Table of contents

It is found that the maximal acceleration of a quantum particle is directly related to the speed of transportation in the projective Hilbert space. The minimum space-time uncertainty is inversely proportional to the maximal acceleration of the particle. It is argued that the most accurate clocks are those which can be maximally accelerated.

Many successive ideal measurements separated with time intervals Δ_k (k = 1,2,..., n; n is the number of measurements) are considered. It is shown that there always exists τ₀ > 0 such that if every 0 < Δ_k < τ₀, then the joint probability of finding the quantity measured taking this same value in each of these measurements tends to zero as n → ∞.

A random sequential adsorption model of arbitrary mixtures of line segments of two different lengths is solved on the one-dimensional lattice in order to study the coverage as a function of the segments lengths and probabilities. The rate of late stage deposition is found to be given, independently of the lengths, by the probability associated with the smallest segment of the mixture. This behaviour, and some convexity properties that we find for the jamming limit as a function of the segment lengths, are probably independent of the lattice dimensionality, as suggested by a comparison of our results with the observations done in a recent Monte Carlo deposition of mixtures on the square lattice.

When an electric charge passes past a stationary current loop the time-varying fields induce changes of the internal energy of the loop. The magnetic flux of the loop also changes and, by Faraday's law, creates a radiation electric field which acts back on the particle. This interaction produces a classical electromagnetic lag which generates the phase shifts of the wave function foreseen by these effects. A possible experimental test is proposed.

Quasi-static anelastic experiments on YBa₂Cu₃O_x (x ≃ 6.45, T = 100 and 120 °C) demonstrate a relaxation process with a wide spectrum of activation enthalpies H for the relaxation times (⟨H⟩ ≃ 1 eV, σ_H = (0.07 ± 0.01) eV) and an extremely large relaxation strength Δ = 0.75 ± 0.15. The large Δ identifies the process as an oxygen redistribution between differently oriented sites in the CuO plane. The relaxation process characterizes the rate-determining step in long-range oxygen diffusion.

We show that the signature of a Fermi surface sheet of YBa₂Cu₃O_7-δ may be obtained unambiguously from twinned crystals. Comparison of electron-positron momentum density from YBa₂Cu₃O_7-δ measured both in insulating and (superconducting) twinned phases leads to a decisive further proof of the existence of the Fermi surface in the metallic YBa₂Cu₃O_7-δ. In addition, measurements on untwinned YBa₂Cu₃O_7-δ single crystals reveal also a "ridge" Fermi surface sheet attributed by band structure calculations to CuO chains.

Newly measured reflectivity data of crystalline α-SiO₂ show better resolved features, a new fine structure, a dependence on polarisation of the incoming radiation and a clearly visible plasma edge. A first analysis of these data is presented. The reflectivity of amorphous SiO₂ is quantitatively different. There is no dependence on polarisation of the incoming radiation and all features are less sharply defined. Finally, the Kramers-Kronig transformed reflectivity data of α-SiO₂ coincide well with earlier transmission measurements.

A concept has been proposed which explains the role and the significance of two-phonon processes in resonance effects in high magnetic fields in semiconductors. Pinnings of magneto-optical effects, cyclotron-phonon and magneto-phonon resonances in InSb as well as Cd_xHg_1-xTe have been studied. It is shown that the variety of two-phonon combinations of these effects leads to the fact that in some special cases they consist of almost continuous background for nonphonon and one-phonon resonances.

The low-temperature contribution to the rate of destruction of phase coherence in the effect of weak localization is studied for 3D amorphous Ca₇₅Al₂₅ and Ca₆₅Al₃₅ alloys at temperatures between 0.18 K and 10 K. The relative accuracy of the magnetoresistivity measurements is better than 5 · 10^-8. The large change by a factor of two of the electrical resistivity between both metals, offers a way of 1) experimentally testing the recently proposed mechanism and model of dephasing by zero-point motion; as expected on theoretical grounds, no such mechanism can be deduced from the experimental data; 2) for the first time, an anomalous dependence of spin orbit scattering on diffusivity is found for the amorphous metals: D scales with τ^-1_so not with τ_so as in crystalline cubic disordered metals. We attribute this to the Dy'akonov-Perel' mechanism of spin relaxation in materials without inversion symmetry.

The transport properties of a 2D-electronic system consisting of series of point contacts separated by large "free volumes" with ballistic transport are considered. The influence of a dynamical chaos and of inelastic scattering on linear and nonlinear resistances as well as on a magnetoresistance is considered.

We have observed ballistic motion of vortices in arrays of superconducting tunnel junctions. In a special geometry vortices are accelerated in one region, then launched into a second region without driving forces. At low temperatures, where damping is small, the vortices are found to propagate in a beam across the force-free region.

Defects cause large electronic and mechanical changes in the structure of Chevrel compounds M_yMo₆S_8-zO_x, which drastically alter their superconductive properties. Joint treatment of both kinds of changes explains a wide range of experimental data for M = Sn, Pb, Eu and Cu. Constraint theory predicts a solid solubility limit of x = x_c = 0.25, in good agreement with experiment. The theory explains quantitatively why dT_c/dx is large and negative for M = Sn, Pb and Eu, but positive for M = Cu, and why BaMo₆S₈, predicted by band theory to be metallic with the highest T_c, is actually semiconductive.

Using results for the 4 × 4 and 6 × 6 lattice, we produce the first finite-size scaling analysis of the frustrated Heisenberg model in two dimensions. The results indicate a continuous phase transition from the ordered phase into an intermediate phase without long-range magnetic order, as for the (2 + 1)-dimensional nonlinear sigma-model. The intermediate phase is stable for 0.4 < J₂/J₁ < 0.65 and exhibits either dimerization or broken chiral symmetry. The transition to the collinear phase at J₂/J₁ ≃ 0.65 is apparently of first order.

We study a simple one-dimensional model of a folded polymer with random self-interactions. A numerical study of the specific heat shows two regimes: at high temperature, the specific heat looks smooth and sample independent, whereas at low temperature it possesses many narrow peaks which change with the sample considered. The model is simple enough to allow for a full description of its ground states. We obtain numerical evidence for the presence of a "weak freezing" transition and derive an upper bound for the transition temperature. Heuristic arguments provide an estimate of a critical exponent γ(T) which varies continuously with the temperature in the low-temperature phase.

We study the motional narrowing of a spin-(1/2) particle, diffusing in a solid with random magnetic fields at sites. At high temperatures, where the particle performs random walk between sites, the spin relaxes according to the well-known classical theory of motional narrowing. At low temperatures, where the particle is delocalized, we show that the classical theory breaks down. In this quantum regime we show that the tunnelling amplitude sets the scale for spin relaxation, but not the diffusion rate as previously thought.

Dynamics of SnCl₄ molecules confined in pores of Vycor glass is studied using Mössbauer spectroscopy. Novel data on the temperature dependence of the recoilless fraction below and above the bulk freezing point T₀ = 240 K are presented. The results show unambiguously the existence of the Mössbauer effect (ME) above T₀. This indicates that collective motion of molecule groups which normally obliterates the ME in the liquid phase is prevented in porous glass. A theoretical description is presented which explains the dynamics of molecules located both in the central space and at the walls of the pores.