Magnetic ordering, electronic structure and magnetic anisotropy energy in the high-spin Mn10_{10} single molecule magnet

We report the electronic structure and magnetic ordering of the single molecule magnet [Mn$_{10}$O$_{4}$(2,2'-biphenoxide)$_{4}$Br$_{12}$]$^{4-}$ based on first-principles all-electron density-functional calculations. We find that two of the ten core Mn atoms are coupled antiferromagnetically to the remaining eight, resulting in a ferrimagnetic ground state with total spin S=13. The calculated magnetic anisotropy barrier is found to be 9 K in good agreement with experiment. The presence of the Br anions impact the electronic structure and therefore the magnetic properties of the 10 Mn atoms. However, the electric field due to the negative charges has no significant effect on the magnetic anisotropy.Comment: 4 pages, submitted to PR

Investigations of the Role of Gaze in Mixed-Reality Personal Computing

This paper investigates how eye tracking andgaze estimation can help create better mixedrealitypersonal computing systems involvingboth physical (real world) and virtual (digital)objects. The role of gaze is discussed in the lightof the situative space model (SSM)determining the set of objects (physical andvirtual) which a given human agent canperceive, and act on, in any given moment intime. The analysis and discussion results inideas for how to extend the SSM model in orderto better incorporate the role of gaze in everydayhuman activities, and for taking advantage ofemerging mobile eye tracking technology

Privatization, Drought, and Fire Exclusion in the Tuul River Watershed, Mongolia

Global wildfire frequency and extent are expected to increase under projected climate change in the twenty-first century, yet little is known about how human activities might affect this trend. In central Mongolia, there has been a 2.5°C rise in spring and summer temperatures during the last 40 years and a decrease in moisture availability during the latter half of the twentieth century. Concurrently, Mongolia has experienced multiple shifts in socioeconomic systems during the twentieth century, most notably the establishment of a Soviet-backed communist economy in the 1920s and a rapid transition to privatization in the 1990s. Observed records of fire in the late twentieth century suggested that fire activity had increased, but no long-term data existed to place these trends in a historical context. Our objective was to identify spatial and temporal patterns in fire occurrence in the forest-steppe ecotone of the Tuul River watershed in the context of changing climatic and social conditions since 1875. We used fire-scarred trees to reconstruct past fire occurrence during the period 1875–2009. Our results indicate a significant association between human activity and fire occurrence independent of climatic variables. The greatest evidence for an anthropogenic fire regime exists following the transition to a free market economy during the early 1990s when land-use intensification near the capital city of Ulaanbaatar resulted in fire exclusion. We emphasize the importance of including socio-political variables in global models of wildfire potential, particularly where fuels limit fire activity

The Hamiltonian of the V15_{15} Spin System from first-principles Density-Functional Calculations

We report first-principles all-electron density-functional based studies of the electronic structure, magnetic ordering and anisotropy for the V$_{15}$ molecular magnet. From these calculations, we determine a Heisenberg Hamiltonian with four antiferromagnetic and one {\em ferromagnetic} coupling. We perform direct diagonalization to determine the temperature dependence of the susceptibility. This Hamiltonian reproduces the experimentally observed spin $S$=1/2 ground state and low-lying $S$=3/2 excited state. A small anisotropy term is necessary to account for the temperature independent part of the magnetization curve.Comment: 4 pages in RevTeX format + 2 ps-figures, accepted by PRL Feb. 2001 (previous version was an older version of the paper

Density-functional-based predictions of Raman and IR spectra for small Si clusters

We have used a density-functional-based approach to study the response of silicon clusters to applied electric fields. For the dynamical response, we have calculated the Raman activities and infrared (IR) intensities for all of the vibrational modes of several clusters (SiN with N=3-8, 10, 13, 20, and 21) using the local density approximation (LDA). For the smaller clusters (N=3-8) our results are in good agreement with previous quantum-chemical calculations and experimental measurements, establishing that LDA-based IR and Raman data can be used in conjunction with measured spectra to determine the structure of clusters observed in experiment. To illustrate the potential of the method for larger clusters, we present calculated IR and Raman data for two low-energy isomers of Si10 and for the lowest-energy structure of Si13 found to date. For the static response, we compare our calculated polarizabilities for N=10, 13, 20, and 21 to recent experimental measurements. The calculated results are in rough agreement with experiment, but show less variation with cluster size than the measurements. Taken together, our results show that LDA calculations can offer a powerful means for establishing the structures of experimentally fabricated clusters and nanoscale systems

Salmonella and Fecal Indicator Bacteria Survival in Soils Amended with Poultry Manure

Minimizing the risks associated with manure-borne pathogenic microorganisms requires an understanding of microbial survival under realistic field conditions. The objective of this 3-year study was to assess the fate of Salmonella (SALM) and fecal indicator bacteria (FIB), E. coli (EC) and enterococci (ENT), in glacial till-derived soils, after application of poultry manure (PM) to cornfields under chisel-plowed (CP) or no-till (NT) management. From 2010 to 2012, soil samples were obtained each spring at 0–15- and 15–30-cm depths, to determine whether over-wintering of target bacteria had occurred. Sampling was followed by application of PM at low (PM1) and high (PM2) rates, based on nitrogen application goals. In 2012, soil samples were collected 21, 42, and 158 days after manure application (DAM), to assess the effects of time, application rates, and tillage on frequency of detection and concentrations of target bacteria. Despite dry conditions, all three target organisms were detected 158 DAM in 2012, and detection of these organisms in spring soil samples from manured plots in 2011 and 2012, nearly a full year after PM application, suggests that these organisms can persist in the soil environment long after application. The highest SALM concentration (790 cfu/g dry weight) and detection rate (25%) was found in PM2 plots 42 DAM. SALM were detected more frequently in CP plots (20%) compared to NT plots (5%). In contrast, tillage practices had no apparent effect on EC or ENT survival, as indicated by both soil, and decay rates estimated from tile-water bacteria concentrations. Decay rate constants (μ) ranged from 0.044 to 0.065 day−1 for EC and 0.010 to 0.054 day−1 for ENT