasing density. The results are in agreement with experimental values of Mead, Desper, and Porter. In the model the amorphous regions are regarded as a mixture of planar and helical segments arranged in nematiclike bundles which additionally may have a certain distribution of orientation. The birefringence is not much affected by defects which, on the other hand, strongly influence the ultimate mechanical properties.     

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Book reviews

     

Bose-Einstein condensation in a cavity

The effect of the physically correct boundary conditions and the nonvanishing ground state energy on Bose-Einstein condensation of quantum particles confined to a cubic volumeV=L ³ is evaluated. The transition point is shifted towards higher temperatures by the confinement, the specific heat below the onset of condensation is no longer proportional toT ^3/2, and the pressure does depend on the volume. Precise expressions for the modification of the ground state population and for the shift of the condensation temperature are derived, together with an expansion of the internal energy and of the specific heat. Numerical computations confirm the accuracy of our analytical approximations.Dedicated to Herbert Wagner, whose work on quantum Fermi liquids proved to be also very stimulating for quantum Bose liquids.     

The pair distribution function of moderately dense quantum fluids

The density expansion for the pair distribution functiong(r) and the structure factorS(k) for interacting quantum systems are given. These functions are thus represented by means of theT-matrices of the two-, three-,... body scattering problem. Possibly existing bound states are taken into account. Explicit expressions for the quantum virial coefficients in terms ofg(r) or ofT-matrices are derived.     

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Ar+Mo. giant fluctuations but small drift

We study recent measurements of Ar+Mo reactions. Transport models do not agree with the data for large energy losses; they predict too small fluctuations and too much drift. Random neck rupture is however in keeping with experiments. We explain random neck rupture by very simple analysis and show that its basic ingredient, the critical ratio of total length over neck radius, is largely independent of angular momentum. We propose experiments to examine random rupture, one of them, a neutron multiplicity experiment, with a prediction of a saw-tooth structure.     

The effect of the core hole on x-ray emission spectra in simple metals

We show that one-particle calculations of x-ray spectra with and without the core hole can give drastically different results, indicating a breakdown of one-particle theory. Only emission spectra obtained in the absence of the core hole consistently show a close resemblance to experiment. We show, that this fact can be explained by the many-body theory of Nozières and DeDominicis. We are then able to give the first interpretation of the L_2,3 satellite spectrum of sodium.     

A test of factorisation predictions for the fragmentation processes pp→π± at p momenta of 9.1 and 4.6 GeV/c

Relations have been derived between the invariant cross sections for various inclusive processes by assuming factorisation of the leading (pomeron) and non-leading (meson) trajectories. In this paper predictions for the invariant cross sections $\text{f(}\text{p}\text{p}\text{→π}{}^{\mathrm{\pm }}\text{)(2E}{}^1\text{/π√s)}\text{d}{}^2\text{σ/}\text{d}\text{x}\text{d}\text{p}{}^2{}_{\mathrm{\perp }}{}^2$ have been tested using data from $\text{p}\text{p}$interactions at 4.6 and 9.1 GeV/c.A large discrepancy between experiment and theory is apparent for the π^? data: in the π⁺ case the discrepancy is less marked but still present.     

Observation of the B meson in K−p interactions at 4.2 GeVc

A πω enhancement at 1245 MeV is observed in the reaction K^?p → Σ⁺π^?ω. Its properties agree with those of a B meson produced by natural-parity exchange thus establishing a coupling of the B to a $\text{K}\text{K}{}^1$ system.     

Shape dependence of correlation times in chaos-induced diffusion

A one-dimensional discrete-time chaotic system generating a diffusive process with strong correlations between successive steps is considered, as realized in brownian motion with directional persistence (observed e.g. for the motion of infusoriae). The resulting intermittency-like behavior without $\text{1}\text{?}$ spectrum may be found in many nonlinear systems with two metastable states.