Table of contents

In this paper we apply the unified dynamics of Ghirardi, Rimini and Weber to the translational and rotational motion of solids in three dimensions. We show that, in a certain approximation, the rotational equations can formally be reduced to the translational ones already known. We point out that the rotation of solids as well as their translation are practically of classical nature without any observable quantum effects.

Quantal and classical spectra are computed for a model 1-D hydrogen atom in an oscillating electrical field. Using a Fokker-Planck equation, the results are discussed in terms of restricted diffusion in action space due to the presence of broken-down KAM tori ("Cantori"). The fluctuations diminish during the onset of chaos and settle down to the expected Thomas-Porter behaviour only upon the final disappearance of Cantori at high field strengths.

We apply concepts and techniques developed in the context of the mean-field theory of spin glasses to networks of random automata. This approach, proposed recently by Derrida and Flyvbjerg, may be useful in understanding the multivalley structure of the Kauffman model.

We study two versions of central-force percolation on a triangular lattice made out of springs that can freely rotate around nodes: 1) random dilution; a proportion 1 - p of springs is missing and 2) random reinforcement; a proportion p of springs is infinitely rigid, while the other ones are all present and have finite strength. We use a transfer-matrix algorithm to deal with strips of width ranging from 2 to 30 in case 1) and 48 in case 2), and length 10⁵. We estimate the threshold p* = 0.642 and we obtain the critical exponents relative to the scaling of the elastic moduli with the strip width in case 1) 3.0 ± 0.4 and 2) 0.97 ± 0.02. These values are very close to the ones found for percolation systems with angular elasticity, supporting the hypothesis that these two critical phenomena belong to the same universality class.

We consider a model of a protein, in which the contact interaction between any pair of amino-acids along the chemical sequence is a quenched random variable. At high dimension, the model is equivalent to a Potts spin glass. The protein has a folding transition, between a Brownian phase at high temperature and a collapsed glassy phase at low temperature. At low dimensions, this equivalence does not hold, and we propose an approximate solution to the mean-field equations.

The effect of a substrate temperature of about twice the Debye temperature on the X-ray standing-wave fields has been explored. An atomic scattering amplitude modified by the Debye-Waller factor appears to be adequate for the description of the standing-wave fields. The high-temperature standing-wave method has been applied to study the temperature dependence of Pb monolayers on Ge(111) surfaces. We obtain evidence that the high-temperature (1 × 1) phase of the reversible phase transition √3 × √3R30° ⇄ 1 × 1 is not consistent with a model of a two-dimensional isotropic liquid Pb layer. Rather it appears to be composed of small islands of the original √3 × √3R30° crystalline overlayer.

In a semi-classical approach to the study of nuclear collective dynamics we analyse the effect of explicit quantum corrections to the Vlasov equation. We show that the lowest multipole response affected is the octupole one. However, quantitatively, these corrections seem to be small for both high- and low-lying octupole states, provided some realistic nuclear potential is used, with surface terms.

Recent results concerning the interaction radius of very neutron-rich He and Li isotopes renewed the interest in measuring the size of a nucleus by means of a strong interacting probe. We stress that various ratios of π⁺ and π^- production can bring valuable information in this respect.

The stretched configurations adopted by densely grafted chains, i.e. chains attached to an interface by a head group only, affect their dynamics. The effect on the characteristic time, τ_exp, required by a single "degrafted" chain in order to leave the layer under equilibrium conditions is analysed. τ_exp is relevant to the understanding of exchange processes in aggregates formed by polymeric surfactants. Because of the chain stretched configuration this motion is reptative in nature. For flat grafted layers τ_exp is determined by driven motion. This arises because of the loss of free energy due to the release from confinement associated with the chain expulsion. As a result τ_exp of a chain consisting of N monomers scales as N² rather than τ_exp ∼ N³ expected for diffusive motion. For extended spherical grafted layers the τ_exp associated with the diffusive and the driven motions are comparable and τ_exp ∼ N^9/5 for the driven case.

The laser containing a saturable absorber (LSA) is known to present optical instabilities in the form of regular pulses of various shapes. We report on the first experimental observation of optical chaos through different scenarios in the CO₂ + CH₃I LSA. By varying the absorber pressure and/or the laser frequency, the LSA output follows a period-doubling cascade to chaos or makes a transition to Shilnikov-type chaos.

We have studied the influence of temperature gradients on the concentration distribution in a microemulsion near a consolute critical point. Temperature inhomogeneities much smaller than the distance to the critical temperature trigger convective flows along the free surface (liquid-vapour interface) and are the cause of a considerable stratification of the bulk sample. All these phenomena disappear if the vapour phase on top of the sample is replaced by a solid phase. We propose an explanation involving a thermodiffusion-induced superficial concentration gradient and the resulting Marangoni convection.

The "multislice" method is used to simulate HREM images of gold particles. The 1024 × 1024 sampling of the supercells and the implementation on a CRAY 1S/2300 computer enable for the first time the accurate calculation of images of icosahedral aggregates containing up to 2869 atoms. The agreement with observations is very good for images along a threefold symmetry axis. For the fivefold symmetry, the computed contrasts clearly explain some anomalous observed features.

The "resonating valence bond" (RVB) state has been proposed as the basis for an explanation of high-temperature superconductivity. Recently, we have described the charge and spin excitations about this state, and have shown that they are solitons, precisely analogous to those found in polyacetylene. Since the charged solitons are + e bosons, it is natural to ask whether flux quantization will occur in units of hc/2e, as in traditional BCS superconductivity, or will come only in larger units of hc/e. We show here that flux quantization in units of hc/2e will occur unless a condensation of cooperative ring exchanges occurs analogous to that found in the fractional quantized Hall effect.

By measuring the structure, superconductivity, magnetoresistance of polycrystalline single-phase YBa₂Cu₃O_7-δ samples, the phenomena of negative magnetoresistance and the hysteresis of the magnetoresistance have been studied. An analysis of the origin of the influence of grain boundaries on the superconductivity and the electronic transport properties are given.

Neutron diffraction experiments on polycrystalline samples of ErBa₂Cu₃O₇ show that the Er moments are ordered three-dimensionally at T = 140 mK. Two independent propagation vectors k₁ = (1/2, 0, 0) and k₂ = (1/2, 0, 1/2) are required to describe the magnetic reflections. The most likely model is one in which all the moments are inclined at an angle of 32° to the crystallographic c-axis and the magnitude of the ordered moment at 140 mK is 4.9(2)μ_B. The two propagation vectors indicate the existence of both ferromagnetic and antiferromagnetic coupling between moments along the c-axis.

Positron lifetime and Doppler broadened lineshape parameter have been measured as a function of quench temperature in Y₁Ba₂Cu₃O_7-x. The mean lifetime, , and the peak parameter of the Doppler broadened lineshape, I, are observed to decrease as the quench temperature (T_Q) is lowered from 925 °C to 325 °C. The variations in and I with T_Q are observed to correlate with the oxygen deficiency (x) measured at a various quench temperatures. The positron lifetime component, τ₂, attributed to trapping at oxygen vacancies, is observed to decrease from (200 ± 1) ps in the tetragonal phase (x = 0.5) to (190 ± 1) ps in the orthorhombic phase (x = 0.0). The variation in positron annihilation parameters with T_Q is qualitatively explained in terms of a change in the charge state of oxygen vacancies.

The spin dependence of the electron current absorbed by a thin Fe(001)-film (target current) has been measured as a function of energy in the range (0 ÷ 22) eV above the vacuum level. The peak value of the spin asymmetry is 7% at 11 eV electron energy. The measured energy dependence of the spin asymmetry is in very good agreement with earlier predictions in the framework of a dynamical LEED calculation by Tamura et al.