3,405 research outputs found
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
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
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
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
A Public Good Version of the Collective Household Model: An Empirical Approach with an Application to British Household Data
In this paper we consider an empirical collective household model of time allocation for two-earner households. The novelty of this paper is that we estimate a version of the collective household model, where the internally produced goods and the externally purchased goods are assumed to be public. The empirical results suggest that: (1) Preferences of men and women differ; (2) Although there are significant individual variations, on average the utility functions of men and women are equally weighted in the household utility function; (3) Differences in the ratio of the partners' hourly wages are explanatory for how individual utilities are weighted in the household utility function. (4) The female's preference for household production is influenced by family size, but this does not hold for the male; (5) Both the male and the female have a backward-bending labor supply curve; (6) Labor-supply curves are forward-bending with respect to the partner's wage rate; (7) Our model rejects the unitary Slutsky symmetry condition.labor supply, collective household models, household behavior,
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
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