Table of contents

The proof that faster-than-light communication is not permitted by quantum mechanics derived some years ago by three of us is extended to cover the case of measurements which do not fit within the standard scheme based on sets of orthogonal projections. A detailed discussion of a recent proposal of superluminal transmission making resort to a CP-violating interaction is presented. It is shown that such a proposal cannot work.

The modified Hopfield model defined in terms of "V-variables" (V = 0; 1), which is appropriate for storage of correlated patterns, is considered. The learning algorithm is proposed to enhance significantly the storage capacity in comparison with previous estimates. At low levels of neural activity, p ≪ 1, we obtain α_c(p) ∼ (p|ln p|)^-1 which resembles Gardner's estimate for the maximum storage capacity.

The travelling salesman problem is investigated by mixed thermodynamic and biological-evolution strategies. The concept of life cycles is introduced. The mixed strategy yields in special cases better results than pure simulated annealing.

The recursive algorithm of the normal coordinate expansion of the nonlinear sigma-model of Mukhi is extended. A simple background field expansion for the Wess-Zumino-Witten nonlinear sigma-model is presented. The general structure of its n-th order term in the expansion is explicitly given. It is shown that this expansion is equivalent to the usual background field expansion in the matrix form of the chiral model.

Using Adler's chromostatics and the leading-logarithm, renormalization group improved Euclidean action for the gluon fields, it is shown that the colour singlet hybrid state of quark-antiquark-gluon (qg) should be as well defined as the corresponding meson (q), baryon (qqq) or glueball (gg) states. For simplicity the colour gauge group is chosen to be SU(2).

Aim of the present work is to show the importance of a detailed microscopic description in the calculation of two-nucleon transfer reaction cross-sections. We also show that pairing correlations give rise to a clusterization on the nuclear surface even in deformed nuclei.

A survey of the interactions of ¹⁶O nuclei of energies 60 and 200 A GeV in nuclear emulsions is presented. Correlations between projectile fragments and target evaporation are studied. The electromagnetic dissociation of ¹⁶O, mainly ascribed to absorption in the giant-resonance region, is found to contribute to a sizeable fraction of the cross-section, and increases with increasing energy.

A joint experimental-theoretical study of the population of the vibrational levels of the a¹Δ_g and b¹Σ_g⁺ states excited in an electron-O₂ collision at low energy is presented. The experimental study is performed on a crossed beam electrostatic spectrometer: the relative population of the vibrational levels is directly determined from the energy loss spectra. The theoretical analysis is performed in the effective range approximation; following our previous work, the electronic excitation to the a and b states is described as a resonant multichannel process involving the O₂^-(²Π_g) intermediate. The vibrational population associated with the excitation of the a and b states is found to be quite different from a Franck-Condon distribution, at low energies. This is attributed to the multichannel character of the resonant excitation process.

In order to modelize the propagation of pressure waves in porous media, we study the diffusion of electric charges on two-dimensional model systems. For homogeneous structures, we show that complex impedance measurements agree with theoretical predictions. In the case of percolation lattices, we observe experimentally an "anomalous diffusion" behaviour. We analyse these observations using classical results from the percolation theory.

The critical temperature of La₂CuO₄ specimens showing a grain surface superconductivity after annealing under normal O₂ pressure can be improved by 2.9 GeV krypton ion bombardment. The midpoint temperature has thus been raised from 32.5 to 35.0 K by a fluence of 4 · 10¹² cm^-2. Such a result probably enhanced by the high value of the electronic stopping power might be related to the pressure effect of the defects induced by irradiation and/or the existence of a spin density wave phase.

It is shown that the anisotropic central-force network of randomly mixed incompressible and compressible springs considered by Roux and Hansen is splay rigid for all nonzero concentrations of incompressible springs. The scaling of the splay elastic constant near the rigidity threshold where the bulk modulus diverges is presented. The exponent governing this divergence is shown to be identical with that for the conductivity of a random network of finite and zero resistances.

Edge melting of submonolayer films (analogous to surface melting in bulk solids) of Fe(CO)₅ adsorbed on graphite causes smooth deviation of the Mössbauer spectral intensity I(T) from exponential temperature dependence for observation in direction parallel to the film-surface. This anomaly starts at T ≃ 0.9T_m (2D) and lasts until T_m (2D) is reached. The gradual I(T) changes are associated with gradual reduction of the average solid island-size at temperatures close to T_m (2D) when peripheral island-molecules become mobile.

Excitons in GaAs/Al_xGa_1-xAs superlattices are investigated by means of photoluminescence and reflectivity measurements carried out in a wide temperature range. Comparison between reflectivity and photoluminescence clearly shows the excitonic character of the radiative recombination in such structures up to room temperature. Several excitonic states due to higher conduction and valence subbands are observed. High excitation intensity photoluminescence spectra are also reported, and cooperative processes due to excitonic band filling effects are shown to modify the excitonic line shapes.

We present and analyse ¹H NMR relaxation (T₁^-1) data of the two-chain organic conductor TTF [Ni(dmit)₂]₂ as a function of temperature and magnetic field at ambient pressure. In the entire temperature domain studied between 1 and 300 K, we report important deviations to the normal metal Korringa law. We show that both T₁ temperature and field dependences can be well understood in terms of a typical balance between 1D uniform field-dependent and diffusive spin correlations, which dominate the relaxation enhancement above 60 K or so, and 1D antiferromagnetic fluctuations that strongly develop below this temperature with a power law temperature dependence. The successful comparison between theory and experiment strongly supports a model in which TTF-conducting chains are dominated by repulsive electron-electron interactions and 1D correlation effects down to the lowest temperature reached.

The mobility of the two-dimensional electron gas in thin GaAs multiple quantum well structures is studied. At low temperatures a strong decrease is found for well widths between 60 Å and 40 Å. This can be explained by interface roughness scattering. The measured electron density dependence of the mobility is used to determine the roughness parameters Λ ≈ 65 Å an Δ' ≈ 2.4 Å. The importance of the penetration of the electron wave function into the barrier is discussed.