Table of contents

The electrical avalanche breakdown of p-doped germanium at low temperatures displays self-generated current oscillations. The investigation of the transition from a stable fixed point to a limit cycle yields a scaling behaviour known from the saddle node bifurcation on a limit cycle.

By studying the time evolution of two configurations submitted to the same thermal noise, we investigate phase space properties of the three-dimensional ±J Ising spin glass. Our results for the distance between states are consistent with the picture of phase space having a multivalley structure below T_c. By fixing the damage at one site, we also study the Hamming distance between the two configurations at its critical point, where the distribution of probability of getting damaged n times is found to be multifractal.

Phase diagram in a b.c.c. lattice with Ising spins is studied. Interactions between nearest-neighbours and between next-nearest-neighbours in one sublattice are included. Monte Carlo results show first-order transition, multicriticality, re-entrance and coexistence of order and disorder. We have also performed a differential renormalization group calculation in the Migdal-Kadanoff approximation. An analysis of the first-order transition line is made in the renormalization group framework. The phase diagram obtained is in agreement with that from Monte Carlo simulation.

It is shown that a Zeno-type effect due to disturbances caused by collisions associated with correlated pairs inside the nucleus may significantly modify double-beta decay rates. Several nuclei for which double-beta decay data is available are studied, and it is shown that the trend of the effect is consistent with this interpretation. The resulting ratio of the lifetimes of ¹²⁸Te to ¹³⁰Te could indicate (in the absence of particle-particle interaction corrections to which the theoretical amplitudes are extremely sensitive) an increased likelihood for the occurrence of the 0ν mode. An upper limit on the neutrino mass of about 4 eV appears to be compatible with the experimental bounds on ⁷⁶Ge, ⁸²Se double-beta decay lifetimes, and the ratio of the lifetimes of the Te isotopes.

Using the two-step decay model of clustering formation and then tunnelling, ⁸⁰Zr is predicted to decay via emission of ²⁴Mg and ²⁸Si clusters with half-life times ∼ 10⁴⁵ s.

The possibility of achieving resolution beyond the classical Rayleigh limit, or "superresolution", has been a subject of interest in various wave propagation problems since Lord Rayleigh first proposed his resolution criterion. In a recent series of papers by Bertero et al. proposals for achieving superresolution in confocal microscopy have been put forward and detailed calculations of the results which may be achieved in various experimental configurations have been presented. The main feature of these proposed new microscope systems is that the single pinhole at the detector of a normal confocal scanning microscope system is replaced by an array of detectors which gather more information about the object at each scanning point. This information is processed by algorithms for inversion based, for example, on the theory of singular systems.

During directional solidification of thin layers (50 μm) of the transparent CBr₄-C₂Cl₆ lamellar eutectic alloy, we observed tilt waves, a type of instability consisting in small domains of tilted lamellae moving transversely along the growth front. This instability is a way the system adjusts its wavelength after a small perturbation, without creating or destroying lamellae.

A crucial aspect of any dislocation theory of plastic flow in crystals is the mean-field description of dislocation reactions (mainly the activation and operation of dislocation sources). Here it is shown how dislocation reaction terms must be included in the elastodynamic theory in order not to violate the basic topological definition of continuously distributed dislocations and a specific physical example, together with the equations connecting the reaction terms with plastic deformation, is given. A rigorous theoretical account of dislocation multiplication phenomena implies a modification (proposed here) of the familiar Orowan equation.

We introduce a multi-order parameter van der Waals theory for surface melting near the triple point T_t which is based on density functional techniques and includes long-range interparticle forces. As a result we establish that long-range attractive forces induce a stretched exponential decay as T → T_t of the residual crystal order in the quasi-liquid interfacial layer. If these forces are weak, a crossover from logarithmic to power logarithmic to power law growth in T_t - T of the interfacial width occurs.

Low-energy electron diffraction (LEED) studies show that the clean Pb(110) surface has a c(2 × 4) superstructure below about 425 K. It is attributed to surface stress relief by increased packing density but a dimensionally enhanced Kohn anomaly-induced lattice distortion cannot be excluded.

Fe_x/Ru_y hexagonal close-packed superlattices have been grown by MBE with various layers thickness 4 ⩽ x ⩽ 12 Å and 4 ⩽ y ⩽ 52 Å. The superlattices show fully h.c.p. structure with sharp interfaces and with a periodic modulation along the [0001] growth axis. The h.c.p.-Fe layers correspond to a specific volume of 12.5 ų per Fe atom, much larger than in regular close packed phases (11.3 ų), which is likely related to a high-spin state for Fe.

Structures of edge dislocations in aluminum are calculated using molecular dynamics and an effective medium theory with many-atom interactions. The local density approximation is used to calculate the positron states at different trapping sites. The partial [211]-dislocation is a very shallow trap with a positron binding energy of less than 80 meV and a positron lifetime similar to the bulk lifetime. Vacancies and single jogs at the dislocation line result in a lifetime of about 224 ps which is in a good agreement with the experimentally observed lifetimes in aluminum containing dislocations.

New exactly solvable SO(q)-invariant q-state vertex models are introduced. We employ a new method using inversion relations which enables us to determine directly the spectra of the transfer matrices. The results are applied to related quantum spin chains. A special case is the pure biquadratic spin-1 Hamiltonian which turns out to be noncritical. Various quantities are calculated, e.g., the energy gap and the correlation length.

The unoccupied Z₂-d-band in Ni(110) has been studied as a function of temperature employing spin-resolved inverse photoemission. This band has only the majority spin states occupied at T = 0 and contributes therefore to the ferromagnetism in nickel. As a function of temperature, our data show majority and minority features merging together upon approaching the Curie temperature T_c. This result indicates either a temperature-dependent exchange splitting vanishing at T_c or–assuming a temperature-independent local exchange splitting–only a small amount of short-range magnetic order in nickel at T_c.

Photoelectron spectroscopy at the electron-doped superconducting cuprate Nd_2-xCe_xCuO_4-δ gives evidence to strong electronic correlations by valence band data as well as by Cu core level spectra which reveal that doping by electrons gives rise to an admixture of Cu 3d¹⁰ to the 3d⁹-state which is pure in Nd₂CuO_4-δ. An enhanced on-site Coulomb repulsion energy U_dd and augmented Cu 3d-ligand hybridization is found for Nd_2-xCe_xCuO_4-δ in comparison with the p-type cuprate superconductors. A XPS study of the valence of the constituents reveals that Nd is trivalent with strong Nd 4f-O 2p hybridization, which is confirmed by resonant photoemission, whereas Ce has a 4f occupation number n_f ≈ 0.45 as in CeO₂.

The new experiments presented in this paper show that the anomalous critical behaviour previously reported in a quaternary microemulsion is due as a matter of fact to a crossover of two critical phenomena. The temperature evolution of the phase diagram and the light diffusion data agree with this interpretation. The system investigated exhibits a phase separation at low and high temperatures. The high-temperature critical point is a consolute critical point characterized by Ising exponents. The critical behaviour found at low temperature is related to a transition from micellar to flow birefringent phases. This interpretation is in agreement with recent models.

Erratum for Europhys. Lett.9 (2) 107-111 (1989)