Table of contents

We study low-temperature effects of frequency-dependent damping on spectral and dynamical properties of biased two-level systems. Inherent inconsistencies of the noninteracting-blip approximation are removed by systematically considering the interblip interactions to first order in the coupling strength. The self-energy contribution from all irreducible multi-blip clusters is taken into account. We discuss the influence of phonons and the effects of the superconducting state.

A coarse-grained spin model is constructed and used to study global phase behavior of ensembles of fluid membranes. This model encompasses both bending rigidity renormalization and steric entropy arising from coarse-graining of short-scale membrane fluctuations. It thus enables us to obtain, in a unified way, phase diagrams containing both uniform and periodic phases in microemulsions and binary systems of nonionic surfactant bilayers in a single solvent.

An approximating procedure for geodesic flows on Riemannian manifolds is proposed in terms of approximating singular manifolds which are locally flat. The corresponding dynamical system is then reduced to a discrete map. The reliability of the approximation is tested by evaluating the maximum Lyapunov characteristic exponent for the flow on a two-handle sphere and showing the extremely smooth tendency to the singular case, which in this case corresponds to the Sinai billiard.

The effect of two-dimensional spatial modulations on spatio-temporal Hopf bifurcations is investigated. In particular, in the case of a spatial forcing corresponding to triangular or hexagonal planforms, a strong resonance between triplets of travelling waves may occur, provided that the wavelength of the forcing is nearly equal to one-third of the critical one. In this case a periodic spatio-temporal pattern of triangular symmetry may be stabilized in regions of the parameter space where it is otherwise unstable. Such a phenomenon may present the onset of turbulence in forced hexagonal vortex lattices.

Three-dimensional gold clusters deposited in UHV on clean natural graphite are imaged at the atomic level using scanning tunnelling microscopy (STM). After STM observation, the samples are characterized by transmission electron microscopy (TEM). The smallest observed gold clusters (1.0 to 1.5 nm) are close-packed, two atomic layers high, and stable over a period of time of at least 5 minutes.

It is shown theoretically that nonclassical correlations between the fundamental and the second-harmonic mode exist for second-harmonic generation in a resonant optical cavity. The fluctuations in the sum of the intensities are reduced relative to the corresponding shot-noise level, even beyond the value expected for two uncorrelated sub-Poissonian beams.

The steady convective flow in a compressible fluid layer is computed, for different values of the parameters, using bifurcation theory. We show how to eliminate parasitic 0 eigenvalues due to the Galileian invariance of the problem, and to its partly hyperbolic structure. We compare results with direct numerical simulation, and we discuss the validity on a truncation at the second order in the amplitude for the velocity vector field.

Rod-coil copolymers form smectic phases in melts and solutions. A mean-field analysis of nonoverlapping lamellae formed by rod-coil copolymers in a selective solvent of low molecular weight is presented. The system exhibits a first-order, smetic A-smectic C phase transition. The transition is due to competition between the deformation free energy of the flexible blocks and the surface free energy of the rodlike segments.

Pure crystalline gallium samples have been irradiated at temperatures lower than 11 K with a high-energy Xe beam (27 MeV/a.m.u.) depositing 2 keV/Å by electronic excitations. The results obtained with the aid of resistance variation measurements are compared with those obtained by an electron irradiation and with published results on pure metals. For the first time in a pure crystalline metal, it is found that high-energy ions are more efficient than electrons or low-energy ions for defect creation. The role of displacement threshold anisotropy is discussed, and it is suggested that the high electronic stopping power is responsible for the observed effect.

We have measured the generalized phonon density of states G(ω) of the quasi-one-dimensional compounds M₂Mo₆Se₆ (M = Tl, In, Rb and vacancy) by inelastic neutron scattering on polycrystalline samples. The d.o.s. spectra are consistent with the specific-heat data and can be interpreted as the superposition of a low-energy peak corresponding to the Einstein-like vibration of the M atom and of the characteristic spectrum of the Mo₆Se₆ skeleton. The particular features of the Tl₂Mo₆Se₆ and In₂Mo₆Se₆ spectra are discussed in connection with the occurrence of superconductivity in these compounds. A preliminary derivation of the true (unweighted) d.o.s. g(ω) in the binary compound Mo₆Se₆ is presented.

We have evaluated the contribution to the temperature dependence of the indirect band gap of silicon arising from electron-phonon interactions using a first-principle pseudo-potential technique. This represents the first parameter-free calculation of the temperature dependence of the band gap of a real material. Numerical results are in reasonable agreement with experiment. At high temperatures we find that each phonon branch gives approximately the same contribution to the temperature dependence of the gap.

An exact analytical solution of the time-dependent Ginzburg-Landau model in the large-N limit is presented for a quench to zero temperature. Standard dynamic scaling is obeyed when the order parameter is not conserved, while a novel form of scaling, due to two (marginally different) divergent lengths and characterized by infinitely many growth exponents, is found when the order parameter is conserved. The new scaling behaviour is expected to be a pattern common to other growth phenomena.

We have found the angular dependence of the surface tension using simplified models including a short-range interaction. The shape of a rounded surface is found for a full range of angles as well as the sizes of rectilinear (smooth) faces depending on the parameters of a model and the temperature. Some dimensionless ratios are shown to be universal.

The formation of titanium-silicon interfaces under different growth and annealing conditions is studied by a combination of techniques. The main emphasis is placed on a detailed comparison between experimental and theoretical determinations of the Si L₂₃VV Auger line shape for different compositions of silicides. It is shown that this greatly enhances the potential of Auger electron spectroscopy for the determination of thin layers composition.

New experimental features of the field-modulated microwave absorption in high-T_c ceramic superconductors are presented. The proposed model for the loss mechanism can explain the complicated field and temperature dependences of the absorption signals.

We study doped La₂CuO₄ modelled by a single-band, strongly coupled Hubbard model. Following a proposal of Inui, Doniach and Gabay, we make use of an effective spin Hamiltonian which includes next-nearest-neighbour frustrating couplings. We describe its long-wavelength properties by an O(3) quantum nonlinear sigma-model. We find that antiferromagnetic order survives doping till a critical value δ_c ≈ 0.067. In this regime we obtain essentially a parameter-free prediction of the correlation length ξ(T) as a function of the doping fraction. It is very substantially reduced with respect to the undoped case, and in very good agreement with the experimental data.