Ab initio study on the magneto-structural properties of MnAs

The magnetic and structural properties of MnAs are studied with ab initio methods, and by mapping total energies onto a Heisenberg model. The stability of the different phases is found to depend mainly on the volume and on the amount of magnetic order, confirming previous experimental findings and phenomenological models. It is generally found that for large lattice constants the ferromagnetic state is favored, whereas for small lattice constants different antiferromagnetic states can be stabilized. In the ferromagnetic state the structure with minimal energy is always hexagonal, whereas it becomes orthorhombically distorted if there is an antiferromagnetic component in the hexagonal plane. For the paramagnetic state the stable cell is found to be orthorhombic up to a critical lattice constant of about 3.7 Angstrom, above which it remains hexagonal. This leads to the second order structural phase transition between paramagnetic states at about 400 K, where the lattice parameter increases above this critical value with rising temperature due to the thermal expansion. For the paramagnetic state an analytic approximation for the magnitude of the orthorhombic distortion as a function of the lattice constant is given. Within the mean field approximation the dependence of the Curie temperature on the volume and on the orthorhombic distortion is calculated. For orthorhombically distorted cells the Curie temperature is much smaller than for hexagonal cells. This is mainly due to the fact that some of the exchange coupling constants in the hexagonal plane become negative for distorted cells. With these results a description of the susceptibility as function of temperature is given

75As NMR Study of Hole-Doped Superconductor Ba1-xKxFe2As2 (Tc = 38K)

We report the 75As nuclear magnetic resonance (NMR) measurement of the hole-doped superconductor Ba1-xKxFe2As2 with different lattice parameters and different superconducting volume fractions (Tc = 38K). 75As-NMR spectra revealed that the magnetically ordered and superconducting phases are microscopically separated. The spin-lattice relaxation rate 1/T1 in the normal state reflects the existence of a large two-dimensional antiferromagnetic spin fluctuation. The 1/T1 in the superconducting state down to the lowest measurement temperature T varies close to T^3. In addition, it exhibits no coherence peak just below Tc. This shows a T dependence similar to those of other iron pnictides.Comment: 5 pages, 4 figres, to be published in J. Phys. Soc. Jpn. 78 (2009) No.3 issu

Air pollution and sick-leaves - is there a connection? A case study using air pollution data from Oslo

In recent years a growing number of studies have been discussing the relationship between air pollution and human health. The evidence in the literature for adverse health effects of several pol-lutants seems convincing. In our article we are concerned with to which extent these health effects in turn induce sick-leaves or other kinds of reduced labour productivity, which is important for as-sessment of air pollution costs. We analyse the association between sick-leaves in a large office in Oslo and the concentration of different air pollutants and find a significant relationship between the concentration of particulate matter and sick-leaves, while the associations with SO2 and NO2 are more ambiguous. Keywords: Air pollution, sick-leaves, logit specificatio