Spin and exchange coupling for Ti embedded in a surface dipolar network

We have studied the spin and exchange coupling of Ti atoms on a Cu$_2$N/Cu(100) surface using density functional theory. We find that individual Ti have a spin of 1.0 (i.e., 2 Bohr Magneton) on the Cu$_2$N/Cu(100) surface instead of spin-1/2 as found by Scanning Tunneling Microscope. We suggest an explanation for this difference, a two-stage Kondo effect, which can be verified by experiments. By calculating the exchange coupling for Ti dimers on the Cu$_2$N/Cu(100) surface, we find that the exchange coupling across a `void' of 3.6\AA\ is antiferromagnetic, whereas indirect (superexchange) coupling through a N atom is ferromagnetic. We confirm the existence of superexchange interactions by varying the Ti-N angle in a model trimer calculation. For a square lattice of Ti on Cu$_2$N/Cu(100), we find a novel spin striped phase

A preliminary evaluation of LANDSAT-4 thematic mapper data for their geometric and radiometric accuracies

Some LANDSAT thematic mapper data collected over the eastern United States were analyzed for their whole scene geometric accuracy, band to band registration and radiometric accuracy. Band ratio images were created for a part of one scene in order to assess the capability of mapping geologic units with contrasting spectral properties. Systematic errors were found in the geometric accuracy of whole scenes, part of which were attributable to the film writing device used to record the images to film. Band to band registration showed that bands 1 through 4 were registered to within one pixel. Likewise, bands 5 and 7 also were registered to within one pixel. However, bands 5 and 7 were misregistered with bands 1 through 4 by 1 to 2 pixels. Band 6 was misregistered by 4 pixels to bands 1 through 4. Radiometric analysis indicated two kinds of banding, a modulo-16 stripping and an alternate light dark group of 16 scanlines. A color ratio composite image consisting of TM band ratios 3/4, 5/2, and 5/7 showed limonitic clay rich soils, limonitic clay poor soils, and nonlimonitic materials as distinctly different colors on the image

Radio Emission from a Young Supernova Remnant Interacting with an Interstellar Cloud: MHD Simulation with Relativistic Electrons

We present two-dimensional MHD simulations of the evolution of a young Type Ia supernova remnant during its interaction with an interstellar cloud of comparable size at impact. We include for the first time in such simulations explicit relativistic electron transport, including spectral information using a simple but effective scheme that follows their acceleration at shocks and subsequent transport. From this information we also model radio synchrotron emission, including spectra. The principal conclusions from these experiments are: 1) Independent of the cloud interaction, the SNR reverse shock can be an efficient site for particle acceleration in a young SNR. 2) At these early times the synchrotron spectral index due to electrons accelerated at the primary shocks should be close to 0.5 unless those shocks are modified by cosmic-ray pressures. However, interaction with the cloud generates regions of distinctly steeper spectra, which may complicate interpretation in terms of global dynamical models for SNR evolution. 3) The internal motions within the SNR become highly turbulent following the cloud interaction. 4) An initially uniform interstellar magnetic field is preferentially amplified along the magnetic equator of the SNR, primarily due to biased amplification by instabilities. Independent of the external field configuration, there is a net radial direction to this field inside the SNR. 5) Filamentary radio structures correlate well with magnetic filaments, while diffuse emission follows the electron distribution. 6) Interaction with the cloud enhances both the electron population and the radio emission.Comment: 29 pages of Latex generated text with 6 figures in gif format. Accepted for publication in the Astrophysical Journal. High resolution postscript figures can be obtained by anonymous ftp from ftp://ftp.msi.umn.edu/pub/users/twj/sn

Classical Stern-Gerlach profiles of Mn5 and Mn6 clusters

Mn5 and Mn6 clusters have recently been found to exhibit Stern-Gerlach profiles marked by a central peak that broadens with the increasing field gradient. The profiles neither exhibit a reminiscence of space quantization as observed through a splitting of beams for the case of free atoms, nor a net deflection characteristic of superparamagnetic relaxations observed in other transition metal clusters. It is proposed that this new behavior results from a weak coupling of localized atomic moments. ab initio electronic structure studies are carried out to show that a Mn5cluster has isomers with spin magnetic moments of 3μB, 13μB, and 23μBwhile a Mn6 cluster has isomers with moments of 2μB, 8μB, 16μB, and 26μB, respectively. The isomers can be obtained by sequential turning of the local atomic moments starting from the ferromagnetic state and can be seen in the negative ion photoelectron spectra of the anions. The weak coupling of the atomic moments, however, leads to unconventional spin dynamics that result in classical broadening of the Stern-Gerlach profiles and lower apparent magnetic moments. The theoretical results illustrate how a combination of the negative ion photodetachment spectroscopy and Stern-Gerlach profiles can provide information on the net spin moment, interatomic spin coupling, and spin dynamics

Structural growth in iron oxide clusters: Rings, towers, and hollow drums

It is shown that the transition from an elementary FeO molecule to the bulk rock-salt FeO proceeds via hollow rings, towers, and drums. Our first-principles electronic structure calculations carried out within a gradient-corrected density functional framework show that small FenOn (n=2,3,4,5) clusters form single, highly stable rings. Starting at Fe6O6, these elementary rings begin to assemble into nano columnar structures to form stable Fe6O6, Fe7O7, Fe8O8, Fe9O9, Fe10O10, and Fe12O12 towers. The rings and the empty towers can be further stabilized by capping O atoms at the ends, leading to FenOn+1 and FenOn+2 sequences. The theoretical results provide insight into the progression of mass intensities in the experimental mass spectra and account for the observed peaks in the negative ion photodetachment spectra of iron oxide clusters

The impact of economic information on medical decision making in primary care

Background: Many general practitioners (GPs) are concerned about the increasing dominance of economic issues in major decisions about clinical care, and feel their opinions on economic matters have not been heard. It is unclear whether this information has any impact on everyday clinical practice in a primary care setting. Aim: To investigate GPs' perspectives on the use of economic information in medical decision making. Design of study: Cross-sectional survey. Setting: GP members of the West of Scotland Primary Care Research and Development Network (WestNet). Methods: Questionnaire survey sent to GPs by post and by email. Results: The overall response rate was 44%, favouring postal over email responses. All respondents indicated that economic information has previously influenced them and should be incorporated into their medical decision making. The most common source of this information was generated by local authorities such as health boards, primary care groups and local prescribing advisors – used by 80% of the respondents. However, publications, such as the <i>British Journal of General Practice</i>, locally produced newsletters and prescribing formularies, and feedback from the General Practice Administration System for Scotland, were used as sources of economic information by 20%, 27% and 33%, respectively. Published materials – in particular, locally specific information and summarized information in leaflet format – were favoured (54%) in comparison to verbally presented material. Conclusions: GPs believe that economic information should be incorporated in medical decision making. The need for precise and summarized information, produced locally, has been highlighted. Better understanding towards the type of economic evidence GPs find useful and comprehensible is required

The MHD Kelvin-Helmholtz Instability II: The Roles of Weak and Oblique Fields in Planar Flows

We have carried out high resolution MHD simulations of the nonlinear evolution of Kelvin-Helmholtz unstable flows in 2 1/2 dimensions. The modeled flows and fields were initially uniform except for a thin shear layer with a hyperbolic tangent velocity profile and a small, normal mode perturbation. The calculations consider periodic sections of flows containing magnetic fields parallel to the shear layer, but projecting over a full range of angles with respect to the flow vectors. They are intended as preparation for fully 3D calculations and to address two specific questions raised in earlier work: 1) What role, if any, does the orientation of the field play in nonlinear evolution of the MHD Kelvin-Helmholtz instability in 2 1/2 D. 2) Given that the field is too weak to stabilize against a linear perturbation of the flow, how does the nonlinear evolution of the instability depend on strength of the field. The magnetic field component in the third direction contributes only through minor pressure contributions, so the flows are essentially 2D. Even a very weak field can significantly enhance the rate of energy dissipation. In all of the cases we studied magnetic field amplification by stretching in the vortex is limited by tearing mode, ``fast'' reconnection events that isolate and then destroy magnetic flux islands within the vortex and relax the fields outside the vortex. If the magnetic tension developed prior to reconnection is comparable to Reynolds stresses in the flow, that flow is reorganized during reconnection. Otherwise, the primary influence on the plasma is generation of entropy. The effective expulsion of flux from the vortex is very similar to that shown by Weiss for passive fields in idealized vortices with large magnetic Reynolds numbers. We demonstrated that thisComment: 23 pages of ApJ Latex (aaspp4.sty) with 10 figures, high resolution postscript images for figs 4-9 available through anonymous at ftp://ftp.msi.umn.edu/pub/twj To appear in the June 10, 1997 Ap

Anomalous Thermal Stability of Metastable C_20 Fullerene

The results of computer simulation of the dynamics of fullerene C_20 at different temperatures are presented. It is shown that, although it is metastable, this isomer is very stable with respect to the transition to a lower energy configuration and retains its chemical structure under heating to very high temperatures, T ~ 3000 K. Its decay activation energy is found to be E_a ~ 7 eV. Possible decay channels are studied, and the height of the minimum potential barrier to decay is determined to be U = 5.0 eV. The results obtained make it possible to understand the reasons for the anomalous stability of fullerene C_20 under normal conditions.Comment: Slightly corrected version of the paper submitted to Phys. Solid Stat