Table of contents

For a system of identical particles (we take fermions as an example), we derive the analogue of the Weyl formula for the average level density. This is done by introducing the projection operator onto states in Hilbert space which have the symmetry imposed by particle identity. The semi-classical result is obtained by using the Wigner representation for this operator and by letting ℏ → 0. Numerical examples for few degrees of freedom show in general good agreement with exact diagonalization even near the ground state.

Dipolar systems with randomly directed anisotropy axes are analysed within a local-mean-field approach and in the Ising limit. The properties of the system are shown to be essentially identical to those characterizing Ising spin glasses. Accounting for some departures from the spin glass behaviour that have been found in experiments on frozen ferrofluids requires more involved modelling.

The method of scaled factorial moments is used to study non-statistical fluctuations present in the emission process of the medium-energy protons, emitted in nucleus-nucleus interactions, in terms of their emission angle (θ). The investigation has been done with our data of ²⁴Mg-Ag/Br interaction at 4.5 GeV/c/n and ¹⁶O-Ag/Br interaction at 2.1 GeV/n. Corrections have been made for the non-uniformity of the distribution of the variable used for the purposes of analysis. A power law growth of the corrected moments with decreasing bin size suggests an intermittent behaviour in both the cases. Multifractal analysis in terms of fractal moments, G_i, has also been performed, which reveals hint of multifractality in the emission process.

We have examined the role of plasmons on the electron energy response of solid-state (Si and Ge) radiation detectors. We found that at the level of parts per thousand, internal-conversion electron calibration techniques do not suffice to yield an adequate response function. In particular, spectral distortions in the detection of low-energy beta-particles have been found which are not accounted for by the usual calibration methods. Thus, a small but significant error can arise from energy loss to low-energy plasmons in Si and Ge detectors. The proximity of the plasmon energy to the end-point singularity and the quadratic form of the beta-decay spectrum may account for the effect interpreted as a 17 keV neutrino. Similar errors can also arise in other subtle solid-state measurements as, for example, in the x-ray edge absorption and emission spectra of metals and semiconductors.

1) We describe first the non-uniform stretching of one long flexible tethered chain (N monomers) immersed in a good solvent moving at velocity V. In strong flows the extension L is predicted to increase quadratically with velocity (L ∼ N³V²). The chain can be pictured as a sequence of blobs of size y decreasing as x⁻¹ (x being the distance measured from the chain end).

2) We extend this analysis to the stretching of a few grafted chains ("mushroom" regime) when a shear flow (rate s) of the solvent is imposed near the wall. Here the elongation L ∼ N^3/2s^1/2 and the blob size decreases as y ∼ x^−1/2. 3) Finally we discuss a chain confined in a slit. We find L ∼ N^4/3V^1/3 and blobs sizes y ∼ x^−3/4. In all these cases, the deformation is progressive and we expect no sharp coil stretch transitions.

We propose a mechanism for non-contact repulsive interaction between elastic bodies moving past each other in a fluid. A lateral flow along the curved surface and the resulting deformation lead to dissipative lubrication forces that act normal to the surfaces. Forces between polymer-bearing surfaces undergoing shear are calculated and seem to be important for the interpretation of surface force apparatus measurements such as those by Klein et al.

The influence of the transverse (magnetic) interaction on the dynamic properties of strongly coupled plasmas is considered. It is shown that, though magnetic interaction almost completely compensates Coulomb interaction in the high-frequency domain, the effect of the former on the Langmuir mode dispersion relation remains negligible.

The spatial evolution of the phase of the potential of a resonance cone excited in a warm magnetoplasma by a point-source antenna has been measured. Accurate phase measurements show that at some distance from the source a 2π phase jump occurs. This is related to the thermal structure of the resonance cone. This phase jump and the overall behaviour of the phase are well reproduced by a numerical calculation in the electrostatic approximation.

Powder neutron diffraction measurements of copper(I) iodide at a temperature of 293 K and pressures up to 27 kbar are presented. A structural phase transition is observed at a pressure of 16.3(1) kbar and involves a volume change of 4.1(1)%. Above the transition CuI is found to adopt a rhombohedral structure, space group Rm with Z = 2, a = 7.236(3) Å and α = 33.37(2)° at p = 17.4(1) kbar. The I^- ions essentially retain the cubic close-packed array adopted in the ambient pressure zincblende phase, whilst the smaller Cu⁺ ions are rearranged over the available tetrahedral voids. No structural evidence has been found for the transitions around 4 and 5 kbar observed by other workers.

Three different incommensurate phases predicted by the phenomenological theory of frustrated smectics were reportedly found in mixtures of DB₈OCN and 8OBCAB and DB₇OCN and 8OCB. The results of our high-resolution x-ray diffraction study of these materials show that these phases are, indeed, coexistences of two or more smectic-A phases. The correct phase diagrams of the two systems are found to be in excellent agreement with the phenomenological models. At this time, there remain no known materials that exhibit an incommensurate smectic-A phase.

The free surface of a stressed solid is not stable and fluctuations appear by surface or bulk diffusion. Qualitative analysis of the fluctuations evolution suggests a mechanism of dislocation nucleation at the surface not requiring an activation energy.

The adhesion interaction of an Ir tip with Ir and Al surfaces has been investigated using the scanning tunneling microscope. Qualitative differences in the adhesion characteristics have been observed. A cluster model based on effective pair potentials has been set up to understand adhesion in atomic-scale contacts. The model calculation provides a semi-quantitative explanation of the experimental data. In particular, it is shown that subtle differences of the atomic-interaction potentials have a significant effect on the contact mechanics.

Erratum of Europhys. Lett.22 (7) pp 517

Erratum of Europhys. Lett.22 (2) pp 145