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See detailScreening of electron-phonon coupling in graphene on Ir(111)
Endlich, M.; Molina-Sanchez, Alejandro UL; Wirtz, Ludger UL et al

in Physical Review. B, Condensed Matter and Materials Physics (2013), 88(205403),

The phonon dispersion of graphene on Ir(111) has been determined by means of angle-resolved inelastic electron scattering and density functional calculations. Kohn anomalies of the highest optical-phonon ... [more ▼]

The phonon dispersion of graphene on Ir(111) has been determined by means of angle-resolved inelastic electron scattering and density functional calculations. Kohn anomalies of the highest optical-phonon branches are observed at the ¯ and ¯K point of the surface Brillouin zone. At ¯K the Kohn anomaly is weaker than observed for pristine graphene and graphite. This observation is rationalized in terms of a decrease of the electron-phonon coupling due to screening of graphene electron correlations by the metal substrate. [less ▲]

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See detailAnomalous quantum confinement of the longitudinal optical phonon mode in PbSe quantum dots
Habinshuti, J.; Kilian, O.; Cristini-Robbe, O. et al

in Physical Review. B, Condensed Matter and Materials Physics (2013), 88(11), 115313

We have investigated the diameter dependence of the Raman spectra of lead selenide nanocrystals. The first-order Raman peak at about 136 cm−1 and its second-order overtone at twice this wavenumber move up ... [more ▼]

We have investigated the diameter dependence of the Raman spectra of lead selenide nanocrystals. The first-order Raman peak at about 136 cm−1 and its second-order overtone at twice this wavenumber move up in energy with decreasing nanocrystal diameter. This anomalous behavior is interpreted in terms of quantum confinement of the longitudinal optical (LO) phonon whose frequency displays a minimum at in the dispersion of bulk PbSe. We perform ab initio calculations of the phonons of PbSe slabs with up to 15 layers. The LO mode perpendicular to the slab shifts indeed upwards with decreasing layer thickness, thus validating the interpretation of the anomalous radius dependence of the Raman spectra in terms of quantum confinement. [less ▲]

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See detailEffect of spin-orbit interaction on the optical spectra of single-layer, double-layer, and bulk MoS2
Molina-Sanchez, Alejandro UL; Sangalli, Davide; Hummer, Kerstin et al

in Physical Review. B, Condensed Matter and Materials Physics (2013)

We present converged ab initio calculations of the optical absorption spectra of single-layer, double-layer, and bulk MoS2. Both the quasiparticle-energy calculations (on the level of the GW approximation ... [more ▼]

We present converged ab initio calculations of the optical absorption spectra of single-layer, double-layer, and bulk MoS2. Both the quasiparticle-energy calculations (on the level of the GW approximation ) and the calculation of the absorption spectra (on the level of the Bethe-Salpeter equation) explicitly include spin-orbit coupling, using the full spinorial Kohn-Sham wave functions as input. Without excitonic effects, the absorption spectra would have the form of a step function, corresponding to the joint density of states of a parabolic band dispersion in two dimensions. This profile is deformed by a pronounced bound excitonic peak below the continuum onset. The peak is split by spin-orbit interaction in the case of single-layer and (mostly) by interlayer interaction in the case of double-layer and bulk MoS2. The resulting absorption spectra are thus very similar in the three cases, but the interpretation of the spectra is different. Differences in the spectra can be seen in the shape of the absorption spectra at 3 eV where the spectra of the single and double layers are dominated by a strongly bound exciton. [less ▲]

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See detailTheory of magnetic small-angle neutron scattering of two-phase ferromagnets
Honecker, Dirk UL; Michels, Andreas UL

in Physical Review. B, Condensed Matter and Materials Physics (2013), 87(224426), 1-10

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See detailAnalysis of magnetic neutron-scattering data of two-phase ferromagnets
Honecker, Dirk UL; Dewhurst, Charles; Suzuki, Kiyonori et al

in Physical Review. B, Condensed Matter and Materials Physics (2013), 88

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See detailQuantification of finite-temperature effects on adsorption geometries of π-conjugated molecules: Azobenzene/Ag(111)
Mercurio, G.; Maurer, R. J.; Liu, W. et al

in Physical Review. B, Condensed Matter and Materials Physics (2013), 88(3),

The adsorption structure of the molecular switch azobenzene on Ag(111) is investigated by a combination of normal incidence x-ray standing waves and dispersion-corrected density functional theory. The ... [more ▼]

The adsorption structure of the molecular switch azobenzene on Ag(111) is investigated by a combination of normal incidence x-ray standing waves and dispersion-corrected density functional theory. The inclusion of nonlocal collective substrate response (screening) in the dispersion correction improves the description of dense monolayers of azobenzene, which exhibit a substantial torsion of the molecule. Nevertheless, for a quantitative agreement with experiment explicit consideration of the effect of vibrational mode anharmonicity on the adsorption geometry is crucial. © 2013 American Physical Society. [less ▲]

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See detailDielectric screening of the Kohn anomaly of graphene on hexagonal boron nitride
Forster, F.; Molina-Sanchez, Alejandro UL; Engels, S. et al

in Physical Review. B, Condensed Matter and Materials Physics (2013), 88

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See detailMagnetic ordering in nanocrystalline gadolinium: A neutron diffraction study
Ryan, D. H.; Michels, Andreas UL; Döbrich, Frank UL et al

in Physical Review. B, Condensed Matter and Materials Physics (2013), 87(064408), 1-6

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See detailNeutron scattering study of the magnetic microstructure of nanocrystalline gadolinium
Döbrich, Frank UL; Kohlbrecher, Joachim; Sharp, Melissa et al

in Physical Review. B, Condensed Matter and Materials Physics (2012), 85(094411), 1-17

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See detailObservation of cross-shaped anisotropy in spin-resolved small-angle neutron scattering
Michels, Andreas UL; Honecker, Dirk UL; Döbrich, Frank UL et al

in Physical Review. B, Condensed Matter and Materials Physics (2012), 85(184417), 1-5

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See detailMicromagnetic modeling and small-angle neutron scattering characterization of magnetic nanocomposites
Erokhin, Sergey; Berkov, Dmitry; Gorn, Nataliya et al

in Physical Review. B, Condensed Matter and Materials Physics (2012), 85(024410), 1-12

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See detailMagnetic neutron scattering on nanocomposites: Decrypting cross-section images using micromagnetic simulations
Erokhin, Sergey; Berkov, Dmitry; Gorn, Nataliya et al

in Physical Review. B, Condensed Matter and Materials Physics (2012), 85(134418), 1-5

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See detailElectronic structure of dye-sensitized TiO 2 clusters from many-body perturbation theory
Marom, Noa; Moussa, Jonathan E.; Ren, Xinguo et al

in Physical Review. B, Condensed Matter and Materials Physics (2011), 84(24),

The development of new types of solar cells is driven by the need for clean and sustainable energy. In this respect dye-sensitized solar cells (DSC) are considered as a promising route for departing from ... [more ▼]

The development of new types of solar cells is driven by the need for clean and sustainable energy. In this respect dye-sensitized solar cells (DSC) are considered as a promising route for departing from the traditional solid state cells. The physical insight provided by computational modeling may help develop improved DSCs. To this end, it is important to obtain an accurate description of the electronic structure, including the fundamental gaps and level alignment at the dye-TiO2 interface. This requires a treatment beyond ground-state density functional theory (DFT). We present a many-body perturbation theory study, within the G(0)W(0) approximation, of two of the crystalline phases of dye-sensitized TiO2 clusters, reported by Benedict and Coppens, [J. Am. Chem. Soc. 132, 2938 (2010)]. We obtain geometries in good agreement with the experiment by using DFT with the Tkatchenko-Scheffler van der Waals correction. We demonstrate that even when DFT gives a good description of the valence spectrum and a qualitatively correct picture of the electronic structure of the dye-TiO2 interface, G(0)W(0) calculations yield more valuable quantitative information regarding the fundamental gaps and level alignment. In addition, we systematically investigate the issues pertaining to G(0)W(0) calculations, namely: (i) convergence with respect to the number of basis functions, (ii) dependence on the mean-field starting point, and (iii) the validity of the assumption that the DFT wave function is a good approximation to the quasiparticle wave function. We show how these issues are manifested for dye molecules and for dye-sensitized TiO2 clusters. [less ▲]

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See detailPhonons in single-layer and few-layer MoS2 and WS2
Molina-Sanchez, Alejandro UL; Wirtz, Ludger UL

in Physical Review. B, Condensed Matter and Materials Physics (2011), 84

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See detailInfluence of crystallite size and temperature on the antiferromagnetic helices of terbium and holmium metal
Michels, Andreas UL; Bick, Jens-Peter UL; Birringer, R. et al

in Physical Review. B, Condensed Matter and Materials Physics (2011), 83

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See detailVariations in the work function of doped single- and few-layer graphene assessed by Kelvin probe force microscopy and density functional theory
Ziegler, D.; Gava, P.; Guettinger, J. et al

in Physical Review. B, Condensed Matter and Materials Physics (2011), 83(23),

We present Kelvin probe force microscopy measurements of single-and few-layer graphene resting on SiO2 substrates. We compare the layer thickness dependency of the measured surface potential with ab ... [more ▼]

We present Kelvin probe force microscopy measurements of single-and few-layer graphene resting on SiO2 substrates. We compare the layer thickness dependency of the measured surface potential with ab initio density functional theory calculations of the work function for substrate-doped graphene. The ab initio calculations show that the work function of single-and bilayer graphene is mainly given by a variation of the Fermi energy with respect to the Dirac point energy as a function of doping, and that electrostatic interlayer screening only becomes relevant for thicker multilayer graphene. From the Raman G-line shift and the comparison of the Kelvin probe data with the ab initio calculations, we independently find an interlayer screening length in the order of four to five layers. Furthermore, we describe in-plane variations of the work function, which can be attributed to partial screening of charge impurities in the substrate, and result in a nonuniform charge density in single-layer graphene. [less ▲]

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See detailAnisotropic excitonic effects in the energy loss function of hexagonal boron nitride
Galambosi, S.; Wirtz, Ludger UL; Soininen, J. A. et al

in 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 ▲]

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See detailCoupling of excitons and defect states in boron-nitride nanostructures
Attaccalite, C.; Bockstedte, M.; Marini, A. 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 ▲]

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See detailDielectric function of colloidal lead chalcogenide quantum dots obtained by a Kramers-Kronig analysis of the absorbance spectrum
Moreels, Iwan; Allan, Guy; De Geyter, Bram et al

in Physical Review. B, Condensed Matter and Materials Physics (2010), 81(23),

We combined the Maxwell-Garnett effective medium theory with the Kramers-Kronig relations to obtain the complex dielectric function epsilon of colloidal PbS, PbSe, and PbTe quantum dots (Qdots). The ... [more ▼]

We combined the Maxwell-Garnett effective medium theory with the Kramers-Kronig relations to obtain the complex dielectric function epsilon of colloidal PbS, PbSe, and PbTe quantum dots (Qdots). The method allows extracting both real (epsilon(R)) and imaginary (epsilon(I)) parts of the dielectric function from the experimental absorption spectrum. This enables the quantification of the size-dependent oscillator strength of the optical transitions at different critical points in the Brillouin zone, strongly improving our understanding of quantum confinement effects in these materials. In addition, the static-limit sum rule yields the electronic dielectric constant from the epsilon(I) spectrum. Interestingly, values for lead chalcogenide Qdots remain close to the bulk dielectric constant. To verify these trends, we determined the dielectric constant of thin lead chalcogenide layers by ab initio calculations, and the results agree with the experimental data. [less ▲]

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See detailTransport through open quantum dots: Making semiclassics quantitative
Brezinova, Iva; Wirtz, Ludger UL; Rotter, Stefan et al

in Physical Review. B, Condensed Matter and Materials Physics (2010), 81(12),

We investigate electron transport through clean open quantum dots (quantum billiards). We present a semiclassical theory that allows to accurately reproduce quantum-transport calculations. Quantitative ... [more ▼]

We investigate electron transport through clean open quantum dots (quantum billiards). We present a semiclassical theory that allows to accurately reproduce quantum-transport calculations. Quantitative agreement is reached for individual energy-dependent and magnetic field dependent elements of the scattering matrix. Two key ingredients are essential: (i) inclusion of pseudopaths which have the topology of linked classical paths resulting from diffraction in addition to classical paths and (ii) a high-level approximation to diffractive scattering. Within this framework of the pseudopath semiclassical approximation (PSCA), typical shortcomings of semiclassical theories such as violation of the anticorrelation between reflection and transmission and the overestimation of conductance fluctuations are overcome. Beyond its predictive capabilities the PSCA provides deeper insights into the quantum-to-classical crossover. [less ▲]

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