First-principles calculations are used to establish that the electronic structure of graphene ribbons with zigzag edges is unstable with respect to magnetic polarization of the edge states. The magnetic interaction between edge states is found to be remarkably long ranged and intimately connected to the electronic structure of the ribbon. Various treatments of electronic exchange and correlation are used to examine the sensitivity of this result to details of the electron-electron interactions, and the qualitative features are found to be independent of the details of the approximation. The possibility of other stablization mechanisms, such as charge ordering and a Peierls distortion, are explicitly considered and found to be unfavorable for ribbons of reasonable width. These results have direct implications for the control of the spin-dependent conductance in graphitic nanoribbons using suitably modulated magnetic fields.

Electronic correlation effects, usually associated with open d or f shells, have so far been considered in p orbitals only sporadically for the localized 2p states of first-row elements. We demonstrate that the partial band occupation and the metallic band-structure character as predicted by local density calculations for II-VI materials containing cation vacancies is removed when the correct energy splitting between occupied and unoccupied p orbitals is recovered. This transition into a Mott-insulating phase dramatically changes the structural, electronic and magnetic properties along the entire series (ZnO, ZnS, ZnSe, and ZnTe), and impedes ferromagnetism. Thus, important correlation effects due to open p shells exist not only for first-row (2p) elements, but also for much heavier anions like Te (5p).

Results obtained from this preliminary work suggest that aortic morphology and primary entry tear size and position exert significant effects on flow and other hemodynamic parameters in the dissected aorta in this preliminary work. Blood flow into the false lumen increases with increasing tear size and proximal location. Morphologic analysis coupled with computational fluid dynamic modeling may be useful in predicting acute type B dissection behavior allowing for selection of proper treatment modalities, and further confirmatory studies are warranted.

These molecular MRI probes constitute a novel imaging tool for in vivo characterization of plaque vulnerability and inflammatory activity in atherosclerosis. Further development and translation into the clinical arena will facilitate more accurate risk stratification in carotid atherosclerotic disease in the future.

Pure MnN and ͑Ga,Mn͒N alloys are investigated using the ab initio generalized gradient approximation +U ͑GGA+ U͒ or the hybrid-exchange density-functional ͑B3LYP͒ methods. These methods are found to predict dramatically different electronic structure, magnetic behavior, and relative stabilities compared to previous density-functional theory ͑DFT͒ calculations. A unique structural anomaly of MnN, in which local-density calculations fail to predict the experimentally observed distorted rocksalt as the ground-state structure, is resolved under the GGA+ U and B3LYP formalisms. The magnetic configurations of MnN are studied and the results suggest the magnetic state of zinc-blende MnN might be complex. Epitaxial calculations are used to show that the epitaxial zinc-blende MnN can be stabilized on an InGaN substrate. The structural stability of ͑Ga,Mn͒N alloys was examined and a crossover from the zinc-blende-stable alloy to the rocksalt-stable alloy at an Mn concentration of ϳ65% was found. The tendency for zinc-blende ͑Ga,Mn͒N alloys to phase separate is described by an asymmetric spinodal phase diagram calculated from a mixed-basis cluster expansion. This predicts that precipitates will consist of Mn concentrations of ϳ5 and ϳ50% at typical experimental growth temperatures. Thus, pure antiferromagnetic MnN, previously thought to suppress the Curie temperature, will not be formed. The Curie temperature for the 50% phase is calculated to be T C = 354 K, indicating the possibility of high-temperature ferromagnetism in zinc-blende ͑Ga,Mn͒N alloys due to precipitates.

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The atomic microstructure of alloys is rarely perfectly random, instead exhibiting differently shaped precipitates, clusters, zigzag chains, etc. While it is expected that such microstructural features will affect the electronic structures ͑carrier localization and band gaps͒, theoretical studies have, until now, been restricted to investigate either perfectly random or artificial "guessed" microstructural features. In this paper, we simulate the alloy microstructures in thermodynamic equilibrium using the static Monte Carlo method and study their electronic structures explicitly using a pseudopotential supercell approach. In this way, we can bridge atomic microstructures with their electronic properties. We derive the atomic microstructures of InGaN using ͑i͒ density-functional theory total energies of ϳ50 ordered structures to construct a ͑ii͒ multibody cluster expansion, including strain effects to which we have applied ͑iii͒ static Monte Carlo simulations of systems consisting of over 27000 atoms to determine the equilibrium atomic microstructures. We study two types of alloy thermodynamic behavior: ͑a͒ under lattice incoherent conditions, the formation enthalpies are positive and thus the alloy system phase-separates below the miscibility-gap temperature T MG , ͑b͒ under lattice coherent conditions, the formation enthalpies can be negative and thus the alloy system exhibits ordering tendency. The microstructure is analyzed in terms of structural motifs ͑e.g., zigzag chains and In n Ga 4−n N tetrahedral clusters͒. The corresponding electronic structure, calculated with the empirical pseudopotentials method, is analyzed in terms of band-edge energies and wave-function localization. We find that the disordered alloys have no electronic localization but significant hole localization, while below the miscibility gap under the incoherent conditions, In-rich precipitates lead to strong electron and hole localization and a reduction in the band gap.

A combination of reactive force field molecular dynamics and hybrid-exchange density functional theory (DFT) generates a defective structure of Rh-C 60 possessing an inter-cage link. Hybrid-exchange DFT is used within periodic boundary conditions to investigate the long-range magnetic coupling between the resulting defects. Inelastic neutron scattering experiments highlight the presence of hydrogen chemically bonded to carbon in the magnetic samples. A simple spin model previously applied to studies of planar conjugated electron systems is used to illustrate the mechanism through which chemically bonded hydrogen leads to a ferromagnetic ground state for this system. The recent observation of high temperature ferromagnetism in carbon materials is of great interest both as it introduces a class of potentially highly tunable materials for use in magnetic devices and because it presents a major challenge to our current understanding of magnetism. At elevated pressures and temperatures cubic C 60 fullerenes form well ordered two-dimensional polymerized phases. Below 9 GPa three distinct phases occur, with orthorhombic, tetragonal ͑T͒ and rhombohedral (Rh) symmetries.1 Further heating of these phases beyond ϳ900 K results in the collapse of the C 60 cages and the formation of hard "graphitic" phases. The ferromagnetic phases occur close to this phase boundary.2,3 A remarkably high Curie temperature ͑T c ͒ of ϳ500 K was reported in initial studies, 3 with recent data indicating an even higher T c of ϳ820 K. 4 Magnetic force microscopy (MFM) measurements 5 and additional experimental evidence 3 imply that the magnetism is an intrinsic property of carbon and not due to metallic impurities such as iron.The experimental characterization of the magnetic phases has proven difficult and currently the detailed atomic structure is not known. In situ x-ray diffraction measurements reveal a thermally activated process which converts the Rh phase (Fig. 1) into the highly disordered graphite-like phase, which displays very broad Bragg peaks. 6 The detailed structure of the magnetic phase cannot be determined from this data. Transmission electron microscopy (TEM) reveals an apparently well ordered crystalline structure in which the C 60 cages are largely intact and still in a Rh-C 60 -like arrangement.2 MFM studies established that only ϳ30% of the material is magnetic with the magnetism occurring in well defined domains. 5 As the characterization of the magnetic phase is problematic, theoretical calculations have an important role to play in determining possible local geometries. A number of previous theoretical studies have addressed the origins of magnetism in nonplanar carbon systems. Tight-binding molecular dynamics and cluster ab initio calculations have recently been used to analyze a single C 60 cage with a carbon vacancy. 7 Density functional theory (DFT) calculations have been used to examine a fullerene molecule during the transition to a nanotube segment via a series of Stone-Wales transformations 8 and a fragment with n...