Episode of unusual high solar ultraviolet radiation over central Europe due to dynamical reduced total ozone in May 2005

In late May 2005 unusual high levels of solar ultraviolet radiation were observed over central Europe. In Northern Germany the measured irradiance of erythemally effective radiation exceeded the climatological mean by more than about 20%. An extreme low ozone event for the season coincided with high solar elevation angles and high pressure induced clear sky conditions leading to the highest value of erythemal UV-radiation ever observed over this location in May since 1994. This hereafter called "ozone mini-hole" was caused by an elevation of tropopause height accompanied with a poleward advection of ozone-poor air from the tropics. The resultant increase in UV-radiation is of particular significance for human health. Dynamically induced low ozone episodes that happen in late spring can considerably enhance the solar UV-radiation in mid latitudes and therefore contribute to the UV-burden of people living in these regions

The reason why doping causes superconductivity in LaFeAsO

The experimental observation of superconductivity in LaFeAsO appearing on doping is analyzed with the group-theoretical approach that evidently led in a foregoing paper (J. Supercond 24:2103, 2011) to an understanding of the cause of both the antiferromagnetic state and the accompanying structural distortion in this material. Doping, like the structural distortions, means also a reduction of the symmetry of the pure perfect crystal. In the present paper we show that this reduction modifies the correlated motion of the electrons in a special narrow half-filled band of LaFeAsO in such a way that these electrons produce a stable superconducting state

The structural distortion in antiferromagnetic LaFeAsO investigated by a group-theoretical approach

As experimentally well established, undoped LaFeAsO is antiferromagnetic below 137K with the magnetic moments lying on the Fe sites. We determine the orthorhombic body-centered group Imma (74) as the space group of the experimentally observed magnetic structure in the undistorted lattice, i.e., in a lattice possessing no structural distortions in addition to the magnetostriction. We show that LaFeAsO possesses a partly filled "magnetic band" with Bloch functions that can be unitarily transformed into optimally localized Wannier functions adapted to the space group Imma. This finding is interpreted in the framework of a nonadiabatic extension of the Heisenberg model of magnetism, the nonadiabatic Heisenberg model. Within this model, however, the magnetic structure with the space group Imma is not stable but can be stabilized by a (slight) distortion of the crystal turning the space group Imma into the space group Pnn2 (34). This group-theoretical result is in accordance with the experimentally observed displacements of the Fe and O atoms in LaFeAsO as reported by Clarina de la Cruz et al. [nature 453, 899 (2008)]

Iron porphyrin molecules on Cu(001): Influence of adlayers and ligands on the magnetic properties

The structural and magnetic properties of Fe octaethylporphyrin (OEP) molecules on Cu(001) have been investigated by means of density functional theory (DFT) methods and X-ray absorption spectroscopy. The molecules have been adsorbed on the bare metal surface and on an oxygen-covered surface, which shows a $\sqrt{2}\times2\sqrt{2}R45^{\circ}$ reconstruction. In order to allow for a direct comparison between magnetic moments obtained from sum-rule analysis and DFT we calculate the dipolar term $7$, which is also important in view of the magnetic anisotropy of the molecule. The measured X-ray magnetic circular dichroism shows a strong dependence on the photon incidence angle, which we could relate to a huge value of $7$, e.g. on Cu(001) $7$ amounts to -2.07\,\mbo{} for normal incidence leading to a reduction of the effective spin moment $m_s + 7$. Calculations have also been performed to study the influence of possible ligands such as Cl and O atoms on the magnetic properties of the molecule and the interaction between molecule and surface, because the experimental spectra display a clear dependence on the ligand, which is used to stabilize the molecule in the gas phase. Both types of ligands weaken the hybridization between surface and porphyrin molecule and change the magnetic spin state of the molecule, but the changes in the X-ray absorption are clearly related to residual Cl ligands.Comment: 17 figures, full articl

Phase diagram of the frustrated, spatially anisotropic S=1 antiferromagnet on a square lattice

We study the S=1 square lattice Heisenberg antiferromagnet with spatially anisotropic nearest neighbor couplings $J_{1x}$, $J_{1y}$ frustrated by a next-nearest neighbor coupling $J_{2}$ numerically using the density-matrix renormalization group (DMRG) method and analytically employing the Schwinger-Boson mean-field theory (SBMFT). Up to relatively strong values of the anisotropy, within both methods we find quantum fluctuations to stabilize the N\'{e}el ordered state above the classically stable region. Whereas SBMFT suggests a fluctuation-induced first order transition between the N\'{e}el state and a stripe antiferromagnet for $1/3\leq J_{1x}/J_{1y}\leq 1$ and an intermediate paramagnetic region opening only for very strong anisotropy, the DMRG results clearly demonstrate that the two magnetically ordered phases are separated by a quantum disordered region for all values of the anisotropy with the remarkable implication that the quantum paramagnetic phase of the spatially isotropic $J_{1}$-$J_{2}$ model is continuously connected to the limit of decoupled Haldane spin chains. Our findings indicate that for S=1 quantum fluctuations in strongly frustrated antiferromagnets are crucial and not correctly treated on the semiclassical level.Comment: 10 pages, 10 figure

Galileo dust data from the jovian system: 2000 to 2003

The Galileo spacecraft was orbiting Jupiter between Dec 1995 and Sep 2003. The Galileo dust detector monitored the jovian dust environment between about 2 and 370 R_J (jovian radius R_J = 71492 km). We present data from the Galileo dust instrument for the period January 2000 to September 2003. We report on the data of 5389 particles measured between 2000 and the end of the mission in 2003. The majority of the 21250 particles for which the full set of measured impact parameters (impact time, impact direction, charge rise times, charge amplitudes, etc.) was transmitted to Earth were tiny grains (about 10 nm in radius), most of them originating from Jupiter's innermost Galilean moon Io. Their impact rates frequently exceeded 10 min^-1. Surprisingly large impact rates up to 100 min^-1 occurred in Aug/Sep 2000 when Galileo was at about 280 R_J from Jupiter. This peak in dust emission appears to coincide with strong changes in the release of neutral gas from the Io torus. Strong variability in the Io dust flux was measured on timescales of days to weeks, indicating large variations in the dust release from Io or the Io torus or both on such short timescales. Galileo has detected a large number of bigger micron-sized particles mostly in the region between the Galilean moons. A surprisingly large number of such bigger grains was measured in March 2003 within a 4-day interval when Galileo was outside Jupiter's magnetosphere at approximately 350 R_J jovicentric distance. Two passages of Jupiter's gossamer rings in 2002 and 2003 provided the first actual comparison of in-situ dust data from a planetary ring with the results inferred from inverting optical images.Comment: 59 pages, 13 figures, 6 tables, submitted to Planetary and Space Scienc

Episode of unusual high solar ultraviolet radiation in central Europe due to dynamical reduced stratospheric ozone in May 2005

In late May 2005 unusual high levels of solar ultraviolet radiation were observed over central Europe. In Northern Germany the measured irradiance of erythemally effective radiation exceeded the climatological mean by more than about 20%. An extreme low ozone event for the season coincided with high solar elevation angles and high pressure induced clear sky conditions leading to the highest value of erythemal UV-radiation ever observed over this location in May since 1994. This hereafter called "ozone mini-hole" was caused by an elevation of tropopause height accompanied with a poleward advection of ozone-poor air from the tropics. The resultant increase in UV-radiation is of particular significance for human health. Dynamically induced low ozone episodes that happen in late spring can considerably enhance the solar UV-radiation in mid latitudes and therefore contribute to the UV-burden of people living in these regions

Relativistic electronic dressing

We study the effects of the relativistic electronic dressing in laser-assisted electron-hydrogen atom elastic collisions. We begin by considering the case when no radiation is present. This is necessary in order to check the consistency of our calculations and we then carry out the calculations using the relativistic Dirac-Volkov states. It turns out that a simple formal analogy links the analytical expressions of the differential cross section without laser and the differential cross section in presence of a laser field.Comment: 11 pages, 18 figures, Late