Enhancement of Antiferromagnetic Correlations Induced by Nonmagnetic Impurities: Origin and Predictions for NMR Experiments

Spin models that have been proposed to describe dimerized chains, ladders, two dimensional antiferromagnets, and other compounds are here studied when some spins are replaced by spinless vacancies, such as it occurs by $Zn$ doping. A small percentage of vacancies rapidly destroys the spin gap, and their presence induces enhanced antiferromagnetic correlations near those vacancies. The study is performed with computational techniques which includes Lanczos, world-line Monte Carlo, and the Density Matrix Renormalization Group methods. Since the phenomenon of enhanced antiferromagnetism is found to occur in several models and cluster geometries, a common simple explanation for its presence may exist. It is argued that the resonating-valence-bond character of the spin correlations at short distances of a large variety of models is responsible for the presence of robust staggered spin correlations near vacancies and lattice edges. The phenomenon takes place regardless of the long distance properties of the ground state, and it is caused by a ``pruning'' of the available spin singlets in the vicinity of the vacancies. The effect produces a broadening of the low temperature NMR signal for the compounds analyzed here. This broadening should be experimentally observable in the structurally dimerized chain systems $Cu(NO_3)_2\cdot2.5H_2O$, $CuWO_4$, $(VO)_2P_2O_7$, and $Sr_{14}Cu_{24}O_{41}$, in ladder materials such as $Sr Cu_2 O_3$, in the spin-Peierls systems $CuGeO_3$ and $NaV_2 O_5$, and in several others since it is a universal effect common to a wide variety of models and compounds.Comment: 18 pages revtex with 26 figures include

Phono-spectrographic analysis of heart murmur in children

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Reconstructing terrestrial nutrient cycling using stable nitrogen isotopes in wood

Although recent anthropogenic effects on the global nitrogen (N) cycle have been significant, the consequences of increased anthropogenic N on terrestrial ecosystems are unclear. Studies of the impact of increased reactive N on forest ecosystems—impacts on hydrologic and gaseous loss pathways, retention capacity, and even net primary productivity— have been particularly limited by a lack of long-term baseline biogeochemical data. Stable nitrogen isotope analysis (ratio of ¹⁵N to ¹⁴N, termed δ¹⁵N) of wood chronologies offers the potential to address changes in ecosystem N cycling on millennial timescales and across broad geographic regions. Currently, nearly 50 studies have been published utilizing wood δ¹⁵N records; however, there are significant differences in study design and data interpretation. Here, we identify four categories of wood δ¹⁵N studies, summarize the common themes and primary findings of each category, identify gaps in the spatial and temporal scope of current wood δ¹⁵N chronologies, and synthesize methodological frameworks for future research by presenting eight suggestions for common methodological approaches and enhanced integration across studies. Wood δ¹⁵N records have the potential to provide valuable information for interpreting modern biogeochemical cycling. This review serves to advance the utility of this technique for long-term biogeochemical reconstructions

Structure and magnetic properties of ultrathin iron films deposited on the CoGa(100) surface

Epitaxial bcc alpha-Fe(100) films are grown on the (100) surface of a CoGa crystal. The misfit between the Fe and the CoGa lattices is very small (<0.5%). Both, at room temperature and at 550 K layer by layer growth is found with thermal energy helium atom scattering (TEAS). The TEAS experiments show that iron films grown at 550 K on reconstructed c(4x2) CoGa(100) are smooth. The iron films are stable up to a temperature of 650 K. The magnetic properties of the Fe films are investigated with the magneto-optical Kerr effect. Fe films with a thickness in excess of 1.1-1.3 ML are ferromagnetic at 300 K, with the easy axis for magnetization oriented parallel to the surface. It is found that the coercive field H-c is sensitive to the order of the film. Upon annealing at 550-600 K subsequent to deposition, the coercive field, measured around room temperature, increases. The ordering of the film appeared to have no effect on the remanent magnetization. For a 1.7 ML Fe film a Curie temperature of 525 K is measured. At higher coverage the Curie temperature increases to above 600 K. The coercive field H-c is found to approach zero at a 50-100 K lower temperature

Interpreting tree responses to thinning and fertilization using tree-ring stable isotopes

Carbon sequestration has focused renewed interest in understanding how forest management affects forest carbon gain over timescales of decades, and yet details of the physiological mechanisms over decades are often lacking for understanding long-term growth responses to management. Here, we examined tree-ring growth patterns and stable isotopes of cellulose (δ13Ccell and δ18Ocell) in a thinning and fertilization controlled experiment where growth increased substantially in response to treatments to elucidate physiological data and to test the dual isotope approach for uses in other locations. δ13Ccell and δ18Ocell results indicated that fertilization caused an increase in intrinsic water-use efficiency through increases in photosynthesis (A) for the first 3 yr. The combination treatment caused a much larger increase in A and water-use efficiency. Only the thinning treatments showed consistent significant increases in δ18Ocell above controls. Changes in canopy microclimate are the likely drivers for δ18Ocell increases with decreases in relative humidity and increases in leaf temperature associated with thinning being the most probable causes. Tree-ring isotopic records, particularly δ13Ccell, remain a viable way to reconstruct long-term physiological mechanisms affecting tree carbon gain in response to management and climate fluctuations