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

Hydrographs by Single Linear Reservoir Model

A single linear reservoir (SLR) model is presented which provides a simple means for developing runoff hydrographs for small, urban watersheds. The model only requires one parameter, K, which can be estimated from watershed and precipitation characteristics. Several methods for estimating K and the results of testing the model on various watersheds are presented.Waller Creek Working Grou

Self-Reports about Homelessness: Single Women with Children

The number of homeless families is on the rise. ln 1989 more than 60,000 families nationwide, four times the number recorded in 1984, were in homeless shelters. Seventy to ninety percent of homeless families are headed by women. To better understand the causes of homelessness this study obtained self-reports from 16 women with children residing in a metropolitan homeless shelter. An exploratory study was conducted using a questionnaire that gathered quantitative and qualitative data regarding demographics, reasons for homelessness, and preventative measures. Consistent with other research, the findings indicate varied reasons for homelessness including: circumstantial events, lack of relational support, lack of community support, and lack of self knowledge. Recommendations for solutions to homelessness include providing a diverse array of services in shelters and in the community that support and empower the people who are homeless to improve their situations. Changes in public policy are needed to provide these services

Conceptual Design Tool to Analyze Electrochemically-Powered Micro Air Vehicles

A multi-fidelity conceptual design tool was developed to assess electrochemically-powered micro air vehicles(MAVs). The tool utilizes four areas of contributing analyses (CAs): aerodynamics, propulsion, power management, and power sources to determine the endurance duration of a given mission. The low-fidelity aerodynamic CA consisted of drag polar calculations and the high-level CA used a vortex theory code called Athena Vortex Lattice (AVL). The propulsion CA employed QPROP and a MATLAB code that used experimental propeller data and motor constants to predict propeller-motor combination performance for the low- and high-fidelity tracks, respectively. The power management CA determined the percentage of required power the power sources needed to provide by a user-defined split or an optimization to maximize endurance duration for the two fidelity options. The power source CA used specific energy and specific power calculations for the low-fidelity track and polarization curves and Ragone plots for the high-fidelity track. Model Center software allowed for integration of each of these CAs into one model. Based on the current state of the art battery and fuel cell technology, the model predicted endurance durations ranging from 88.5 to 107.3 min. The mission simulations that led to these durations used a generic MAV (GenMAV) configuration and the complete spectrum of fidelity combinations

Vibrational signatures for low-energy intermediate-sized Si clusters

We report low-energy locally stable structures for the clusters Si20 and Si21. The structures were obtained by performing geometry optimizations within the local density approximation. Our calculated binding energies for these clusters are larger than any previously reported for this size regime. To aid in the experimental identification of the structures, we have computed the full vibrational spectra of the clusters, along with the Raman and IR activities of the various modes using a recently developed first-principles technique. These represent, to our knowledge, the first calculations of Raman and IR spectra for Si clusters of this size

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