457 research outputs found

    Surgical management of gingival recession using autogenous soft tissue grafts

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    One of the chief goals of periodontal plastic surgery is establishment of ideal pink esthetics through the reconstruction of gingival recessions. A gold standard treatment approach for coverage of gingival recession with predictable esthetic outcomes is the transplantation of autogenous soft tissue grafts. Various surgical techniques can be used in combination with autogenous soft tissue grafts for gingival recession coverage

    Precision Measurement of the 29Si, 33S, and 36Cl Binding Energies

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    The binding energies of 29Si, 33S, and 36Cl have been measured with a relative uncertainty <0.59×10−6< 0.59 \times 10^{-6} using a flat-crystal spectrometer. The unique features of these measurements are 1) nearly perfect crystals whose lattice spacing is known in meters, 2) a highly precise angle scale that is derived from first principles, and 3) a gamma-ray measurement facility that is coupled to a high flux reactor with near-core source capability. The binding energy is obtained by measuring all gamma-rays in a cascade scheme connecting the capture and ground states. The measurements require the extension of precision flat-crystal diffraction techniques to the 5 to 6 MeV energy region, a significant precision measurement challenge. The binding energies determined from these gamma-ray measurements are consistent with recent highly accurate atomic mass measurements within a relative uncertainty of 4.3×10−74.3 \times 10^{-7}. The gamma-ray measurement uncertainties are the dominant contributors to the uncertainty of this consistency test. The measured gamma-ray energies are in agreement with earlier precision gamma-ray measurements.Comment: 13 pages, 4 figure

    Transition-metal interactions in aluminum-rich intermetallics

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    The extension of the first-principles generalized pseudopotential theory (GPT) to transition-metal (TM) aluminides produces pair and many-body interactions that allow efficient calculations of total energies. In aluminum-rich systems treated at the pair-potential level, one practical limitation is a transition-metal over-binding that creates an unrealistic TM-TM attraction at short separations in the absence of balancing many-body contributions. Even with this limitation, the GPT pair potentials have been used effectively in total-energy calculations for Al-TM systems with TM atoms at separations greater than 4 AA. An additional potential term may be added for systems with shorter TM atom separations, formally folding repulsive contributions of the three- and higher-body interactions into the pair potentials, resulting in structure-dependent TM-TM potentials. Towards this end, we have performed numerical ab-initio total-energy calculations using VASP (Vienna Ab Initio Simulation Package) for an Al-Co-Ni compound in a particular quasicrystalline approximant structure. The results allow us to fit a short-ranged, many-body correction of the form a(r_0/r)^{b} to the GPT pair potentials for Co-Co, Co-Ni, and Ni-Ni interactions.Comment: 18 pages, 5 figures, submitted to PR

    Ab initio study of the beta$-tin->Imma->sh phase transitions in silicon and germanium

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    We have investigated the structural sequence of the high-pressure phases of silicon and germanium. We have focussed on the cd->beta-tin->Imma->sh phase transitions. We have used the plane-wave pseudopotential approach to the density-functional theory implemented within the Vienna ab-initio simulation package (VASP). We have determined the equilibrium properties of each structure and the values of the critical parameters including a hysteresis effect at the phase transitions. The order of the phase transitions has been obtained alternatively from the pressure dependence of the enthalpy and of the internal structure parameters. The commonly used tangent construction is shown to be very unreliable. Our calculations identify a first-order phase transition from the cd to the beta-tin and from the Imma to the sh phase, and they indicate the possibility of a second-order phase-transition from the beta-tin to the Imma phase. Finally, we have derived the enthalpy barriers between the phases.Comment: 12 pages, 16 figure

    Structure of the icosahedral Ti-Zr-Ni quasicrystal

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    The atomic structure of the icosahedral Ti-Zr-Ni quasicrystal is determined by invoking similarities to periodic crystalline phases, diffraction data and the results from ab initio calculations. The structure is modeled by decorations of the canonical cell tiling geometry. The initial decoration model is based on the structure of the Frank-Kasper phase W-TiZrNi, the 1/1 approximant structure of the quasicrystal. The decoration model is optimized using a new method of structural analysis combining a least-squares refinement of diffraction data with results from ab initio calculations. The resulting structural model of icosahedral Ti-Zr-Ni is interpreted as a simple decoration rule and structural details are discussed.Comment: 12 pages, 8 figure

    Flat photonic bands in guided modes of textured metallic microcavities

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    M. G. Salt and William L. Barnes, Physical Review B, Vol. 61, pp. 11125-11135 (2000). "Copyright © 2000 by the American Physical Society."A detailed experimental study of how wavelength-scale periodic texture modifies the dispersion of the guided modes of λ/2 metal-clad microcavities is presented. We first examine the case of a solid-state microcavity textured with a single, periodic corrugation. We explore how the depth of the corrugation and the waveguide thickness affect the width of the band gap produced in the dispersion of the guided modes by Bragg scattering off the periodic structure. We demonstrate that the majority of the corrugation depths studied dramatically modify the dispersion of the lowest-order cavity mode to produce a series of substantially flat bands. From measurements of how the central frequency of the band gap varies with direction of propagation of the guided modes, we determine a suitable two-dimensional texture profile for the production of a complete band gap in all directions of propagation. We then experimentally examine band gaps produced in the guided modes of such a two-dimensionally textured microcavity and demonstrate the existence of a complete band gap for all directions of propagation of the lowest-order TE-polarized mode. We compare our experimental results with those from a theoretical model and find good agreement. Implications of these results for emissive microcavity devices such as light-emitting diodes are discussed
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