Table of contents

We present a novel method for simulating hydrodynamic phenomena. This particle-based method combines features from molecular dynamics and lattice-gas automata. It is shown theoretically as well as in simulations that a quantitative description of isothermal Navier-Stokes flow is obtained with relatively few particles. Computationally, the method is much faster than molecular dynamics, and the at same time it is much more flexible than lattice-gas automata schemes.

The reptational dynamics of a charged Rouse chain under electric field is studied. Internal modes are taken into account, and it is shown that the chain mobility is decreasing due to the effect of these modes.

The microscopic shell model of nuclear physics is used to calculate the electronic structure and excitations of spherical atomic clusters described within the jellium model. The ground-state energies and ionization potentials compare well with those of the local-spin-density approximation. The spherical clusters do not show odd-even staggering in the ionization potential. The calculated photoabsorption cross-section has two dominant peaks for clusters of 9 and 10 atoms but only peak of the 8-atom cluster, in agreement with experiment.

The dynamics of the fragmentation of a liquid drop falling in a miscible fluid is studied in a Hele-Shaw geometry. We show that this configuration allows quantitative measurements of the characteristic sedimentation velocity and internal velocity of the drop. The break-up process observed in 3D is also found in this quasi-2D configuration. The relation between the internal circulation pattern and this process is also discussed.

Experimental observations of azimuthally traveling waves in rotating Rayleigh-Bénard convection in a circular container are presented and described in terms of the theory of bifurcation with symmetry. The amplitude of the convective states varies as √ε and the traveling-wave frequency depends linearly on ε with a finite value at onset. Here ε = R/R_c - 1, where R_c is the critical Rayleigh number. The onset value of the frequency decreases to zero as the dimensionless rotation rate Ω decreases to zero. These experimental observations are consistent with the presence of a Hopf bifurcation from the conduction state expected to arise when rotation breaks the reflection symmetry in vertical planes of the nonrotating apparatus.

The fractal dimension of iso-vorticity surfaces is estimated from a 3-dimensional simulation of homogeneous turbulence at moderate Reynolds numbers, performed by Vincent and Meneguzzi. The results are found to be compatible with a recently proposed theory which predicts a crossover from a 2-dimensional geometry at small scales to a fractal geometry at larger scales, with a dimension D = 2.5 + ζ/2, with ζ being the exponent characterizing the scaling of velocity differences.

Aluminum targets irradiated with intense (I ∼ 10¹⁷W/cm²) 80 fs laser pulses are shown to emit strong Kα lines in the kV spectral region. The analysis of the spectra indicates that suprathermal electrons play an important role in the laser energy deposition and transport. Hydrodynamic simulations taking into account hot electron transport are in quantitative agreement with the measurements. Based on these simulations, the duration of the suprathermal X-ray emission is shown to be comparable to the incident optical pulse.

The kinetics of the end crosslinking process in linear-polymer melts is studied using molecular-dynamics simulations. Starting from an equilibrated melt, tetrafunctional crosslinkers are attached randomly to a fraction x of the chain ends. The system is then allowed to evolve. When a free end comes within a short reaction radius r_x from an unsaturated crosslinker, the chain ends are attached. Two cases can be distinguished. In the first, with a stoichiometric number (x_s = 1/4) of crosslinkers present, the time dependence of such quantities as the number of free ends and the number of unsaturated crosslinkers decays as a power law in time t^-a with α ≈ 0.5. However, for x ≠ x_s, the decay is significantly faster.

The anomalous low-energy vibrational behaviour exhibited by amorphous solids (a peak in the vibrational density of states, in C_v/T³ and in the Raman spectrum–the boson peak–and a plateau in the thermal conductivity) is ascribed to the phonon scattering caused by intrinsic density fluctuation domains in the structure, within which short- and medium-range order is maintained and beyond which the material is structurally isotropic and homogeneous. Phonon localization occurs when the mean-free path is comparable to the size of the domains. This model can also explain the correlation between the boson-peak frequency and the position of the first sharp diffraction peak in the structure factor observed in a number of inorganic and polymeric amorphous solids.

The dispersion of collective models in the amorphous solid, Mg₇₀Zn₃₀, has been determined down to momentum transfers below the first pseudo-Brillouin zone for the first time using epithermal neutrons at the high-resolution time-of-flight spectrometer HET at the spallation source ISIS. In the region of Q values, where the previous experiment done with thermal neutrons and the present experiment overlap, the two sets of results agree very well. Also the previous discrepancies with the dispersion determined from the calculated one-phonon spectral function for the same metallic glass reported earlier are now less severe in the comparison with a more recent and more complete theoretical investigation of the dynamics of this glass. The extension into the first pseudo-Brillouin zone by neutron Brillouin scattering now allows this dispersion branch to be assigned to predominantly longitudinal excitations. Previously, it was erroneously assumed to be a transverse branch due to the discrepancies with the earlier theoretical results.

We report the observation of a novel phase on the Pb(110) surface below 400 K using the high-resolution low-energy electron diffraction technique. The surface contains two layers of atoms separated by double steps. We suggest that the observed phase may be related to a class of disordered flat phases predicted recently by Rommelse and den Nijs. As the temperature approaches 415 K, the double-step surface thermally transforms into a Kosterlitz-Thouless rough surface in which multilayers of atoms are involved and the steps have predominantly the single-atomic-step height.

This letter reports upon transport properties of the (EDT-TTF)₂ [Pd(dmit)₂]₂ compound. A nonlinear effect is observed below 50 K at ambient pressure. A resistivity bump, observed at ca. 40 K at ambient pressure, is shifted towards lower temperature under pressure and a metallic ground state is stabilized up to 10 kbar. In order to understand lack of superconductivity above 500 mK, low-dimensional electronic correlations are stressed on.

Basic results concerning oxygen ordering in the superconducting compound YBa₂Cu₃O_z are briefly summarized. It is shown that, at equilibrium, only infinite-chain structures can be stabilized and those models based on hypothetical (and actually nonphysical) screened Coulomb interactions cannot produce stable ground states. It is suggested that diffraction data (neutrons, X-rays, electrons) from oxygen-lean samples are indicative of metastable displacive transformations, and are not directly related to oxygen ordering.

The magnetic coupling between 4 nm thick layers of the spin glass Cu_1-xMn_x is found to depend strongly upon the metal separating the spin glass layers. The spin glass layers couple across ⩾ (20 ÷ 30) nm of Cu or Ag, which is longer than the distances that ferromagnetic layers couple across Cu. The coupling across Cu_0.95Ge_0.05 is somewhat weaker; that for Al is weaker still; and those for Mo, Nb, and V are the weakest. Potential sources of these couplings include–in analogy with Dy/Y multilayers–the possibility that the SG layers drive Cu or Ag interlayers into a finite correlation length spin-density-wave state.

A two-layered neural network that organizes itself in response to a set of external stimuli is considered. The output layer is a stochastic discrete model of a neural network. The synaptic weights between layers obey a competitive learning law. It is shown that the Long-Term Memory dynamics (weight dynamics) always stabilize by minimizing a Lyapounov function. In the special case where all lateral interactions are uniform, negative and sufficiently strong, this Lyapounov function, within the limit of low noise, is interpreted as the Kullback discrepancy between the probability distribution of the environment and the internal representation of the network.

Erratum for Europhys. Lett.16 (3) 283-288 (1991)