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We model the remarkable thin-film Ba-Ti-O structures formed by heat treatment of an initial perovskite BaTiO3 thin film on a Pt(111) surface. All structures contain a rumpled Ti-O network with all Ti threefold coordinated with O and with Ba occupying the larger. mainly Ti7O7, pores. The quasicrystal structure is a simple decoration of three types of tiles: square, triangle, and 30° rhombus, with edge lengths 6.85 Å, joined edge-to-edge in a quasicrystalline pattern; observed periodic crystals in ultrathin film Ba-Ti-O are built from these and other tiles. Simulated STM images reproduce the patterns seen experimentally and identify the bright protrusions as Ba atoms. The models are consistent with all experimental observations. [ABSTRACT FROM AUTHOR]

First-principles density functional theory DFT+U calculations and experimental neutron difiraction structure analyses were used to determine the low-temperature crystallographic and magnetic structure of bixbyite a-Mn2O3. The energies of various magnetic arrangements, calculated from first principles, were fit to a cluster-expansion model using a Bayesian method that overcomes a problem of underfitting caused by the limited number of input magnetic configurations. The model was used to predict the lowest-energy magnetic states. Experimental determination of magnetic structure benefited from an optimized sample synthesis, which produced crystallite sizes large enough to yield a clear splitting of peaks in the neutron powder diffraction patterns, thereby enabling magnetic-structure refinements under the correct orthorhombic symmetry. The refinements employed group theory to constrain magnetic models. Computational and experimental analyses independently converged to similar ground states, with identical antiferromagnetic ordering along a principal magnetic axis and secondary ordering along a single orthogonal axis, differing only by a phase factor in the modulation patterns. The lowest-energy magnetic states are compromise solutions to frustrated antiferromagnetic interactions between certain comer-sharing [MnO6] octahedra. [ABSTRACT FROM AUTHOR]

The structure of finite-area topological defects in graphene is described in terms of both the direct honeycomb lattice and its dual triangular lattice. Such defects are equivalent to cutting out a patch of graphene and replacing it with a different patch with the same number of dangling bonds. An important subset of these defects, bound by a closed loop of alternating 5- and 7-membered carbon rings, explains most finite-area topological defects that have been experimentally observed. Previously unidentified defects seen in scanning tunneling microscope (STM) images of graphene grown on SiC are identified as isolated divacancies or divacancy clusters. [ABSTRACT FROM AUTHOR]

Topological defects can affect the physical properties of graphene in unexpected ways. Harnessing their influence may lead to enhanced control of both material strength and electrical properties. Here we present a class of topological defects in graphene composed of a rotating sequence of dislocations that close on themselves, forming grain boundary loops that either conserve the number of atoms in the hexagonal lattice or accommodate vacancy or interstitial reconstruction, while leaving no unsatisfied bonds. One grain boundary loop is observed as a "flower" pattern in scanning tunneling microscopy studies of epitaxial graphene grown on SiC(0001). We show that the flower defect has the lowest energy per dislocation core of any known topological defect in graphene, providing a natural explanation for its growth via the coalescence of mobile dislocations. [ABSTRACT FROM AUTHOR]

Local structures in BaTi1-xZrxO3 solid solutions were analyzed using x-ray absorption fine structure (XAFS) measurements and density-functional theory (DFT) calculations. We demonstrate that for low concentrations of Ti, isolated Ti atoms in the relatively large octahedral sites of the BaZrO3 lattice acquire centrosymmetric coordination with average Ti-O distances shorter than those in BaTiO3. In contrast for higher concentrations of Ti, Ti atoms having one or more Ti as their B-site nearest neighbors undergo polar off-center displacements. Our DFT calculations confirm both effects. These results combined with the previously published data suggest that isolated polarizable ions on the B sites of a relatively expanded host perovskite lattice remain nonpolar by symmetric relaxation of the nearest-neighbor oxygen atoms to yield nearly ideal bond lengths around the dopant species. For neighboring Ti atoms, such symmetric relaxation is impossible, and these atoms are displaced off center. Our XAFS measurements did not detect any significant deviations from a random distribution of Ti and Zr in the present samples except for compositions close to BaTiO3. The DFT calculations suggest that the dominant effect of the local displacements on band-gap values for this system is determined by the shortest Ti-O bonds due to strong Ti 3d-O 2p hybridization; however, local displacements have only a secondary effect on the band-gap behavior. [ABSTRACT FROM AUTHOR]

Hard x-ray photoelectron spectroscopy (HAXPES) has been used to study metal-oxide-semiconductor (MOS) structures fabricated with both high (Ni) and low (Al) work-function metals on 8-nm thick Al203 dielectric layers, deposited on sulfur passivated n- and p-doped GaAs substrates. A binding energy difference of 0.6 eV was measured between the GaAs core levels of the n- and p-doped substrates in the absence of gate metals, indicating different Fermi level positions in the band gap. Subsequent photoemission measurements made on the MOS structures with the different work-function metals displayed very limited change in the GaAs core level binding energies, indicating that the movement of the Fermi level at the Al203/GaAs interface is restricted. Using a combination of HAXPES measurements and theoretical calculations, the Fermi level positions in the band gap have been determined to be in the range of 0.4-0.75 eV and 0.8-1.11 eV above the valence band maximum for p- and n-type GaAs, respectively. Analysis of capacitance voltage (C-V) measurements on identically prepared samples yield very similar Fermi level positions at zero applied gate bias. The C-V analysis also indicates a higher interface defect density (Dit) in the upper half of the GaAs bandgap. [ABSTRACT FROM AUTHOR]

We study Ti 1s near-edge spectroscopy in PbTiO 3 at various temperatures above and below its tetragonal-to-cubic phase transition, and in SrTiO 3 at room temperature. Ab initio molecular dynamics (AIMD) runs on 80-atom supercells are used to determine the average internal coordinates and their fluctuations. We determine that one vector local order parameter is the dominant contributor to changes in spectral features: the displacement of the Ti ion with respect to its axial O neighbors in each Cartesian direction, as these displacements enhance the cross section for transitions to E g -derived core-hole exciton levels. Using periodic five-atom structures whose relative Ti-O displacements match the root-mean-square values from the AIMD simulations, and core-hole Bethe-Salpeter equation (BSE) calculations, we quantitatively predict the respective Ti 1s near-edge spectra. Properly accounting for atomic fluctuations greatly improves the agreement between theoretical and experimental spectra. The evolution of relative strengths of spectral features vs temperature and electric field polarization vector are captured in considerable detail. This work shows that local structure can be characterized from first-principles sufficiently well to aid both the prediction and the interpretation of near-edge spectra.

Molecular dynamics simulations of first-principles-based effective Hamiltonians for Pb(Sc{1/2}Nb{1/2})O(3) under hydrostatic pressure and for Pb(Mg{1/3}Nb{2/3})O(3) at ambient pressure show clear evidence of a relaxor state in both systems. The Burns temperature is identified as the temperature below which dynamic nanoscale polar clusters form, pinned to regions of quenched chemical short-range order. The effect of pressure in Pb(Sc{1/2}Nb{1/2})O(3) demonstrates that the stability of the relaxor state depends on a delicate balance between the energetics that stabilize normal ferroelectricity and the average strength of random local fields which promote the relaxor state.