Table of contents

We present a practical method to construct a caustic produced by shining light through a natural pattern in a transparent medium. This caustic is studied using for the first time the global theory of singularities. We can identify every type of singularity and describe the global topology of the configuration. Also an example of caustic bifurcation is presented. One has then a tool that allows the experimentalist to exploit the information contained in caustics and the mathematician to test applications to concrete cases.

The behavior of very large clusters of diffusion-limited aggregation (DLA) was investigated to help discriminate between the two geometric scenarios recently described by Mandelbrot: finite transient and infinite drift. Using 50 DLA clusters of 1 million particles, we follow the increase during growth of the maximum radius of the clusters and of various relative moments. One can distinguish two regions: an inactive completely grown core and an active growing region. In the growing region, scale factors were defined the moments of the atoms distances from the original "seed". They do not cross-over to the behavior characteristic of self-similarity for finite sizes and support the novel "drift" scenario that postulate an infinite continuing "transient". The moment's "misbehavior" may help understand the disagreement between previous estimates of the clusters' fractal dimension.

It is shown that the presence of the surface causes circular dichroic effects in angular-resolved photoemission (CDAD) from spherical symmetric electronic states of adsorbates. The origin of this new class of CDAD effects is electron reflection. A simple reflection mechanism including inelastic processes is introduced into the theory of the angular-resolved photoemission from adsorbates excited by polarised light. This model is expanded by including electron diffraction. Both models show that interference of partial waves of the same orbital quantum number causes CDAD for spherical initial states of adsorbed atoms. The interference is due to the difference in the path length of the direct and the reflected wave and in the case of diffraction additionally to the scattering phase shift. The CDAD derived theoretically is in agreement with experiments with free laser aligned and adsorbed alkali atoms.

Electric-field effects are investigated in non-ionic fluids near the critical point. We first derive an upward shift of the critical temperature proportional to the square of the field. Its sign is opposite to that predicted by Debye and Kleboth. Second, we derive an induced dipolar interaction among the critical fluctuations, which suppresses the fluctuations and yields another upward shift. The fluctuation contribution to the macroscopic dielectric constant is also examined.

The first pulsed microbeam for positrons in the keV energy range is described. The principle of operation and the construction details are given. The performance achieved is: 20 μm (FWHM) spot diameter; 350 ps (FWHM) time resolution; 5000 e⁺ /(s mCi) at 5 keV final energy.

We present the first tight-binding molecular-dynamics simulation of the defect-induced crystal-to-amorphous transition in crystalline silicon. The process is studied by monitoring the response of the crystal lattice to a random insertion of self-interstitial defects. Both structural and electronic properties are investigated and a comparison to amorphous silicon as obtained by quenching from the melt is presented. Finally, we study the effects of the post-implantation thermal annealing on the implanted sample.

The electron addition spectrum of the intermediate-valence material CeRh₃ exhibits a dramatic dependence on the excitation energy near the CeM₅ (3d → 4f) absorption edge. The cerium 4f spectral intensity is first strongly depressed, and then resonantly enhanced with respect to the band contribution, with an overall cross-section variation of almost three orders of magnitude. These results show that, in close analogy with resonant photoemission, a comparison of on- and off-resonance spectra can be used to identify and separate the signal from the localized and the extended electronic states in Ce systems.

The study of the magnetoresistance in the organic quasi-one-dimensional conductor (TMTSF)₂NO₃ at a pressure (8 kbar) exceeding the critical pressure required for the stabilization of a conducting state at low temperature has revealed the existence of quantum oscillations with a frequency of 190 T in good agreement with an extended Hückel-type calculation of the band structure. Unlike other members of the (TMTSF)₂X series the ambient pressure SDW ground state in (TMTSF)₂NO₃ can be understood in terms of an instability involving the nesting of 2D semi-metallic Fermi surfaces.

The zero-field muon spin relaxation (ZF-μSR) technique is employed to study the magnetic properties of the non-superconducting borocarbide SmNi₂B₂C. Coherent ordering of the Sm electronic magnetic moments appears below 9.86(3) K, leading to a local field at the muon site of 1056.2(2) G at 3.3 K. Measurements on superconducting YNi₂B₂C reveal no evidence of either slowing-down of fluctuating or the presence of quasi-static Ni electronic moments down to 3 K.

Scattered-wave calculations of X-ray absorption near-edge structure are presented for the K-edge of molecular chlorine. The calculations are based on different choices of one-electron potential: using either overlapped-free-atom or self-consistent-field (SCF) molecular electron densities; and either making the muffin-tin (MT) approximation or relaxing it (non-MT). The SCF non-MT results approach quantitative agreement with experiment, indicating the necessity of both SCF densities and the non-MT approach.

In the paper by Kolovsky (Europhys. Lett., 27 (1994) 79) an equation for a mesoscopic particle, with internal structure, was proposed. Nevertheless, that equation is in confrontation with the stability of such a mesoscopic particle. This instability is independent of the chaotic character of the corresponding classical system.

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