Table of contents

It is shown that the matching problem and the low autocorrelated binary string problem belong to the same universality class of value landscapes as the TSP and the graph bipartitioning problem.

The amount of the Hawking radiation in Hartle-Hawking vacuum around the Schwarzschild black hole is calculated in the presence of the cloud of virtual-cosmic-string loops near the black hole. Recently Coleman, Preskill and Wilczek have discussed the black-hole temperature in such situation. We show, in this letter, it is hardly possible to find any deviation in the Hawking radiation even if the virtual cosmic strings exist.

Elastic scattering of protons from ⁶Li and ⁶He is studied in the intermediate-energy region (E_p ⩾ 100 MeV) in a folding model with "realistic" target densities which reproduces the r.m.s. radii and a range of electroweak data. The scattering clearly reflects the size of the systems, but is only marginally able to discriminate between details of the correlated valence structures of the two nuclei: at forward angles, where the direct fingerprint of the presence of a halo structure is expected, the cross-section differences are small; at large angles, where the response of the α-core on the halo structure is probed, the differences found are accompanied by a falling cross-section magnitude. Likewise, only a weak dependence is found to differences in the halo tail, corresponding to three calculations for ¹¹Li which all reproduce the experimental r.m.s. radius.

With the aid of quasi-elastic neutron scattering we have investigated the dynamics of three small, related, aromatic molecules (benzene, toluene and ethyl benzene) in their respective solid states. We have observed distinct changes, which we interpret as being due to the onset and slowing-down of reorientational motions of different parts of the molecules.

We have measured, using nuclear-reaction analysis, the concentration depth profiles of polymer brushes consisting of end-tethered deuterated polystyrene tails within a polystyrene homopolymer, as a function of the surface coverage σ of tails and of the degree of polymerisation P of the polymer matrix. We find that the onset of brush swelling is shifted to higher σ values at higher P as expected from theory. Within the range of our parameters the L(σ) and L(P) variations are consistent with predictions of scaling and mean-field models, where L is the effective brush thickness.

When a long hot wire is set horizontally just below the free surface of a liquid, propagative patterns are observed when the heat power, supplied electrically to the wire, exceeds a critical value Q_c. The stability diagram, i.e. the dependence of Q_c on the distance h of the wire from the surface, is determined. The spatial and temporal properties of the waves are studied in the plane of the two parameters Q and h, together with the influence of defects. The results agree very well with a phenomenological approach based on Ginzburg-Landau equations.

The interactions of point vortices in electromagnetically coupled superconducting layers are renormalized by thermally excited vortex-antivortex pairs. We calculate these renormalized interactions in a self-consistent linear-response theory. Thereby we derive the renormalized dissociation temperatures of vortex-antivortex pairs and of single flux lines. It has been noted previously that both temperatures are equal in the absence of thermally created vortex-antivortex pairs. We find the equality even in the renormalized theory and prove it as a direct consequence of flux quantization. Finally we derive the phase diagram of the layered system for zero external field and apply it to experimental data for Bi-type high-T_c superconductors.

We present the first quantitative study of the "tears of wine" phenomenon. The ethanol-water film presents a self-similar profile of parabolic shape and obeys a diffusivelike wetting law. Both the fluid velocity and the surface tension gradient along the film are deduced. In addition, we relate the existence of a star instability at the edge of the reservoir and a fingering instability at the front of the film.

We examine the microscopic pinning of a contact line on a single heterogeneity. We find that many features of this behavior are predicted by models based on the same energy functionals which describe macroscopic contact angle hysteresis on surfaces with many such imperfections. As the defect emerges from the undistorted contact line, the contact line pins, increasingly distorts, slides across the defect, and then jumps back to an undistorted position. The distortion of a stably pinned contact line is described by a logarithmic dependence. While the force pinning the contact line depends on the defect microstructure, the elastic restoring force follows Hooke's law and is dependent on the properties of the fluid and contact angle on the underlying surface. The sliding motion and unstable jumps of the contact line occur at small capillary numbers.

The system size dependence of the conductance in a class of tight-binding quasi-periodic potentials V₀cos(παn^ν), where 0 < ν < 1, has been investigated numerically. It has been shown that at the metal-insulator transition (MIT), V₀^c = 2−|E|, the conductance follows a clear power law decay vs. system size, g ∼ N^−δ, which corroborates the existence of a pronounced power law localization at the MIT.

The defect structure of Monte Carlo simulations is analyzed in terms of Delaunay triangulations. A small density of dislocations starts to appear close to the melting temperature. The density increases gradually as the temperature is further increased; eventually, isolated disclinations are generated. The change is more rapid in YBCO than in BSCCO. This is consistent with the dislocation mechanism of melting we proposed. The dynamics of the transition is discussed in terms of jog motions of dislocations. Its relevance to the Monte Carlo simulation is discussed.

Fast-neutron irradiation at low temperature was employed to introduce stable collision cascades, produced by high-energy recoil atoms, into an YBa₂Cu₃O_7−δ single crystal. The critical current densities were inferred from magnetization measurements before and after warming-up to room temperature. The results show large J_c enhancements in the "cold" state and only small reductions of these J_c values upon annealing. At the same time, some recovery of the radiation-induced T_c depression is observed, which is attributed to oxygen reordering. Both of these observations provide evidence for the overwhelming contribution of collision cascades and large-defect clusters to the improved flux pinning capability of neutron-irradiated YBCO.

We present the Monte Carlo phase diagram of Ising surfaces with intrasurface (J_s) and surface-bulk (J₁) interactions modified with respect to the bulk exchange interaction J. The new parameter J₁ strongly affects the position of the Binder-Landau multi critical point and the crossover from three-dimensional to two-dimensional behaviour. The surface critical exponents β₁ and ϕ are determined by crossover scaling analysis. A detailed comparison with the results of the mean-field equations is also presented. The Monte Carlo phase diagram shows that stronger modifications of the surface couplings, with respect to those obtained by mean field, are needed to produce an ordered surface phase on a paramagnetic bulk. This is particularly relevant at low values of J_s; in the Monte Carlo simulation the surface is disordered for all values of J₁ when J_s < (0.4 ÷ 0.5) J, while the mean field predicts that the ordered phase is present even in the limit J_s → 0.

We report the observation of intense and spectrally broad luminescence from polycrystalline C₆₀ for laser excitation intensities exceeding a particular threshold value. On the basis of time-resolved luminescence studies, we attribute this "white" luminescence to transitions between higher electronic band states, populated by two-step absorption processes. The observation of white luminescence for excitation in the infrared verifies this interpretation.

A new treatment of the phase behaviour of a colloid + nonadsorbing polymer mixture is described. The calculated phase diagrams show marked polymer partitioning between coexisting phases, an effect not considered in the usual effective-potential approaches to this problem. We also predict that under certain conditions an area of three-phase coexistence should appear in the phase diagram.

We propose a network which has the ability to retrieve not only a sequence of patterns but also their rhythms. The network is a deterministic system in which neither delay nor noise is required. The elements constituting the network are bursting neurons modelled in a relatively simple way. In the retrieval process, the network at first exhibits a retrieval pattern for a finite time. It is then followed by a period in which all neurons are quiescent until the network switches to the next retrieval pattern. The duration of each pattern and the duration of each resting period can be independently adjusted to an arbitrary length. We confirm the excellent retrieval performance of the network by numerical simulation.

Erratum of Europhys. Lett.17 (4) pp 343-348