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   Anisotropic excitonic effects in the energy loss function of hexagonal boron nitride; Wirtz, Ludger  ; et alin Physical Review. B, Condensed Matter and Materials Physics (2011), 83(8), 081413 The anisotropy of the valence energy-loss function of hexagonal boron nitride (hBN) is shown to be largely enhanced by the highly inhomogeneous character of the excitonic states. The energy loss with ... [more ▼] The anisotropy of the valence energy-loss function of hexagonal boron nitride (hBN) is shown to be largely enhanced by the highly inhomogeneous character of the excitonic states. The energy loss with momentum transfer parallel to the BN layers is dominated by strongly bound, quasi-two-dimensional excitons. In contrast, excitations with momentum transfer perpendicular to the layers are influenced by weakly bound three-dimensional excitons. This striking phenomenon is revealed by a combined study using high-precision nonresonant inelastic x-ray scattering measurements supported by ab initio calculations. The results are relevant in general to layered insulating systems. [less ▲] Detailed reference viewed: 150 (7 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: 115 (2 UL)Vibrational properties of hexagonal boron nitride: Inelastic X-ray scattering and ab initio calculations; ; et al in Physical Review Letters (2007), 98(9), 095503 The phonon dispersion relations of bulk hexagonal boron nitride have been determined from inelastic x-ray scattering measurements and analyzed by ab initio calculations. Experimental data and calculations ... [more ▼] The phonon dispersion relations of bulk hexagonal boron nitride have been determined from inelastic x-ray scattering measurements and analyzed by ab initio calculations. Experimental data and calculations show an outstanding agreement and reconcile the controversies raised by recent experimental data obtained by electron-energy loss spectroscopy and second-order Raman scattering. [less ▲] Detailed reference viewed: 136 (2 UL) |
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