Mixed Ising ferrimagnets with next-nearest-neighbour couplings on square lattices.

We study Ising ferrimagnets on square lattices with antiferromagnetic exchange couplings between spins of values S = 1/2 and 1 on neighbouring sites, couplings between S = 1 spins at next-nearest-neighbour sites of the lattice and a single-site anisotropy term for the S = 1 spins. Using mainly ground state considerations and extensive Monte Carlo simulations, we investigate various aspects of the phase diagram, including compensation points, critical properties and temperature-dependent anomalies. In contrast to previous belief, the next-nearest-neighbour couplings, when being of antiferromagnetic type, may lead to compensation points.

Monte Carlo study of mixed-spin S = (1/2, 1) Ising ferrimagnets.

We investigate Ising ferrimagnets on square and simple cubic lattices with exchange couplings between spins of values S = 1/2 and 1 on neighbouring sites and an additional single-site anisotropy term on the S = 1 sites. Mainly on the basis of a careful and comprehensive Monte Carlo study, we conclude that there is no tricritical point in the two-dimensional case, in contradiction to mean-field predictions and recent series results. However, evidence for a tricritical point is found in the three-dimensional case. In addition, a line of compensation points is found for the simple cubic, but not for the square lattice.

Critical Binder cumulant in a two-dimensional anisotropic Ising model with competing interactions.

The Binder cumulant at the phase transition of Ising models on square lattices with ferromagnetic couplings between nearest neighbors and with competing antiferromagnetic couplings between next-nearest neighbors, along only one diagonal, is determined using Monte Carlo techniques. In the phase diagram a disorder line occurs separating regions with monotonically decaying and with oscillatory spin-spin correlations. Findings on the variation of the critical cumulant with the ratio of the two interaction strengths are compared to related recent results based on renormalization-group calculations.

Ising model with periodic pinning of mobile defects.

A two-dimensional Ising model with short-range interactions and mobile defects describing the formation and thermal destruction of defect stripes is studied. In particular, the effect of a local pinning of the defects at the sites of straight equidistant lines is analyzed using Monte Carlo simulations and the transfer matrix method. The pinning leads to a long-range ordered magnetic phase at low temperatures. The dependence of the phase transition temperature, at which the defect stripes are destabilized, on the pinning strength is determined. The transition seems to be of first order, with and without pinning.

Ising cubes with enhanced surface couplings

Using Monte Carlo techniques, Ising cubes with ferromagnetic nearest-neighbor interactions and enhanced couplings between surface spins are studied. In particular, at the surface transition, the corner magnetization shows nonuniversal, coupling-dependent critical behavior in the thermodynamic limit. Results on the critical exponent of the corner magnetization are compared to previous findings on two-dimensional Ising models with three intersecting defect lines.

[On the state of population policy discussion in spatial and regional planning]. / Zum Stand der bevolkerungspolitischen Diskussion in Raumordnung und Landesplanung

PIP: This article is concerned with projected regional differences in population decline in the Federal Republic of Germany. A general overview of fertility decline and its effects is first presented. The assumptions and results of model calculations of regional population trends up to the year 2030 are then discussed. Finally, the implications for regional planning are considered, with particular attention to the impact of changes in the dependency burden.^ieng