Table of contents

Various possibilities for the space experiments on the 5th force are discussed. It is shown that practically no methods exist to obtain the values of parameters for the 5th force by space experiments.

For the quantum particle in a double-well potential, approximated by a two-level spin-boson model, we prove the unicity of the equilibrium state at all temperatures, i.e. the absence of localization. This can be understood on the basis of an exact computation of the transition probability between the thermal wave functions of the two minima.

Random walks on tree structures are as useful tools in physics as they are interesting themselves. Here we show that for a certain class of models they can undergo a transition from being recurrent to being transient depending on the temperature. At the transition the relaxation is logarithmic. The significance of the pole in the relaxation exponent is also discussed.

A new graphical tool for measuring the time constancy of dynamical systems is presented and illustrated with typical examples.

In the framework of spin glass models with symmetric interactions a local dynamical learning process is studied, by which the energy landscape is modified in such a way that even strongly correlated noisy patterns can be recognized. Additionally the basins of attraction of the patterns can be systematically enlarged. After completion of the learning process the system can recognize as many patterns as there are neurons (p = N), and for small systems even more (p > N). The dependence of the learning time R on the parameters of the system (e.g., the average correlation, the noise level, and the number p of patterns) is studied and it is found that R increases as p^x, with x ≈ 3.5, as long as p < N, whereas for p > N the increase is more drastic.

Inclusive production of π⁺ on four nuclear targets has been studied using a 40 GeV/c π^- beam at the Serpukhov accelerator. No noticeable A-dependence has been found.

The medium-range nuclear force is studied in terms of a nonrelativistic quark model, which is free from the van der Waals force. An effective potential method is developed to describe quark rearrangement processes. The role of isobar and hidden-colour states for the medium-range nuclear force is discussed.

It is shown that subthreshold pion production in a collision between two nuclei at intermediate energy has to be attributed to an early stage of the collision and cannot take place together with a multifragmentation break-up at a later stage of the collision.

A computer simulation technique has been used to analyse the effects of ion microfield fluctuations on the Lyman-α fine-structure doublet of hydrogenic ions for the plasma conditions found in inertial confinement fusion (ICF). For a large domain of plasma parameters of interest in ICF our calculations demonstrate that the ion broadening mechanism belongs to a regime which is intermediate between the limiting impact or quasi-static approximations which have been used previously. Our simulation calculations are compared to quasi-static ion profiles in the case of the Ar⁺¹⁷ Lyman-α doublet for several densities and both proton or Ar⁺¹⁷ perturbers.

Gradients of electric fields induce in some media a macroscopic density of electric quadrupolar momentum. Thermodynamics and electrostatics of such media are investigated, showing that an external point charge suffers a short-range screening. The r^-1 divergence of the Coulombic potential is weakened, and cancelled if the induced quadrupolar polarization is simply proportional to the field gradient. Estimates for liquid crystals show that the expected screening length is greater than the elastic coherence length, and that an important part of the external charge is screened.

We study the spatial stability of systems at onset of the temporal intermittent transition to chaos. We show that the phase dynamics during laminar episodes, described as orbits near a saddle-node bifurcation of limit cycles, is second order in time and can lead to propagative behaviour.

Ba₂Y₂CuPtO₈ is orthorhombic (space group Pnma) with a = 13.207(2), b = 5.680(2) and c = 10.321(2) Å. The structure was refined from 2654 x-ray independent reflections to R = 0.050 (R_w = 0.059). The lattice is built up from double chains of corner-sharing platinum octahedra and copper pyramids running along the b-axis. Yttrium and barium ions ensure the cohesion of the structure. The crystal chemistry of Ba₂Y₂CuPtO₈ is discussed and compared to the structure of KPbCr₂F₉.

An equiatomic Ni-Al ordered alloy is irradiated at 10 K with 360 KeV Xe ions and transmission electron microscope observations are performed in situ. Two phase transformations are observed during ion irradiation. In a first stage (up to 3 · 10¹³ ions · cm⁻²) the dominant radiation damage consists in a chemical disordering with the simultaneous occurrence of a new premartensitic phase. Above 3 · 10¹³ ions · cm⁻², diffuse rings are clearly visible and complete transformation to the amorphous state occurs at about 10¹⁴ ions · cm⁻². These results are discussed with respect to those obtained in previous channelling experiments.

The q-dependence of plasmon energy and width for the heavy alkali metals K, Rb, and Cs were measured by electron energy loss spectroscopy. For Rb, we found almost no plasmon dispersion while for Cs, the small-q dispersion turned out to be even negative. These results can be explained qualitatively but not quantitatively by recent electron liquid models including exchange and correlations. Going from K to Rb and Cs, the q-dependence of the plasmon width changes from quadratic to linear.

The properties of various interfaces in helium and, in particular, the interface between liquid ³He and a solution of ³He in ⁴He, may be studied using spin waves. Assuming no transverse relaxation, the boundary condition for the transverse magnetization contains one complex kinetic coefficient, b. For the normal ³He to ³He-⁴He interface, b is related to the ³He quasi-particle transmission probability , which we estimate from a simple model. A calculation of the spin wave absorption spectrum for a typical geometry shows that b and may be measured by NMR. Neither b nor is greatly affected when the pure ³He enters the A phase, but both are strongly reduced in the B phase.

It is shown, from the diffusion model for fluorine-based plasma etching of silicon, how the anisotropy evolution, as a function of the partial pressure of atomic fluorine, may account for adsorption and desorption mechanisms on silicon surfaces. In addition to the anisotropy-isotropy transition, sequential multilayer adsorption is demonstrated experimentally. Etching mechanisms are discussed in the light of the observed results.

It is shown that the change of the optical absorption of reduced undoped LiNbO₃ under heating can be explained as the dissociation of Nb⁴⁺-Nb⁴⁺ bipolarons. By this treatment the well-known absorption band of the material at 500 nm decreases in favour of a new band at 760 nm, attributed to Nb⁴⁺ small polarons. The bipolaron dissociation energy is determined to be 0.27 eV. Also other properties of reduced LiNbO₃ are consistent with the model.

It is conjectured, with the aid of numerical studies, that the site random tight-binding model with the long-range hopping term (decaying more slowly than any exponential function) exhibits nonexponentially localized eigenstates. For example, if the hopping term decays as exp[−√x], then the eigenfunctions decay as exp[−√x].

We report results of Monte Carlo simulations of a three-dimensional dilute Ising antiferromagnet in a field which is the experimental realization of random field systems. We found that at dilution concentration p = 0.2 the existence of a tricritical point is related to the inclusion of next-nearest-neighbour interactions. The results are discussed.

A soliton magnetic resonance (SMR) experiment has been performed on the antiferromagnetic Ising quantum spin chains of CsCoCl₃, at the frequency of 35 GHz, with a magnetic field parallel (H_∥) or perpendicular (H_⊥) to the chains. For the first time SMR signals have been identified in the 1D paramagnetic phase (T > T_N1 ≃ 21 K). They reveal a strong soliton damping and an appreciable resonant frequency renormalization, both associated with the soliton dynamics. For H_∥, the SMR signals provide an evidence for the dyon precession described recently for magnetic chains. These SMR signals have been followed below T_N1, confirming the persistence of solitons in the ordered phases of this compound.

The field-induced motion of Néel lines is optically observed in high-purity iron whisker single crystals. The interaction of these lines with locally produced dislocation arrangements yields a considerable line pinning with characteristic pinning fields of about 2.5 to 3.0 kA/m.

General rules are given for the branching ratio of x-ray absorption lines. The branching ratio between two spin-orbit split absorption edges of a deep core level will deviate from its statistical value only if the spin-orbit splitting in the valence band is large compared to the band width. The branching ratio obtains a maximum value for the Hund's rule ground state and decreases gradually for the other J values. This regular behaviour can be used to assess the spin-orbit splitting in the 4d and 5d band of transition metal compounds.