Synchrotron X-ray imaging via ultra-small-angle scattering: principles of quantitative analysis and application in studying bone integration to synthetic grafting materials

Optimized experimental conditions for extracting accurate information at subpixel length scales from analyzer-based X-ray imaging were obtained and applied to investigate bone regeneration by means of synthetic beta-TCP grafting materials in a rat calvaria model. The results showed a 30% growth in the particulate size due to bone ongrowth/ingrowth within the critical size defect over a 1-month healing period.

Magnetism of Co overlayers and nanostructures on W(001): A first-principles study

The magnetic properties of Co nanostructures and a Co monolayer on W(001) have been studied in the framework of density functional theory. Different geometries such as planar and three-dimensional clusters have been considered, with cluster sizes varying between 2 and 13 atoms. The calculations were performed using the real-space linear muffin-tin orbital method (RS-LMTO-ASA). With respect to the stability of the magnetic state, we predict an antiferromagnetic (AFM) structure for the ground state of the planar Co clusters and a ferromagnetic (FM) state for the three-dimensional clusters. For the three-dimensional clusters, one of the AFM arrangements leads to frustration due to the competing FM and AFM exchange interactions between different atoms in the cluster, and gives rise to a non-collinear state with energy close to that of the FM ground state. The relative role of the Co-Co and Co-W exchange interactions is also investigated. (C) 2007 Elsevier B.V. All rights reserved.

First-principles calculation of the AlAs/GaAs interface band structure using a self-energy-corrected local density approximation

We apply a self-energy-corrected local density approximation (LDA) to obtain corrected bulk band gaps and to study the band offsets of AlAs grown on GaAs (AlAs/GaAs). We also investigate the Al(x)Ga(1-x)As/GaAs alloy interface, commonly employed in band gap engineering. The calculations are fully ab initio, with no adjustable parameters or experimental input, and at a computational cost comparable to traditional LDA. Our results are in good agreement with experimental values and other theoretical studies. Copyright (C) EPLA, 2011

Boron and nitrogen functionalized diamondoids: A first principles investigation

Chemically functionalized adamantane molecules have been investigated by first principles total energy calculations. Boron and nitrogen functionalized molecules were found to be very stable, consistent with available experimental data. Two hypothetical molecular crystals, involving functionalized adamantane, were investigated. These molecular crystals presented direct electronic bandgaps and large bulk moduli, which suggested a possible road for molecular self-assembly using functionalized diamondoids. (C) 2010 Elsevier B.V. All rights reserved.

Furan interaction with the Si(001)-(2 x 2) surface: structural, energetics, and vibrational spectra from first-principles

In this work we employ the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory, to investigate the adsorption process of furan on the silicon (001) surface. A direct comparison of different adsorption structures with x-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS), high resolution electron energy loss spectroscopy (HREELS), near edge x-ray absorption fine structure (NEXAFS), and high resolution spectroscopy experimental data allows us to identify the [4 + 2] cycloaddition reaction as the most probable adsorbate. In addition, theoretical scanning tunnelling microscopy (STM) images are presented, with a view to contributing to further experimental investigations.

The electronic and optical properties of sodalite (Na(8)Al(6)Si(6)O(24)Cl(2)) from first principles

This study reports the results of ab initio electronic and optical calculations for pure socialite crystal using the linear augmented plane wave (LAPW) method within density functional theory (DFT). The calculated electronic structure revealed predominantly orbital characters of the valence band and the conduction band, and enabled us to determine the type and the value of the fundamental gap of the compound. The imaginary part of the dielectric tensor, extinction coefficient and refraction index were calculated as functions of the incident radiation wavelength. It is shown that the O 2p states and Na 3s states play the major role in optical transitions as initial and final states, respectively. The absorption spectrum is localized in the ultraviolet range between 40 and 250 nm. Furthermore, we concluded that the material does not absorb radiation in the visible range. (C) 2009 Elsevier Ltd. All rights reserved.

Orbital-polarization terms: From a phenomenological to a first-principles description of orbital magnetism in density-functional theory

Phenomenological orbital-polarizition (OP) terms have been repeatedly introduced in the single-particle equations of spin-density-functional theory, in order to improve the description of orbital magnetic moments in systems containing transition metal ions. Here we show that these ad hoc corrections can be interpreted as approximations to the exchange-correlation vector potential A(xc) of current-density functional theory (CDFT). This connection provides additional information on both approaches: phenomenological OP terms are connected to first-principles theory, leading to a rationale for their empirical success and a reassessment of their limitations and the approximations made in their derivation. Conversely, the connection of OP terms with CDFT leads to a set of simple approximations to the CDFT potential A(xc), with a number of desirable features that are absent from electron-gas-based functionals. (C) 2008 Wiley Periodicals, Inc.

The low-lying electronic states of BeAs: a first principles characterization

The electronic structure and spectroscopic properties of a manifold of states of a new molecular species, BeAs, have been investigated theoretically at the complete active space self-consistent field/multireference single and double excitations configuration interaction (CASSCF/MRSDCI) approach, using the aug-cc-pV5Z-PP basis set for arsenic, which includes a relativistic effective core potential, and the cc-pV5Z set for beryllium. Potential energy curves of five quartet and eight doublet (I > + S) states correlating with the five lowest-lying dissociation limit are constructed. The effect of spin-orbit coupling is also included in the description of the ground state, and of the doublet states correlating with the second dissociation channel. Dipole moment functions and vibrationally averaged dipole moments are also evaluated. The similarities and differences between BeAs, BeP, and BeN are analyzed. Spin-orbit effects are small for the ground state close to the equilibrium distance, but avoided crossings between Omega = 1/2 states, and between Omega = 3/2 states changes significantly the I > + S curves for the lowest-lying doublets.