Orbital-Peierls State in NaTiSi2O6

Does the quasi one-dimensional titanium pyroxene NaTiSi2O6 exhibit the novel {\it orbital-Peierls} state? We calculate its groundstate properties by three methods: Monte Carlo simulations, a spin-orbital decoupling scheme and a mapping onto a classical model. The results show univocally that for the spin and orbital ordering to occur at the same temperature --an experimental observation-- the crystal field needs to be small and the orbitals are active. We also find that quantum fluctuations in the spin-orbital sector drive the transition, explaining why canonical bandstructure methods fail to find it. The conclusion that NaTiSi2O6 shows an orbital-Peierls transition is therefore inevitable.Comment: 4 pages, 3 figure

Are Dutch Water Safety Institutions Prepared for Climate Change?

This paper deals with the question to what extent the historically grown Dutch water safety institutions have the capacity to cope with the ‘new’ challenges of climate change. The Adaptive Capacity Wheel provides the methodological framework. The analysis focuses on three recent and major planning practices in the Dutch water safety domain: the development and implementation of the Room for the River project, the flood risk approach, and the Second Delta Plan, respectively. The paper concludes that for the Netherlands, to be prepared for climate change, it is necessary to take a new institutional path, by building capacity to improvise, by investing in and by creating room for collaborative leaders, and by finding ways to generate financial resources for long term innovative measure

Absence of magnetic monopoles in Maxwellian magnetoelectrics

The electromagnetic response of topological insulators is governed by axion electrodynamics, which features a topological magnetoelectric term in the Maxwell equations. As a consequence magnetic fields become the source of electric fields and vice-versa, a phenomenon that is general for any material exhibiting a linear magnetoelectric effect. Axion electrodynamics has been associated with the possibility to create magnetic monopoles, in particular by a electrical charge that is screened above the surface of a magnetoelectric material. Here we present the exact solution for the electromagnetic fields in this geometry and show that while vortex-like magnetic screening fields are generated by the electrical charge their divergence is identically zero at every point in space which implies a strict absence of magnetic monopoles. Although magnetic image charges can be made explicit in the problem, no bound state with electric charges yielding a dyon arises. A dyon-like angular momentum follows from our analysis, but is quantized in a universal way, because of its dependence on the dielectric constant. This is consistent with a general argument that precludes magnetic monopoles to be generated in Maxwell magnetoelectrics.Comment: v2: 9 pages, 3 figures; improved presentation and more detailed appendices; added calculation of angular momentum; appendix is made more pedagogical and now includes the detailed solution for a point charge in the presence of a topological dielectric sphere; several references are adde

Intra-Household Work Timing: The Effect on Joint Activities and the Demand for Child Care

This study examines if couples time their work hours and how this work timing influences child care demand and the time that spouses jointly spend on leisure, household chores and child care. By using a innovative matching strategy, this studies identifies the timing of work hours that cannot be explained by factors other than the partners' potential to communicate on the timing of their work. The main findings are that couples with children create less overlap in their work times and this effect is more pronounced the younger the children. We find evidence for a togetherness preference of spouses, but only for childless couples. Work timing also influences the joint time that is spent on household chores, but the effect is small. Finally, work timing behavior affects the demand for informal child care, but not the demand for formal child care.labor supply, work timing, time allocation

Electronic Correlations in Oligo-acene and -thiophene Organic Molecular Crystals

From first principles calculations we determine the Coulomb interaction between two holes on oligo-acene and -thiophene molecules in a crystal, as a function of the oligomer length. The relaxation of the molecular geometry in the presence of holes is found to be small. In contrast, the electronic polarization of the molecules that surround the charged oligomer, reduces the bare Coulomb repulsion between the holes by approximately a factor of two. In all cases the effective hole-hole repulsion is much larger than the calculated valence bandwidth, which implies that at high doping levels the properties of these organic semiconductors are determined by electron-electron correlations.Comment: 5 pages, 3 figure

Environmental variables, pesticide pollution and meiofaunal community structure in two contrasting temporarily open/closed false bay estuaries

Environmental variables (including natural and anthropogenic stressors) and meiobenthic communities were sampled in a ‘natural’ (Rooiels) and a ‘disturbed’ (Lourens) estuary in the Western Cape, South Africa, bimonthly for 20 months. A primary aim of the study was to assess if the meiobenthic community structure is driven by different variables when comparing ‘natural’ versus ‘disturbed’ system. Due to the much smaller catchment of the Rooiels Estuary, many environmental variables were significantly different (p<0.001) from the variables in the Lourens Estuary, e.g. salinity, temperature, pH, total suspended solids, nitrate and depth. No pesticide concentrations were expected in the Rooiels Estuary due to the absence of agricultural development in the catchment. However, chlorpyrifos (8.9 µg/kg), prothiofos (22.0 µg/kg) and cypermethrin concentrations (0.42 µg/kg) were detected frequently, with the highest concentrations recorded during the summer months. Principal response curve analysis showed that temporal variability between sampling dates explained 42% of the variance in environmental variables and pesticide concentrations and spatial variability between the 2 estuaries explained 58%. Variables contributing most to the differences were higher concentrations of endosulfan, p,p-DDE and nitrate concentrations in the Lourens Estuary and larger grain size and higher salinity at the bottom in the Rooiels Estuary. In general the meiofaunal community in the Rooiels Estuary showed a significantly higher number of taxa (p<0.001), a significantly higher Shannon Wiener Diversity Index (