Table of contents

It is shown that the simplex of classical states, i.e. the set of those statistical operators which admit an integral representation by means of coherent states, can be explained as a result of the quantum analogue of the Poisson limit of de Finetti's theorem.

The entropy of the response of a system to a probe is proposed as a quantitative measure for the fraction of the total phase space accessed by that probe. The measure has the required dynamic range and its value depends both on the Hamiltonian of the system and on the probe. Different probes can be considered. The role of quantal fluctuations is explicitly taken into account. The computational procedure can be implemented in general, but an algebraic approach offers a convenient framework for its interpretation and evaluation. An example is provided. The measure is suited for application to experimental data.

The equilibrium statistics of self-avoiding Euclidean surfaces of fractional dimension D and fixed connectivity in a d-dimensional space is investigated. An ε = 4D - d(2 - D) expansion about the upper critical curve d = 4D/(2 - D) is developed by generalizing the direct renormalization treatment of polymers (D = 1). The exponents ν and γ are computed to O(ε). There are O(ε) corrections to γ only for D near an integer.

In the reaction of π^- dissociation into a μ⁺μ^-π^- system on a Cu nucleus, a clear enhancement in the ρ⁰π^- effective-mass distribution has been observed in the π₂(1680) region. Partial-wave analysis shows that this enhancement is due to the P-wave and has a resonant nature. Coherent and incoherent cross-sections for this enhancement have been measured.

Empirical evidence suggests that neutron pairing plays an important role for the stability of nuclei near the neutron drip line. It is shown that the low binding of these nuclei will lead to a neutronization of the nuclear surface and possibly to large cross-sections for Coulomb dissociation, which then offers a new way to study clusters and their binding energies.

We re-examine the charge longitudinal response, as measured in inelastic electron scattering, within the framework of a semi-classical RPA theory, utilizing a Skyrme force. The effects of the Hartree-Fock field are analysed. A comparison with an exact RPA calculation is presented.

We have obtained a well resolved spectrum of the Q-branch of the ν₁ band of CO₂ in natural abundance in the vicinity of 1285.5 cm^-1, with the help of a high-resolution CARS spectrometer. It was possible for the first time to give an attribution of the lines and to determine their position for J values from 8 to 38. Comparison with a calculated spectrum exhibits a very good agreement except for the low J values.

The photon statistics at the output of a nondegenerate optical parametric oscillator is studied. The noise spectrum S_I(ω) of the difference I between the two signal beam intensities is evaluated theoretically. A simple "corpuscular" method, in which photons are considered as classical particles, and a full quantum treatment lead to the same conclusion: photon noise on S_I(ω) is suppressed at the frequencies which are multiple of the free spectral range and it is reduced inside the Airy peaks.

We report on the first observation of optical Stark splitting in noncollinear, nondegenerate multiwave mixing, in Doppler-broadened gas media. We demonstrate this optical line splitting to be the dominant wave mixing process at large e.m. field intensities. Polarization characteristics of this type of high-order mixing processes are investigated, and applications to methods of optical phase conjugation with frequency up-conversion are outlined.

The phase diagram of a binary layer of grafted, laterally mobile polymers (amphiphilic diblock copolymers at a liquid interface) is investigated using both Flory-type, mean-field approach and scaling arguments. The system is found to behave as a two-dimensional regular solution, with an interaction parameter that is a decreasing function of the average distance between grafting sites (D). Below the overlap threshold (D > R_F, where R_F is the Flory radius) ∼ (R_F/D)². For high grafting densities (D < R_F) we find ∼ (R_F/D)^4/3 when using the mean-field approach and ∼ (R_F/D)^5/3 when scaling methods are employed. The existence of the two regimes is due to the coupling of chain conformation and grafting density in the high-density regime.

We present here what we believe to be the first experimental observation of the singular behaviour of the refractive index of a critical microemulsion in the vicinity of the liquid-liquid phase separation. As predicted by two-scale factor universality, and confirmed by independent constant-pressure heat capacity measurement, such anomaly cannot be explained by the well-known heat capacity one. Moreover, in spite of very large values of the correlation length in such samples, singular local-field correction does not seem to play the main role in the observed behaviour. Such a result confirms the assumptions generally used in the descriptions of critical binary mixtures near their consolute points.

A new many-body theory for the de Haas-van Alphen (dHvA) effect in strongly interacting electron systems is proposed, in terms of the statistical quasi-particle theory of interacting fermions, which leads to important deviations from standard theory. These deviations are related to various interaction effects in heavy fermions, and I show how they may be used to test the Fermi liquid hypothesis for their low-temperature behaviour.

We report the optical determination of exciton binding energies in small-period GaAs/Ga_0.7Al_0.3As superlattices by means of low-temperature photoluminescence excitation spectroscopy and photoluminescence spectroscopy as a function of temperature. The heavy-hole exciton binding energy decreases with decreasing superlattice period. Our experimental findings are in reasonable agreement with a variational calculation.

T_c values for triplet pairing superconductivity can be much higher in quasi-two-dimensional nearly magnetic fermion systems than in three-dimensional ones.

The magnetic screening and flux distribution within a polycrystalline sample of La_1.85Sr_0.15CuO₄ has been measured by means of muon spin rotation spectrocopy (μSR) between 7.6 K and T_c = 34 K in applied fields of 10 Oe to 2 kOe. Above 500 Oe the transverse field relaxation rate of the μ⁺ spin polarization is quasi-field-independent, as expected for a superconducting material in the mixed state. At low field, a clear reduction of the relaxation rate is observed, interpreted as the disappearance of the mixed state. However, the average internal field at the muon sites, as deduced from the μ⁺ Larmor frequency does not reveal a sizeable Meissner effect in clear contradiction with the experimental results of the d.c. magnetization carried out on the same sample.

The upper storage capacity of a neural network for patterns of fixed magnetization m is calculated. The optimal capacity increases with the correlation m² between the patterns.

We measure ¹¹⁹Sn Mössbauer spectrum of a (110) face of chromium and demonstrate that it can be properly described in terms of sinusoidally or snoidally modulated spin-density-waves (SDW) if ∼ 2% contribution of the third harmonic of SDW is taken into account.

A model is developed where the Mössbauer ion is subjected to two independently fluctuating hyperfine fields: the changes of the spectrum with respect to the case of a stochastic field alone are shown and a practical application of this model is suggested.

A molecular relaxation mechanism is proposed that accounts quantitatively for the viscosity of liquid sulfur above its polymerization point. The mechanism involves an interplay between reversible bond-breaking and polymer diffusion. The resulting viscosity is about five orders of magnitude less than would be predicted by reptation theories of molecular entanglement in the absence of chain breakage.

We realized a model which self-organizes to perform a task via a learning-by-example scheme. The system is a network of Boolean operators which, in some of our computations, has been able to achieve an error-free design for addition between integer binary numbers when shown only a small subset of all possible additions. The training procedure, based on optimizing the network on the given sampling using simulated annealing, is completely general and allows in principle to treat any binary mapping. We recognize different regimes in learning, i.e. the system can both memorize patterns (with a capacity which is numerically estimated) and generalize information to construct rules and algorithms. Some scaling relations are conjectured and numerically tested for these different regimes.

Erratum for Europhys. Lett.4 (1) 97-102 (1987)