Table of contents

Fractal aggregates simulating a dendritic growth can be obtained using the Witten and Sander model. This one neglects conduction properties of the aggregate itself. Using the electrodeposition example, we demonstrate that such model can only describe a real system for length scales smaller than ξ. ξ depends on the ratio of the conductivities of the aggregate and the surrounding medium through a relationship with a critical exponent ν. ν is written as a function of the Euclidean (d), fractal (D) and spectral (d) dimensions.

Thin-film superconducting tunnel junctions (Sn/SnO_x/Sn) operated at a temperature of 0.3 K were irradiated by a ⁵⁵Fe X-ray source. Pulses due to photo-absorption in the devices were observed and the Mn K_α, K_β lines at 5.89 keV and 6.49 keV, respectively, could clearly be resolved by pulse-height analysis. An energy resolution of ΔE(FWHM) = 250 eV was obtained in the first tests reported here. Improvements may be possible by straightforward techniques.

Accurate variational calculations of the photodetachment cross-section of Ps^- are reported. Both length and velocity forms are reported. The sum rule S_-1 is satisfied to within 2%. The cross-section has a maximum of about 68 · 10^-18 cm² at a wave-length of between 1.8 · 10⁴ and 2.1 · 10⁴ Å and shows a significant peak just below the threshold for exciting Ps (n = 2).

We report on two experiments using an atomic cascade as a light source, and a triggered detection scheme for the second photon of the cascade. The first experiment shows a strong anticorrelation between the triggered detections on both sides of a beam splitter. This result is in contradiction with any classical wave model of light, but in agreement with a quantum description involving single-photon states. The same source and detection scheme were used in a second experiment, where we have observed interferences with a visibility over 98%.

A new periodic antiphase structure is proposed concerning the ordering of the counterions in the 1D Pt chain compounds M_x[Pt(C₂O₄)₂]yH₂O. Its diffraction spectrum, which is superimposed on the overall modulation spectrum previously described, is shown to be in agreement with the observed extinction rules. This type of ordering further supports the idea, introduced some years ago, postulating a fragmentation of these structures into periodic microdomains locally commensurate, but statistically incommensurate with the underlying lattice.

We present the results of a numerical simulation of a three-dimensional nearest-neighbour J = ± 1 Ising spin glass model, and show strong evidence for the existence of many states at low temperature. The internal energy, the magnetization and the Edwards-Anderson order parameter are found to be self-averaging and state independent, while characteristic times are not. We also find strong evidence for 1/f magnetic noise.

Statistical properties of the Hopfield model of content addressable memory are considered. It is shown that in large systems "phase transition" between regions of "well-functioning" and "chaotic" memory occurs. The phase diagram on the plane (γ, T) is found (γ is the relative number of stored prototype neuron configurations, T is the temperature, i.e. the noise intensity).