Spin-flop transitions and spin-wave gaps in La_2CuO_4

We study the spin-wave spectrum and the spin-flop transitions in La_2CuO_4 in a uniform magnetic field at zero temperature. Using the non-linear sigma-model, we show that a field applied along the orthorhombic b direction leads to a two-step rotation of the staggered magnetization, first in the bc and then in the ac plane, until the order parameter is completely aligned along the c axis. In contrast, for a perpendicular magnetic field, we find a conventional spin-flop transition induced by the competition between the field and the interlayer coupling. A comparison with recent measurements of the field-dependence of the in-plane spin-wave gap shows a beautiful agreement between theory and experiments.Comment: 7 pages, 3 figures; added referenc

Emergent multipolar spin correlations in a fluctuating spiral - The frustrated ferromagnetic S=1/2 Heisenberg chain in a magnetic field

We present the phase diagram of the frustrated ferromagnetic S=1/2 Heisenberg J_1-J_2 chain in a magnetic field, obtained by large scale exact diagonalizations and density matrix renormalization group simulations. A vector chirally ordered state, metamagnetic behavior and a sequence of spin-multipolar Luttinger liquid phases up to hexadecupolar kind are found. We provide numerical evidence for a locking mechanism, which can drive spiral states towards spin-multipolar phases, such as quadrupolar or octupolar phases. Our results also shed light on previously discovered spin-multipolar phases in two-dimensional $S=1/2$ quantum magnets in a magnetic field.Comment: 4+ pages, 4 figure

Rapid and High Seed Germination and Large Soil Seed Bank of Senecio aquaticus in Managed Grassland

Senecio aquaticus, regionally a Red List species in Europe, has become increasingly abundant in agricultural grassland of medium to high management intensity in Switzerland, Southern Germany, and Austria in recent years, where it is a threat for animal and human health due to its toxicity. In this study, we investigated the seed ecology of S. aquaticus to help protection of the species in relic populations while improving its control when abundant in managed grassland. Germination percentages of fresh ripe seeds of S. aquaticus were on average 68% in 2008, but only 45% in 2010, indicating yearly variation. Germination was generally fast: ten days after the onset of the tests, often more than 45% of all seeds had germinated. When covered with a soil layer of 5 mm, germination was only 16% compared to 63% in full light. Seeds buried in the soil for one and two years showed a germination of 78%, significantly higher than that of fresh ripe seeds, thus suggesting a stimulating effect of cold-wet stratification on germination and long seed survival in the soil. In grasslands with established populations of S. aquaticus, the number of germinable seeds of the species ranged from 361 to 1875 m−2 in topsoil (0–10 cm) with an average of 1139 m−2. The large seed bank and the rapid and high germination of S. aquaticus suggest that allowing seed formation is important for its preservation in relic populations. With respect to agricultural grassland, strategies to control the species should initially target hindering seed production and dispersal

Human endothelial dysfunction: EDRFs

Since the detection of nitric oxide two and a half decades ago, there has been an incredible boost in endothelial function research, which is fascinating the research community. Physiologically, endothelial cells synthesize a number of vasoactive substances. In particular, several endothelium-derived relaxing factors (EDRFs) have been characterized, whereby nitric oxide is the most important. In humans, endothelial dysfunction is one of the first clinically detectable alterations in the development of atherosclerosis and is characterized by an imbalance in the release of vasoactive substances. Thus, it is the aim of this article to give an overview about endothelial function in humans, to summarize the different possibilities to assess endothelial function in this species, and to give an overview of the role of EDRFs in different cardiovascular disease

On-Farm Risikoanalysen zur Entwicklung von Kontrollmaßnahmen bei Kreuzkraut-Arten

Poisonous Senecio species occur in grasslands of various countries. Their further spread into farmland must be prevented, but efficient methods for their control are often lacking, especially under organic farming. We propose on-farm surveys designed as case-control studies to assess the risk for the occurrence of such species and present results of a study on S. aquaticus. Following a case-control design, 72 plots were evaluated for their botanical composition, half the plots contained S. aquaticus. For all the plots, the soil nutrients and the details of management practice, such as type and intensity of management and fertiliser application, were recorded from the farmer. There was a high risk for occurrence of S. aquaticus with low nitrogen fertilisation, with a decrease of management intensity in the preceding 15 years, high inclination, and gaps in the sward. For the long-term control of S. aquaticus, we suggest promoting dense swards and preventing sward damage as much as possible. This paper demonstrates the great power of case-control studies in on-farm research. The design allows thorough statistical testing using generalised linear models and provides reliable results in relatively short time. Various questions can be linked to the management practice on farms and over a wide range of environmental conditions. Such data can hardly be obtained from small plot experiments

The Charm Quark on the Lattice

We formulate lattice fermions in a way that encompasses Wilson fermions as well as the static and non-relativistic approximations. In particular, we treat $m_qa$ systematically ($m_q$ is the fermion mass) showing how to understand the Wilson action as an effective action for systems with \vek{p}\ll m_q. The results show how to extract matrix elements and the spectrum from simulations with $m_qa\approx1$, which is relevant for the charm quark.Comment: 4 pages LaTeX using espcrc2.sty and epsf.sty. FERMILAB-CONF-92/329-

Destruction of Neel order and local spin spirals in insulating La_{2-x}Sr_xCuO_4

Starting from the t-J model, we derive an effective field theory describing the spin dynamics in the insulating phase of La_{2-x}Sr_xCuO_4, x < 0.055, at low temperature. Using Monte Carlo simulations, we show that the destruction of Neel order is driven by the single-hole localization length kappa. A phase transition at 2% doping is consistent with the value of kappa known from the variable range hopping conductivity. The static spin structure factor obtained in our calculations is in perfect agreement with neutron scattering data over the whole range of doping. We also demonstrate that topological defects (spin vortex-antivortex pairs) are an intrinsic property of the spin-glass ground state.Comment: 4 pages, 2 figure

The FFLO state in the one-dimensional attractive Hubbard model and its fingerprint in the spatial noise correlations

We explore the pairing properties of the one-dimensional attractive Hubbard model in the presence of finite spin polarization. The correlation exponents for the most important fluctuations are determined as a function of the density and the polarization. We find that in a system with spin population imbalance, Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-type pairing at wavevector Q=|k_{F,\uparrow}-k_{F,\downarrow}| is always dominant and there is no Chandrasekhar-Clogston limit. We then investigate the case of weakly coupled 1D systems and determine the region of stability of the 1D FFLO phase. This picture is corroborated by density-matrix-renormalization-group (DMRG) simulations of the spatial noise correlations in uniform and trapped systems, unambiguously revealing the presence of fermion pairs with nonzero momentum Q. This opens up an interesting possibility for experimental studies of FFLO states.Comment: 8 pages, 4 figure

Below ground nitrogen dynamics in the sequence clover-grass maize in the DOK long term experiment

We investigated the effect of organic versus conventional cropping systems on the below ground nitrogen inputs of Trifolium pratense L., its transfer to corresponding grass and the fate in the soil organic matter in the clover-grass ley of the DOK long term experiment, Switzerland. BGN tended to be largest in conventional and organic treatments with standard fertilisation and decreased with lower fertilisation intensity. The largest amount of clover N transferred to grass was observed in the minerally fertilised conventional treatment. Clover N derived from rhizodeposition was rapidly stabilised in all treatments to clay rich fractions and thus clover N will have a relatively low direct N contribution to subsequent nonlegumes