On The Violation Of Marshall-Peierls Sign Rule In The Frustrated J1−J2J_{1}-J_{2} Heisenberg Antiferromagnet

We present a number of arguments in favor of the suggestion that the Marshall-Peierls sign rule survives the frustration in the square-lattice Heisenberg antiferromagnet with frustrating next-nearest-neighbor (diagonal) bonds ($J_{1}-J_{2}$ model) for relatively large values of the parameter $J_{2}/J_{1}$. Both the spin-wave analysis and the exact-diagonalization data concerning the weight of Marshall states support the above suggestion.Comment: 8 pages, LaTex, 2 figurs on reques

Marshall-Peierls sign rule for excited states of the frustrated J1-J2 Heisenberg antiferromagnet

We present analytical and numerical calculations for some exited states of the frustrated J1-J2 spin-1/2 Heisenberg model for linear chains and square lattices. We consider the lowest eigenstates in the subspaces determined by the eigenvalue M of the spin operator S_total^z. Because of the reduced number of Ising basis states in the subspaces with higher M we are able to diagonalize systems with up to N=144 spins. We find evidence that the Marshall-Peierls sign rule survives for a relatively large frustration parameter J2.Comment: 7 pages, LaTeX, 4 eps figures, to appear in Physica

The impact of nitrogen contamination and river modification on a Mississippi River floodplain lake

Anthropogenic nitrogen contamination has increased in ecosystems around the world (frequently termed the “nitrogen cascade”). Coke production for steel manufacturing is often overlooked as a source of nitrogen to natural ecosystems. We examined sediment cores from a Horseshoe Lake, a floodplain lake located just East of St. Louis Missouri (USA) to test whether a coking plant effluent could be traced using stable isotopes of nitrogen and diatom microfossils. The distribution of δ15N values in surface sediment samples from the lake shows the highest values near the coking plant effluent. Analysis stable isotopes of nitrogen from sediment cores using a binary mixing model showed three sources of nitrogen since 1688 CE. The first source (active between 1688 and 1920 CE) had a calculated δ15N value of -.8±.4‰. After 1920 a second source with a δ15N of 20.2±2.0 ‰ became active. The diatom microfossil assemblages present from 1688 CE to the late 1800’s are dominated by the planktonic species Aulacoseira granulata and periphytic and benthic genera Gomphonema, Cocconeis, and Lyrella. After the late 1800’s the diatom assemblages are dominated by Staurosira species indicating a shift of species from high flow riverine environments to epipelic species from a lake environment. Our results show how stable isotopes of nitrogen can be used to track nitrogen inputs from industrial sources. They suggest that the high nitrogen input has not been a major source of eutrophication in the lake

Global transcript levels respond to small changes of the carbon status during progressive exhaustion of carbohydrates in Arabidopsis rosettes

The balance between the supply and utilization of carbon (C) changes continually. It has been proposed that plants respond in an acclimatory manner, modifying C utilization to minimize harmful periods of C depletion. This hypothesis predicts that signaling events are initiated by small changes in C status. We analyzed the global transcriptional response to a gradual depletion of C during the night and an extension of the night, where C becomes severely limiting from 4 h onward. The response was interpreted using published datasets for sugar, light, and circadian responses. Hundreds of C-responsive genes respond during the night and others very early in the extended night. Pathway analysis reveals that biosynthesis and cellular growth genes are repressed during the night and genes involved in catabolism are induced during the first hours of the extended night. The C response is amplified by an antagonistic interaction with the clock. Light signaling is attenuated during the 24-h light/dark cycle. A model was developed that uses the response of 22K genes during a circadian cycle and their responses to C and light to predict global transcriptional responses during diurnal cycles of wild-type and starchless pgm mutant plants and an extended night in wild-type plants. By identifying sets of genes that respond at different speeds and times during C depletion, our extended dataset and model aid the analysis of candidates for C signaling. This is illustrated for AKIN10 and four bZIP transcription factors, and sets of genes involved in trehalose signaling, protein turnover, and starch breakdown

J1−J2J_1-J_2 Quantum Heisenberg Antiferromagnet: Improved Spin-Wave Theories Versus Exact-Diagonalization Data

We reconsider the results cocerning the extreme-quantum $S=1/2$ square-lattice Heisenberg antiferromagnet with frustrating diagonal couplings ($J_1-J_2$ model) drawn from a comparison with exact-diagonalization data. A combined approach using also some intrinsic features of the self-consistent spin-wave theory leads to the conclusion that the theory strongly overestimates the stabilizing role of quantum flutcuations in respect to the N\'{e}el phase in the extreme-quantum case $S=1/2$. On the other hand, the analysis implies that the N\'{e}el phase remains stable at least up to the limit $J_{2}/J_{1} = 0.49$ which is pretty larger than some previous estimates. In addition, it is argued that the spin-wave ansatz predicts the existence of a finite range ($J_{2}/J_{1}<0.323$ in the linear spin-wave theory) where the Marshall-Peierls sigh rule survives the frustrations.Comment: 13 pages, LaTex, 7 figures on reques

Percolation in real Wildfires

This paper focuses on the statistical properties of wild-land fires and, in particular, investigates if spread dynamics relates to simple invasion model. The fractal dimension and lacunarity of three fire scars classified from satellite imagery are analysed. Results indicate that the burned clusters behave similarly to percolation clusters on boundaries and look more dense in their core. We show that Dynamical Percolation reproduces this behaviour and can help to describe the fire evolution. By mapping fire dynamics onto the percolation models the strategies for fire control might be improved.Comment: 8 pages, 3 figures, epl sytle (epl.cls included

Sign Rules for Anisotropic Quantum Spin Systems

We present new and exact ``sign rules'' for various spin-s anisotropic spin-lattice models. It is shown that, after a simple transformation which utilizes these sign rules, the ground-state wave function of the transformed Hamiltonian is positive-definite. Using these results exact statements for various expectation values of off-diagonal operators are presented, and transitions in the behavior of these expectation values are observed at particular values of the anisotropy. Furthermore, the effects of sign rules in variational calculations and quantum Monte Carlo calculations are considered. They are illustrated by a simple variational treatment of a one-dimensional anisotropic spin model.Comment: 4 pages, 1 ps-figur

HD 77407 and GJ 577: two new young stellar binaries detected with the Calar Alto Adaptive Optics system ALFA

We present the first results from our search for close stellar and sub-stellar companions to young nearby stars on the northern sky. Our infrared imaging observations are obtained with the 3.5 m Calar Alto telescope and the AO system ALFA. With two epoch observations which were separated by about one year, we found two co-moving companion candidates, one close to HD 77407 and one close to GJ 577. For the companion candidate near GJ 577, we obtained an optical spectrum showing spectral type M4.5; this candidate is a bound low-mass stellar companion confirmed by both proper motion and spectroscopy. We estimate the masses for HD 77407 B and GJ 577 B to be ~0.3 to 0.5 Msun and ~0.16 to 0.2 Msun, respectively. Compared to Siess al.(2000) models, each of the two pairs appears co-eval with HD 77407 A,B being 10 to 40 Myrs old and GJ 577 A,B being older than 100 Myrs. We also took multi-epoch high-resolution spectra of HD 77407 to search for sub-stellar companions, but did not find any with 3 Mjup as upper mass (msin(i)) limit (for up to 4 year orbits); however, we detected a long-term radial velocity trend in HD 77407 A, consistent with a ~ 0.3 Msun companion at ~ 50 AU separation, i.e. the one detected by the imaging. Hence, HD 77407 B is confirmed to be a bound companion to HD 77407 A. We also present limits for undetected, but detectable companions using a deep image of HD 77407 A and B, also observed with the Keck NIRC2 AO system; any brown dwarfs were detectable outside of 0.5 arcsec (17 AU at HD 77407), giant planets with masses from ~ 6.5 to 12 Mjup were detectable at > 1.5 arcsec.Comment: in pres

Photo- and Electro-Disintegration of 3He at Threshold and pd Radiative Capture

The present work reports results for: pd radiative capture observables measured at center-of-mass (c.m.) energies in the range 0--100 keV and at 2 MeV by the TUNL and Wisconsin groups, respectively; contributions to the Gerasimov-Drell-Hearn (GDH) integral in 3He from the two- up to the three-body breakup thresholds, compared to experimental determinations by the TUNL group in this threshold region; longitudinal, transverse, and interference response functions measured in inclusive polarized electron scattering off polarized 3He at excitation energies below the threshold for breakup into ppn, compared to unpolarized longitudinal and transverse data from the Saskatoon group. The calculations are based on a realistic Hamiltonian with two- and three-nucleon interactions and a realistic current operator, including one- and two-body components. The theoretical predictions obtained by including only one-body currents are in violent disagreement with data. These differences between theory and experiment are, to a large extent, removed when two-body currents are taken into account, although some rather large discrepancies remain in the c.m. energy range 0--100 keV, particularly for the pd differential cross section and tensor analyzing power at small angles, and contributions to the GDH integral. A rather detailed analysis indicates that these discrepancies have, in large part, a common origin, and can be traced back to an excess strength obtained in the theoretical calculation of the E1 reduced matrix element associated with the pd channel having L,S,J=1,1/2,3/2. It is suggested that this lack of E1 strength observed experimentally might have implications for the nuclear interaction at very low energies. Finally, the validity of the long-wavelength approximation for electric dipole transitions is discussed.Comment: 47 pages RevTex file, 10 PostScript figures, submitted to Phys. Rev.

Analysing Large Scale Structure: I. Weighted Scaling Indices and Constrained Randomisation

The method of constrained randomisation is applied to three-dimensional simulated galaxy distributions. With this technique we generate for a given data set surrogate data sets which have the same linear properties as the original data whereas higher order or nonlinear correlations are not preserved. The analysis of the original and surrogate data sets with measures, which are sensitive to nonlinearities, yields information about the existence of nonlinear correlations in the data. We demonstrate how to generate surrogate data sets from a given point distribution, which have the same linear properties (power spectrum) as well as the same density amplitude distribution. We propose weighted scaling indices as a nonlinear statistical measure to quantify local morphological elements in large scale structure. Using surrogates is is shown that the data sets with the same 2-point correlation functions have slightly different void probability functions and especially a different set of weighted scaling indices. Thus a refined analysis of the large scale structure becomes possible by calculating local scaling properties whereby the method of constrained randomisation yields a vital tool for testing the performance of statistical measures in terms of sensitivity to different topological features and discriminative power.Comment: 10 pages, 5 figures, accepted for publication in MNRA