Structural distortions of frustrated quantum spin lattices in high magnetic fields

We study the stability of some strongly frustrated antiferromagnetic spin lattices in high magnetic fields against lattice distortions. In particular, we consider a spin-s anisotropic Heisenberg antiferromagnet on the square-kagom\'{e} and kagom\'{e} lattices. The independent localized magnons embedded in a ferromagnetic environment, which are the ground state at the saturation field, imply lattice instabilities for appropriate lattice distortions fitting to the structure of the localized magnons. We discuss in detail the scenario of this spin-Peierls instability in high magnetic fields which essentially depends on the values of the exchange interaction anisotropy \Delta and spin s

The spin 1/2 Heisenberg star with frustration II: The influence of the embedding medium

We investigate the spin 1/2 Heisenberg star introduced in J. Richter and A. Voigt, J. Phys. A: Math. Gen. {\bf 27}, 1139 (1994). The model is defined by $H=J_1 \sum_{i=1}^{N}{{\bf s}_0{\bf s}_i} + J_2 H_{R}\{{\bf s}_i\}$ ; $J_1,J_2 \ge 0$ , $i=1,...,N$. In extension to the Ref. we consider a more general $H_{R}\{{\bf s}_i\}$ describing the properties of the spins surrounding the central spin ${\bf s}_0$. The Heisenberg star may be considered as an essential structure element of a lattice with frustration (namely a spin embedded in a magnetic matrix $H_R$) or, alternatively, as a magnetic system $H_R$ with a perturbation by an extra spin. We present some general features of the eigenvalues, the eigenfunctions as well as the spin correlation $\langle {\bf s}_0{\bf s}_i \rangle$ of the model. For $H_R$ being a linear chain, a square lattice or a Lieb-Mattis type system we present the ground state properties of the model in dependence on the frustration parameter $\alpha=J_2/J_1$. Furthermore the thermodynamic properties are calculated for $H_R$ being a Lieb--Mattis antiferromagnet.Comment: 16 pages, uuencoded compressed postscript file, accepted to J. Phys. A: Math. Ge

Influence of soil properties on archaeal diversity and distribution in the McMurdo Dry Valleys, Antarctica

Archaea are the least understood members of the microbial community in Antarctic mineral soils. Although their occurrence in Antarctic coastal soils has been previously documented, little is known about their distribution in soils across the McMurdo Dry Valleys, Victoria Land. In this study, terminal-restriction fragment length polymorphism (t-RFLP) analysis and 454 pyrosequencing were coupled with a detailed analysis of soil physicochemical properties to characterize archaeal diversity and identify environmental factors that might shape and maintain archaeal communities in soils of the three southern most McMurdo Dry Valleys (Garwood, Marshall, and Miers Valley). Archaea were successfully detected in all inland and coastal mineral soils tested, revealing a low overall richness (mean of six operational taxonomic units [OTUs] per sample site). However, OTU richness was higher in some soils and this higher richness was positively correlated with soil water content, indicating water as a main driver of archaeal community richness. In total, 18 archaeal OTUs were detected, predominately Thaumarchaeota affiliated with Marine Group 1.1b (> 80% of all archaeal sequences recovered). Less abundant OTUs (2% of all archaeal sequences) were loosely related to members of the phylum Euryarchaeota. This is the first comprehensive study showing a widespread presence and distribution of Archaea in inland Antarctic soils

Order-Parameter Fluctuations in the Frustrated Heisenberg Model on the Square Lattice

The T=0 dynamics of the two‐dimensional s=1/2 Heisenberg model with competing nearest‐neighbor (J 1) and next‐nearest‐neighbor (J 2) interactions is explored via the recursion method, specifically the frequency‐dependent fluctuations of the order parameters associated with some of the known or suspected ordering tendencies in this system, i.e., Néel, collinear, dimer, and chiral order. The results for the dynamic structure factors of the respective fluctuation operators show a strong indication of collinear order at J 2/J 1≳0.6 and a potential for dimer order at 0.5≲J 2/J 1≲0.6, whereas the chiral ordering tendency is observed to be considerably weaker

Исследование этапов жизненного цикла конкурентного преимущества машиностроительного предприятия

В статье рассмотрены вопросы создание научно-обоснованого механизма формирования и оценки конкурентных преимуществ машиностроительного предприятия. Важной задачей статьи является разработка предложений по формированию жизненного цикла конкурентного преимущества, обоснование его этапов и их экономического содержания, прогнозирование изменения силы конкурентного преимущества по этапам его жизненного цикла.In the article questions are considered creation of the scientific-grounded mechanism of forming and estimation of competitive edges of machine-building enterprise. The important task of the article is development of suggestions on forming of life cycle of competitive edge, ground of his stages and their economic maintenance, prognostication of change of force of competitive edge on the stages of his life cycle

Cosmic-Ray Acceleration Efficiency vs Temperature Equilibration: the Case of SNR 0509-67.5

We study the 0509-67.5 supernova remnant in the Large Magellanic Cloud with the VLT/FORS2 spectrograph. We detect a broad component in the H-alpha emission with a FWHM of 2680 \pm 70 km/s and 3900 \pm 800 km/s for the southwest (SW) and northeast (NE) shocks respectively. For the SW, the proton temperature appears to be too low for the shock velocity, which we attribute to a cosmic-ray pressure behind the shock front of at least 20% of the total pressure. For the NE, the post-shock proton temperature and the shock velocity are compatible, only if the plasma behind the shock front has a degree of thermal equilibrium of over 20%, which is at odds with current models for temperature equilibration behind fast shocks, which do not accelerate cosmic rays. If we assume the electron temperature to be less than 10% of the proton temperature, we find a post-shock cosmic-ray pressure of at least 7%.Comment: Accepted for publication in ApJ Letters, 5 pages, 4 figures and 1 tabl

Graphs with at most one crossing

The crossing number of a graph $G$ is the least number of crossings over all possible drawings of $G$. We present a structural characterization of graphs with crossing number one