Field dependent anisotropy change in a supramolecular Mn(II)-[3x3] grid

The magnetic anisotropy of a novel Mn(II)-[3x3] grid complex was investigated by means of high-field torque magnetometry. Torque vs. field curves at low temperatures demonstrate a ground state with S > 0 and exhibit a torque step due to a field induced level-crossing at B* \approx 7.5 T, accompanied by an abrupt change of magnetic anisotropy from easy-axis to hard-axis type. These observations are discussed in terms of a spin Hamiltonian formalism.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let

Ferromagnetic coupling and magnetic anisotropy in molecular Ni(II) squares

We investigated the magnetic properties of two isostructural Ni(II) metal complexes [Ni4Lb8] and [Ni4Lc8]. In each molecule the four Ni(II) centers form almost perfect regular squares. Magnetic coupling and anisotropy of single crystals were examined by magnetization measurements and in particular by high-field torque magnetometry at low temperatures. The data were analyzed in terms of an effective spin Hamiltonian appropriate for Ni(II) centers. For both compounds, we found a weak intramolecular ferromagnetic coupling of the four Ni(II) spins and sizable single-ion anisotropies of the easy-axis type. The coupling strengths are roughly identical for both compounds, whereas the zero-field-splitting parameters are significantly different. Possible reasons for this observation are discussed.Comment: 7 pages, 7 figure

Phylogenetischer Vergleich von chromosomal- und plasmid-kodierten Genabschnitten in enterohämorrhagischen Escherichia coli

In einem repräsentativen Kollektiv HUS-assoziierter EHEC wurde untersucht, in welchem Verhältnis sich das Plasmid und das Chromosom zueinander entwickelt haben. Dazu wurden 41 Stämme hinsichtlich der plasmid-kodierten Gene EHEC-hlyA und espP untersucht und sequenziert. Diese plasmidalen Sequenzen sowie die konkatenierten Sequenzen der sieben chromosomalen Haushaltsgene aus dem E. coli-MLST-Schema wurden auf ihre Phylogenie untersucht um einen Vergleich der Entwicklung durchzuführen. Von den 42 untersuchten HUSEC-Stämmen waren 23 EHEC-hlyA-positiv und 14 espP-positiv, darunter 12 sowohl EHEC-hlyA als auch espP-positiv. Der Vergleich der phylogenetischen Bäume ergab, dass die Struktur der plasmidalen Sequenzen gegenüber den chromosomalen Sequenzen keine signifikanten Übereinstimmungen aufweisen. Zusammenfassend haben sich somit die chromosomalen Gene unabhängig von den plasmidalen Genen entwickelt.<br

Ground state properties of antiferromagnetic Heisenberg spin rings

Exact ground state properties of antiferromagnetic Heisenberg spin rings with isotropic next neighbour interaction are presented for various numbers of spin sites and spin quantum numbers. Earlier work by Peierls, Marshall, Lieb, Schultz and Mattis focused on bipartite lattices and is not applicable to rings with an odd number of spins. With the help of exact diagonalization methods we find a more general systematic behaviour which for instance relates the number of spin sites and the individual spin quantum numbers to the degeneracy of the ground state. These numerical findings all comply with rigorous proofs in the cases where a general analysis could be carried out. Therefore it can be plausibly conjectured that the ascertained properties hold for ground states of arbitrary antiferromagnetic Heisenberg spin rings.Comment: 13 pages, 5 figures, uses epsfig.sty, submitted to Phys. Rev. B. More information at http://www.physik.uni-osnabrueck.de/makrosysteme

Exchange-coupling constants, spin density map, and Q dependence of the inelastic neutron scattering intensity in single-molecule magnets

The Q dependence of the inelastic neutron scattering (INS) intensity of transitions within the ground-state spin multiplet of single-molecule magnets (SMMs) is considered. For these transitions, the Q dependence is related to the spin density map in the ground state, which in turn is governed by the Heisenberg exchange interactions in the cluster. This provides the possibility to infer the exchange-coupling constants from the Q dependence of the INS transitions within the spin ground state. The potential of this strategy is explored for the M = +-10 -> +- 9 transition within the S = 10 multiplet of the molecule Mn12 as an example. The Q dependence is calculated for powder as well as single-crystal Mn12 samples for various exchange-coupling situations discussed in the literature. The results are compared to literature data on a powder sample of Mn12 and to measurements on an oriented array of about 500 single-crystals of Mn12. The calculated Q dependence exhibits significant variation with the exchange-coupling constants, in particular for a single-crystal sample, but the experimental findings did not permit an unambiguous determination. However, although challenging, suitable experiments are within the reach of today's instruments.Comment: 11 pages, 6 figures, REVTEX4, to appear in PR

Herd-level risk factors influencing serological Yersinia prevalence in fattening pig herds

Yersiniosis is the third most frequent zoonosis reported in the European Union with pork as an important source. Identifying risk factors in swine production which may decrease the risk of pork production contamination during pre-harvest is an important step prior to controlling Yersinia spp.. Therefore, management strategies and production processes which might be associated with fattening pigs testing seropositive for pathogenic Yersinia spp. were investigated on 80 fattening pig farms. Although more than 70 farm characteristics were included in the risk assessment, there were only a few which seemed to be connected with serological prevalence: housing on a fully slatted floor and the use of municipal water were observed in herds with low serological Yersinia prevalence, whereas recurring health problems and a low daily weight gain compared to the mean of the herds included in the study were identified more often in herds with a high prevalence

The Weyl bundle as a differentiable manifold

Construction of an infinite dimensional differentiable manifold ${\mathbb R}^{\infty}$ not modelled on any Banach space is proposed. Definition, metric and differential structures of a Weyl algebra and a Weyl algebra bundle are presented. Continuity of the $\circ$-product in the Tichonov topology is proved. Construction of the $*$-product of the Fedosov type in terms of theory of connection in a fibre bundle is explained.Comment: 31 pages; revised version - some typoes have been eliminated, notation has been simplifie

Observation of plaquette fluctuations in the spin-1/2 honeycomb lattice

Quantum spin liquids are materials that feature quantum entangled spin correlations and avoid magnetic long-range order at T = 0 K. Particularly interesting are two-dimensional honeycomb spin lattices where a plethora of exotic quantum spin liquids have been predicted. Here, we experimentally study an effective S=1/2 Heisenberg honeycomb lattice with competing nearest and next-nearest neighbor interactions. We demonstrate that YbBr$_3$ avoids order down to at least T=100 mK and features a dynamic spin-spin correlation function with broad continuum scattering typical of quantum spin liquids near a quantum critical point. The continuum in the spin spectrum is consistent with plaquette type fluctuations predicted by theory. Our study is the experimental demonstration that strong quantum fluctuations can exist on the honeycomb lattice even in the absence of Kitaev-type interactions, and opens a new perspective on quantum spin liquids.Comment: 32 pages, 7 Figure

A population study of gaseous exoplanets

We present here the analysis of 30 gaseous extrasolar planets, with temperatures between 600 and 2400 K and radii between 0.35 and 1.9 $R_\mathrm{Jup}$. The quality of the HST/WFC3 spatially scanned data combined with our specialized analysis tools allow us to study the largest and most self-consistent sample of exoplanetary transmission spectra to date and examine the collective behavior of warm and hot gaseous planets rather than isolated case-studies. We define a new metric, the Atmospheric Detectability Index (ADI) to evaluate the statistical significance of an atmospheric detection and find statistically significant atmospheres around 16 planets out of the 30 analysed. For most of the Jupiters in our sample, we find the detectability of their atmospheres to be dependent on the planetary radius but not on the planetary mass. This indicates that planetary gravity plays a secondary role in the state of gaseous planetary atmospheres. We detect the presence of water vapour in all of the statistically detectable atmospheres, and we cannot rule out its presence in the atmospheres of the others. In addition, TiO and/or VO signatures are detected with 4$\sigma$ confidence in WASP-76 b, and they are most likely present in WASP-121 b. We find no correlation between expected signal-to-noise and atmospheric detectability for most targets. This has important implications for future large-scale surveys.Comment: 14 pages, 12 figures, 3 tables, published in A