Table of contents

A method is introduced for studying thermal relaxation in multiminima energy landscapes. All the configurations connected to a given energy minimum by paths never exceeding a chosen "energy lid" are found, each equipped with a set of pointers to its neighbours. This information defines a phase space pocket around the minimum, in which the master equation for the relaxation process is directly solved. As an example we analyse some instances of the Travelling-Salesman Problem. We find that i) the number of configurations accessible from a given suboptimal tour grows exponentially with the energy lid, ii) the density of states within the pocket also shows exponential growth, iii) the low-temperature dynamical behaviour is characterized by a sequence of local equilibrations in increasingly larger regions of phase space and finally iv) the propagator decays algebraically with a temperature-dependent exponent. These observations are related to both theoretical models and experimental findings on relaxation in complex systems.

We analyse the fracture dynamics of a thin film which covers an elastic substrate subject to stretching. The system is modelled through its one-dimensional electrical analogue: a two-layer system (ladder network) with large conductors on one level and random fuses on the other. The fragmentation process exhibits two different regimes, depending on whether the mean fragment size is large or small compared to an intrinsic correlation length. In the latter case we find a power law dependence of the mean fragment size on the externally applied current, the exponent depending on the distribution of the fuse strengths.

The statistical properties of the many-body energy spectrum of a double-barrier heterostructure are studied. Different types of interactions between the electrons are considered. For the cases where the interactions are homogeneous, the energy levels exhibit properties associated with regular system behaviour. On the other hand, when interactions are limited to electrons in the well region, the statistical properties of the energy spectrum are typical of systems which exhibit chaotic behaviour in the classical limit.

We study the scenario of Hopf bifurcations in the electrical conductivity of barium-sodium-niobat (BSN) crystals at high temperatures between 500 and 800 K. After the third Hopf bifurcation in parameter space we find a region with stable three-frequency quasi-periodicity showing three incommensurate frequencies, an attractor dimension D₂ = 3 and an entropy converging to K₂ = 0.

We study the time evolution of the Hamming distance and survival probability between two configurations submitted to the same thermal noise for the antiferromagnetic Ising model on a triangular lattice. The dynamical phase transition temperature T_d we obtained is very close to the percolation transition temperature T_p which is obtained by studying the percolation properties of defined clusters of spin pairs satisfying the interaction. All our results support the idea that the dynamical transition and the percolation transition may be related.

The differential cross-sections of the charge exchange reaction ⁶Li(⁷Li, ⁷Be) ⁶He were measured at beam energies 78 and 82 MeV. In addition to the 0⁺ bound state and sharp 2⁺ resonance, three wide bumps were identified in the experimental continuum spectrum of ⁷Be at small angles. The maximum at excitation energy of ∼ 6 MeV (Γ ∼ 5 MeV FWHM) is consistent with a soft-dipole response, expected in nuclei with neutron halo.

A new scheme for temperature measurements in a magneto-optical trap is reported. It allows non-destructive on-line measurements of spring constant, friction coefficient, and temperature in a magneto-optical trap. By this method, sub-Doppler temperatures as low as 20 μK have been measured on-line in a rubidium trap.

A two-dimensional model for particle size segregation by shaking has been developed and applied to a binary mixture of discs with radius ratio Φ. The simulation results can be understood in terms of a simple theoretical model that becomes exact in the limit Φ → ∞. When a single large disc is imbedded in a regular packing of small discs, it is shown that segregation occurs only for values of Φ above a critical value Φ_c which depends on the angle of repose α and varies between 2.993... and 12 when α varies from 30° to 60°.

The epitaxial growth of a cubic Fe silicide phase on Si(111) has been confirmed by means of X-ray photoelectron diffraction (XPD) and surface-extended X-ray absorption fine-structure (SEXAFS) experiments. XPD experiments show that a 5-monolayer Fe film deposited on Si(111) and subsequently annealed at ∼ 500 °C has a cubic structure. SEXAFS measured at the Fe K edge (7110 eV) reveals that Fe atoms are coordinated with eight Si atoms with bond length of (2.38 ± 0.04) Å and with six Fe atoms with bond length of (2.71 ± 0.04) Å. All measurements lead to the conclusion that this cubic silicide has a CsCl-type structure.

We calculate the level statistics by finding the eigenvalue spectrum for a variety of one-dimensional many-body models, namely the Heisenberg chain, the t-J model and the Hubbard model. In each case the generic behaviour is GOE, however at points corresponding to models known to be exactly integrable Poisson statistics are found, in agreement with an argument we outline.

Scanning-angle reflectometry around the Brewster angle has been used to measure the reflectivity of a flat silica surface sparsely seeded with latex particles. The reflectivity for such surfaces is composed of 1) the direct (Fresnel) reflection from the silica-water interface, 2) the (Mie) scattering of the adsorbed particles and 3) the interference between the two previously mentioned components. The analysis of the obtained curves of reflectivity vs. incidence angle yields three parameters: particle size, refractive index of the particle, and the adsorbed particle density.

We discuss the deformation of a fluid droplet in an emulsion under external forces, such as those exerted by contact with neighboring droplets. We find that the deformation energy associated with a small droplet-droplet contact scales as f² ln (1/f) with the force f exerted between droplets. We consider the equation of state of an emulsion in which droplets are assumed to interact only via such contact forces, and obtain an osmotic compressibility which diverges logarithmically with the osmotic pressure in the limit of small pressure.