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Coupling of excitons and defect states in boron-nitride nanostructures ; ; et al in Physical Review. B, Condensed Matter and Materials Physics (2011), 83(14), 144115 The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory. A single BN-sheet serves as a model for different layered BN ... [more ▼] The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory. A single BN-sheet serves as a model for different layered BN nanostructures and crystals. In the sheet we embed prototypical defects such as a substitutional impurity, isolated boron and nitrogen vacancies, and the divacancy. Transitions between the deep defect levels and extended states produce characteristic excitation bands that should be responsible for the emission band around 4 eV, observed in luminescence experiments. In addition, defect bound excitons occur that are consistently treated in our ab initio approach along with the "free" exciton. For defects in strong concentration, the coexistence of both bound and free excitons adds substructure to the main exciton peak and provides an explanation for the corresponding feature in cathodo- and photoluminescence spectra. [less ▲] Detailed reference viewed: 77 (4 UL)Phonon surface mapping of graphite: Disentangling quasi-degenerate phonon dispersions ; ; et al in Physical Review B (2009), 80(8), The two-dimensional mapping of the phonon dispersions around the K point of graphite by inelastic x-ray scattering is provided. The present work resolves the longstanding issue related to the correct ... [more ▼] The two-dimensional mapping of the phonon dispersions around the K point of graphite by inelastic x-ray scattering is provided. The present work resolves the longstanding issue related to the correct assignment of transverse and longitudinal phonon branches at K. We observe an almost degeneracy of the three TO-, LA-, and LO-derived phonon branches and a strong phonon trigonal warping. Correlation effects renormalize the Kohn anomaly of the TO mode, which exhibits a trigonal warping effect opposite to that of the electronic band structure. We determined the electron-phonon coupling constant to be 166 (eV/A degrees)(2) in excellent agreement to GW calculations. These results are fundamental for understanding angle-resolved photoemission, double-resonance Raman and transport measurements of graphene-based systems. [less ▲] Detailed reference viewed: 45 (1 UL)Tight-binding description of the quasiparticle dispersion of graphite and few-layer graphene ; ; Wirtz, Ludger et al in Physical Review B (2008), 78(20), A universal set of third-nearest-neighbor tight-binding (TB) parameters is presented for calculation of the quasiparticle (QP) dispersion of N stacked sp(2) graphene layers (N=1...infinity) with AB ... [more ▼] A universal set of third-nearest-neighbor tight-binding (TB) parameters is presented for calculation of the quasiparticle (QP) dispersion of N stacked sp(2) graphene layers (N=1...infinity) with AB stacking sequence. The present TB parameters are fit to ab initio calculations on the GW level and are universal, allowing to describe the whole pi "experimental" band structure with one set of parameters. This is important for describing both low-energy electronic transport and high-energy optical properties of graphene layers. The QP bands are strongly renormalized by electron-electron interactions, which results in a 20% increase in the nearest-neighbor in-plane and out-of-plane TB parameters when compared to band structure from density-functional theory. With the new set of TB parameters we determine the Fermi surface and evaluate exciton energies, charge carrier plasmon frequencies, and the conductivities which are relevant for recent angle-resolved photoemission, optical, electron energy loss, and transport measurements. A comparision of these quantitities to experiments yields an excellent agreement. Furthermore we discuss the transition from few-layer graphene to graphite and a semimetal to metal transition in a TB framework. [less ▲] Detailed reference viewed: 69 (0 UL)Electron-electron correlation in graphite: A combined angle-resolved photoemission and first-principles study ; ; et al in Physical Review Letters (2008), 100(3), 037601 The full three-dimensional dispersion of the pi bands, Fermi velocities, and effective masses are measured with angle-resolved photoemission spectroscopy and compared to first-principles calculations. The ... [more ▼] The full three-dimensional dispersion of the pi bands, Fermi velocities, and effective masses are measured with angle-resolved photoemission spectroscopy and compared to first-principles calculations. The band structure by density-functional theory underestimates the slope of the bands and the trigonal warping effect. Including electron-electron correlation on the level of the GW approximation, however, yields remarkable improvement in the vicinity of the Fermi level. This demonstrates the breakdown of the independent electron picture in semimetallic graphite and points toward a pronounced role of electron correlation for the interpretation of transport experiments and double-resonant Raman scattering for a wide range of carbon based materials. [less ▲] Detailed reference viewed: 61 (1 UL)Low energy quasiparticle dispersion of graphite by angle-resolved photoemission spectroscopy ; ; et al in PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS (2007), 244(11), 4129-4133 The low energy electron dispersion in graphite is measured by angle-resolved photoemission spectroscopy. The measured photoemission intensity maxima are compared to a tight-binding calculation of the ... [more ▼] The low energy electron dispersion in graphite is measured by angle-resolved photoemission spectroscopy. The measured photoemission intensity maxima are compared to a tight-binding calculation of the electronic band structure. We observe a strong trigonal warping of the equi-energy contour which is well reproduced by the calculations. Furthermore we clearly show that the concept of Dirac Fermions breaks down for AB stacked graphite. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. [less ▲] Detailed reference viewed: 48 (1 UL) |
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