Excluded-volume effect for various chain models. I. Application to optical anisotropy and end-to-end distance of linear polymers

A study of the effect of excluded volume on the mean-square optical anisotropy 〈γ²〉 and the mean-square end-to-end distance 〈R²〉 has been carried out for polymer chains of up to 2000 bonds. The calculations have been made for models assuming (a) equiprobability of internal rotations and (b) interdependence of short-range intramolecular interactions. All the results indicate that 〈γ²〉 is practically insensitive to the excluded-volume effect. Concomitantly the behavior of 〈R²〉 has been analyzed.     

Random generalized solutions to the heat equation

A solution for the heat conduction problem with random source term and random initial and boundary conditions is defined. Existence, uniqueness, properties, and asymptotic behavior of such a solution are investigated. Applications to one-dimensional problems are presented.     

The linear combination of localized orbitals methods (LCLO) for polymers

The application of the Linear Combination of Localized Orbitals (LCLO) method to the band structure of polyethylene in the T, TG, TGTG' and G conformations is reported. The method uses full ab initio results on molecules to construct Fock matrix elements and deduce the energy bands. The main advantages of the technique are the low cost and the simplicity. Results are compared with those obtained from published ab initio and empirical calculations.Chargé de Recherches of the Fonds National Belge de la Recherche Scientifique.Titulaire d'une bourse de spécialisation I.R.S.I.A.     

Spectroscopic signatures of a bandwidth-controlled Mott transition at the surface of 1T-TaSe2

High-resolution angle-resolved photoemission data show that a metal-insulator Mott transition occurs at the surface of the quasi-two-dimensional compound 1T-TaSe2. The transition is driven by the narrowing of the Ta 5d band induced by a temperature-dependent modulation of the atomic positions. A dynamical mean-field theory calculation of the spectral function of the half-filled Hubbard model captures the main qualitative feature of the data, namely, the rapid transfer of spectral weight from the observed quasiparticle peak at the Fermi surface to the Hubbard bands, as the correlation gap opens up.     

Improvement of the spatial beam quality of laser sources with an intracavity Bragg grating

We demonstrate a novel method for improvement of spatial beam quality by use of the angular selectivity of a thick phase grating inserted into various laser cavities. Preliminary experiments with an intentionally degraded beam from a Nd:YVO4 laser have enhanced beam propagation factor M2 by a factor of 4.5. Operation with a broad-area laser diode in an external cavity with a thick grating was achieved. We obtained 100 mW of cw power in a beam with an M2 of 1.3. Moreover, a reduction of the output spectrum by as much as a factor of 3.8 was obtained.     

Distortion of a spherical gaseous interface accelerated by a plane shock wave

The evolution of a spherical gaseous interface accelerated by a plane weak shock wave has been investigated in a square cross section shock tube via a multiple exposure shadowgraph diagnostic. Different gaseous bubbles, i.e., helium, nitrogen, and krypton, were introduced in air at atmospheric pressure in order to study the Richtmyer-Meshkov instability in the spherical geometry for negative, close to zero, and positive initial density jumps across the interface. We show that the bubble distortion is strongly different for the three cases and we present the experimental velocity and volume of the developed vortical structures. We prove that at late times the bubble velocities reach constant values which are in good agreement with previous calculations. Finally, we point out that, in our flow conditions, the gaseous bubble motion and shape are mainly influenced by vorticity and aerodynamic forces.