Table of contents

A quantum cryptography set-up is described. It uses polarized photons to code the key. The photons remain guided from the semiconductor laser diode until the photon counter module. The feasibility of establishing a key over more than 1 km by this method has been experimentally demonstrated.

The question whether the correlations in DNA sequences are long ranged or short ranged is addressed by an extensive analysis of genetic sequences. Our results do not support the notion of critical properties of sequential distribution of nucleotides. Continuous segments of DNA (chromosomes) can be viewed as a mosaic structure of shorter segments with distinct functional role and, consequently, with distinct compositional and sequential characteristics. It is shown that the correlation functions within well-defined sequential entities decay exponentially after one or two hundred nucleotides. The main contribution to the correlations stems from the correlated distributions of repetitions of similar nucleotides. If the analysis of correlations is performed without respect to the borders between functional entities one obtains the signs of long-range correlations.

The quantum calculations of diffusion coefficients and mixture viscosity were carried out, for the first time, for molecular mixtures that included a non-linear, polyatomic molecule, methane, interacting with helium. The interaction forces were taken from earlier crossed-beam experiments and the generalized cross-sections were obtained within the Wang-Chang-Ulenhbeck (WCU) description of the transport properties. Different angular-momentum coupling schemes are tested for the relevant inelastic collisions and the coefficients are finally compared with experiments. Rather good agreement is found between calculated quantities and the existing data. The present results show for the first time the computational feasibility of the WCU theory for testing molecular interactions via non-equilibrium properties in systems more complex than simple diatomics.

The dynamics of Bénard-Marangoni convection with unidimensional heating in a pure fluid is studied experimentally. Convection begins with rolls parallel to the heater. The characteristics of these primary rolls have been determined. When the temperature difference across the liquid layer is increased beyond a critical value a secondary instability appears. Motions transverse to the heater with a definite wavelength can be seen. Moreover, for small angles between the heater and the fluid surface, the pattern drifts along the heater with a velocity that depends almost linearly on the inclination. A phenomenological phase equation is proposed to interpret this observation.

We propose an integral method to deal with the spherically symmetric non-linear Fokker-Planck equation appearing in plasma physics. A probability transition expression is obtained, which takes into account the proper domain for the radial velocity component. The analytical and computational results are new, and the time evolution is completely satisfactory. The main achievement of the method is conservation of both the initial norm and energy for unlimited times, which has not been attained in the differential approach to the problem.

A novel formulation of X-ray diffraction and refraction is derived from the dynamical theory of X-ray diffraction. All the expressions of the dispersion relation, wave field amplitudes and reflectivities are given in analytic forms which are valid for the whole angular range of the symmetric diffraction geometry. In addition to putting Fresnel's reflectivity and Darwin's curve into a unified scheme, this framework provides a precise physical picture of diffraction from periodic media.

We show that by measuring force and stiffness on a constant-current scanning tunnelling microscopy (STM) contour a deformation-free topography can be extracted. With reference to mono- and bicomponent self-assembled monolayers, we find that the characteristic depression pattern and the protrusions on a multicomponent film found in STM are to a great extent due to electronic effects.

In addition to the already known softening of the lowest transverse optic mode (TO) in KTa_{1 - x} Nb_x O₃ a pronounced temperature dependence of the transverse acoustic (TA) mode in the [100] direction of the Brillouin zone has been observed by inelastic-neutron-scattering measurements for Nb concentrations of x = 0.008 and 0.012. In the vicinity of the phase transition the dispersion curve of the TA branch exhibits peculiarities at small wave vectors. The observed anomalies are attributed to hybridization effects of modes belonging to the same representation and to an exchange of eigenvectors between TO and TA modes.

A density-functional approach is developed to evaluate the phonon frequencies and the elastic constants of the 3D quantal electron lattice near melting from the exchange-correlation local field factor of the electron fluid near crystallization. Results are presented for both the b.c.c. and the f.c.c. structure. Mechanical stability against small deformations is demonstrated for the b.c.c. electron lattice.

We investigate theoretically the "spin"-flip and "spin"-conserving processes for holes in single quantum wells. The presence of a spatial asymmetry in the hole envelope function Hamiltonian or the small linear-in-|k| perturbation lifts the parity degeneracy of the Luttinger dispersions of the symmetric quantum well. We show that in this case a D'Yakonov and Perel-like mechanism holds for the relaxation of the hole "spin".

A homologous series of cuprates, Sr_{n -1} Cu_{n + 1} O_2n, formed by introducing a parallel array of planar defects into the infinite-layer cuprate, SrCuO₂, have been reported by Takano et al. In each CuO₂ plane line defects consisting of CuO double chains result. An analysis of the electronic properties of such planes demonstrates that the stoichiometric compounds with n = 3, 7, 11,... will be frustrated quantum antiferromagnets and spin liquids. When lightly doped with holes the spin gap will remain and singlet superconductivity should occur on a separate but high temperature scale. This prediction may shed new light on the origin of the separate energy scales for the spin gap and superconductivity in other lightly doped cuprates.

We present a statistical-mechanical analysis of a system of randomly cross-linked macromolecules, focusing on the emergence of random static-density fluctuations as the solidification transition is approached. In mean-field theory the problem can be formulated in terms of a single macromolecule in an effective random potential. The distribution of the random potential, which must be calculated self-consistently, determines all moments of the static-density fluctuations. We find that the solidification transition occurs when an infinite network of cross-linked macromolecules is formed, and that a sequence of nonlinear density response functions diverges simultaneously at this transition.

The undulatory excitations of highly ordered stacks of lipid bilayers at a high hydration level (19 weight % of water; water thickness d_w ≈ 10 Å) was studied by spin echo spectroscopy in the wave vector range 0.03 Å^-1 < q < 0.125 Å^-1, i.e. at qd_w ⩽ 1. The linewidth of the excitations exhibits a scaling law Γ_q ∝ q^2.5±0.5. Undulations of a free membrane are expected to show a q³-behaviour and stacks of coupled bilayers a q²-dependence. Although both cases are still possible according to our scaling analysis, they can be ruled out as the frequencies expected from theory are a factor of 100 to 1000 higher than the observed ones. Our data seem to favour a recent theory by Seifert and Langer taking into account that the bilayer consists of two single layers coupled by friction. A new mode with a (ω ∼ q²)-dependence is predicted and its calculated frequencies are in accordance with the experimentally observed order of magnitude.