References of "Wirtz, Ludger 50003339"
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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 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 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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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 detailRumpling of LiF(001) surface from fast atom diffraction
Schueller, A.; Wethekam, S.; Blauth, D. et al

in Physical Review. A (2010), 82(6), 062902

Quantum diffraction of fast atoms scattered from the topmost layer of surfaces under grazing angles of incidence can be employed for the analysis of detailed structural properties of insulator surfaces ... [more ▼]

Quantum diffraction of fast atoms scattered from the topmost layer of surfaces under grazing angles of incidence can be employed for the analysis of detailed structural properties of insulator surfaces. From comparison of measured and calculated diffraction patterns we deduce the rumpling of the topmost surface layer of LiF(001) (i.e., an inward shift of Li(+) ions with respect to F(-) ions). The effect of thermal vibrations on the measurement of rumpling is accounted for by ab initio calculations of the mean-square vibrational amplitudes of surface ions. At room temperature this leads to a reduction of the apparent rumpling by 0.008 angstrom. We then obtain a rumpling of ( 0.05 +/- 0.04) angstrom, which improves its accuracy achieved in previous work. [less ▲]

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See detailDoped Graphene as Tunable Electron-Phonon Coupling Material
Attaccalite, Claudio; Wirtz, Ludger UL; Lazzeri, Michele et al

in Nano Letters (2010), 10(4), 1172-1176

we present a new way to tune the electron phonon coupling (EPC) in graphene by changing the deformation potential with electron/hole doping. We show the EPC for highest optical branch at the high symmetry ... [more ▼]

we present a new way to tune the electron phonon coupling (EPC) in graphene by changing the deformation potential with electron/hole doping. We show the EPC for highest optical branch at the high symmetry point K acquires a strong dependency on the doping level due to electron electron correlation not accounted in mean-field approaches. Such a dependency influences the dispersion (with respect to the laser energy) of the Raman D and 2D lines and the splitting of the 2D peak in multilayer graphene. Finally this doping dependence opens the possibility to construct tunable electronic devices through external control of the EPC. [less ▲]

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See detailGraphene on Metallic Substrates: Suppression of the Kohn Anomalies in the Phonon Dispersion
Allard, Adrien; Wirtz, Ludger UL

in Nano Letters (2010), 10(11), 4335-4340

The phonon dispersion of graphene is known to display two strong Kohn Anomalies (kinks) in the highest optical branch (HOB) at the high-symmetry points Gamma and K [Piscanec, S.; et al. Phys. Rev. Lett ... [more ▼]

The phonon dispersion of graphene is known to display two strong Kohn Anomalies (kinks) in the highest optical branch (HOB) at the high-symmetry points Gamma and K [Piscanec, S.; et al. Phys. Rev. Lett. 2004, 93, 185503]. The phonon slope around the Kohn anomalies is related to the electron phonon coupling (EPC) with the graphene it bands. we show that this EPC, which has strong impact, for example, on Raman scattering and electron transport, can be strongly modified due to interaction with a metallic substrate. For graphene grown on a Ni(111) surface, a total suppression of the Kohn anomaly occurs: the HOB around Gamma and K becomes completely flat. This is due to the strong hybridization of the graphene pi-bands with the nickel d bands that lifts the linear crossing of the pi bands at K. In addition, the out-of-plane modes are also found to be strongly affected by the binding to the substrate. For other metallic substrates, where the distance between the graphene sheet and the substrate is larger, hybridization is much less pronounced and the Kohn anomaly is only weakly perturbed. From experimental phonon dispersions, one can therefore draw conclusions about the interaction strength between graphene and its different substrates. [less ▲]

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See detailElectronic structure and radial breathing mode for carbon nanotubes with ultra-high curvature
Plank, W.; Pfeiffer, R.; Scharman, C. et al

in Physica Status Solidi B. Basic Research (2010), 247(11-12), 2774-2778

Ultra-high curvature single-walled carbon nanotubes (SWCNTs) with diameters down to 0.37 nm were prepared by transformation of FeCp(2) peapods to double-walled CNTs (DWCNTs). Results from resonance Raman ... [more ▼]

Ultra-high curvature single-walled carbon nanotubes (SWCNTs) with diameters down to 0.37 nm were prepared by transformation of FeCp(2) peapods to double-walled CNTs (DWCNTs). Results from resonance Raman scattering and high resolution transmission electron microscopy (TEM) were compared to calculations on the molecular dynamical, many electron corrected extended tight binding, and density functional theory (DFT) level. The growth process was found to be catalytic from Fe(3)C particles inside the tubes with dimensions of a crystallographic unit cell. The electronic structure showed dramatic deviations from tight binding results. The family behavior leads to level crossing already for E(11) and E(22) transitions. Experimental results fitted well into a Kataura-Popov plot and allowed assignment for the observed Raman lines of the inner tubes. Experimental and calculated radial breathing mode (RBM) frequencies showed a systematic difference indicating a radial expansion of the smallest inner tubes and a radial compression for the larger tubes. (c) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [less ▲]

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See detailNear Kohn anomalies in the phonon dispersion relations of lead chalcogenides
Kilian, Ondrej; Allan, Guy; Wirtz, Ludger UL

in Physical Review. B (2009), 80(24),

We present ab initio phonon dispersion relations for the three lead chalcogenides PbS, PbSe, and PbTe. The acoustic branches are in very good agreement with inelastic neutron-scattering data and ... [more ▼]

We present ab initio phonon dispersion relations for the three lead chalcogenides PbS, PbSe, and PbTe. The acoustic branches are in very good agreement with inelastic neutron-scattering data and calculations of the specific heat give good agreement with experimental data. The pronounced minimum of the transverse-optical branch at Gamma due to the near ferroelectricity of the lead chalcogenides is qualitatively reproduced. In addition, we find a pronounced dip in the longitudinal-optical branch at Gamma. This dip was previously explained as the effect of "free carriers" (due to the presence of impurities). The calculations demonstrate that it persists also in the case of pure lead chalcogenides. We explain the dip as a "near Kohn anomaly" which is associated with the small electronic band gap at the high-symmetry point L. [less ▲]

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See detailFast-Atom Diffraction at Surfaces
Aigner, F.; Simonovic, N.; Solleder, B. et al

in Journal of Physics: Conference Series (2009), 194

Fast helium atoms diffracted at alkali-halide surfaces under grazing angles of incidence exhibit intriguing diffraction patterns. The persistence of quantum coherence is remarkable, considering high ... [more ▼]

Fast helium atoms diffracted at alkali-halide surfaces under grazing angles of incidence exhibit intriguing diffraction patterns. The persistence of quantum coherence is remarkable, considering high surface temperatures and high (keV) kinetic energies of the incident atoms. Dissipative and decohering effects such as the momentum transfer between the incident helium atoms and the surface influence the diffraction patterns and control the width of the diffraction peaks, but they are weak enough to preserve the visibility of the diffration patterns. We perform an ab initio simulation of the quantum diffraction of fast helium beams at a LiF (100) surface in the < 110 > direction. Our results agree well with recent experimental diffraction data. [less ▲]

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See detailVanishing gap in LiF for electronic excitations by slow antiprotons
Solleder, Beate; Wirtz, Ludger UL; Burgdörfer, Joachim

in Physical Review. B (2009), 79(12),

We study the influence of antiprotons and protons traveling through LiF on the band structure of the insulator using the embedded-cluster method. The crystal is represented by F(m)(-)Li(n)(+) clusters ... [more ▼]

We study the influence of antiprotons and protons traveling through LiF on the band structure of the insulator using the embedded-cluster method. The crystal is represented by F(m)(-)Li(n)(+) clusters with up to 19 fluorine ions embedded in a lattice of point charges representing the remainder of the crystal. The minimum excitation energy of LiF perturbed by the (anti)proton impurity is calculated employing the multiconfiguration self-consistent field method. The repulsive potential of the antiproton causes a dramatic local perturbation of the LiF band structure. We find a strong suppression of the excitation energy by more than an order of magnitude compared to that of the unperturbed crystal. The present results provide a simple explanation of recent stopping-power experiments for antiprotons in LiF which, surprisingly, found a "metal-like" behavior of the wide-band-gap insulator LiF. Our results also agree with recent experimental data indicating a deviation from metallic behavior of the stopping power of proton projectiles. [less ▲]

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See detailPhonon surface mapping of graphite: Disentangling quasi-degenerate phonon dispersions
Grueneis, A.; Serrano, J.; Bosak, A. 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 ▲]

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See detailExcitation energy and pair correlation function of trions near an LiF surface
Solleder, Beate; Wirtz, Ludger UL; Burgdörfer, Joachim

in Physical Review. B (2008), 78(15),

We investigate the excitation spectrum of a singly charged LiF surface using an embedded cluster approach. The spectrum is determined by multiconfiguration self-consistent field calculations. We find ... [more ▼]

We investigate the excitation spectrum of a singly charged LiF surface using an embedded cluster approach. The spectrum is determined by multiconfiguration self-consistent field calculations. We find stable trionic states, i.e., quasimolecular states of a three-quasiparticle complex consisting of two holes and one electron interacting through the Coulomb potential. The excitation energy of the lowest trionic state is approximately 12.1 eV. The mean hole-hole spacing is about 7.6 a.u. (or 4 A). Our results confirm the interpretation of experiments [P. Roncin , Phys. Rev. Lett. 89, 043201 (2002)] where trion excitation was suggested as effective energy-loss channel during the neutralization of Ne(+) ions at an LiF surface. [less ▲]

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See detailImpact of the electron-electron correlation on phonon dispersion: Failure of LDA and GGA DFT functionals in graphene and graphite
Lazzeri, Michele; Attaccalite, Claudio; Wirtz, Ludger UL et al

in Physical Review. B (2008), 78(8),

We compute the electron-phonon coupling (EPC) of selected phonon modes in graphene and graphite using various ab initio methods. The inclusion of nonlocal exchange-correlation effects within the GW ... [more ▼]

We compute the electron-phonon coupling (EPC) of selected phonon modes in graphene and graphite using various ab initio methods. The inclusion of nonlocal exchange-correlation effects within the GW approach strongly renormalizes the square EPC of the A(1)' K mode by almost 80% with respect to density-functional theory in the LDA and GGA approximations. Within GW, the phonon slope of the A(1)' K mode is almost two times larger than in GGA and LDA, in agreement with phonon dispersions from inelastic x-ray scattering and Raman spectroscopy. The hybrid B3LYP functional overestimates the EPC at K by about 30%. Within the Hartree-Fock approximation, the graphene structure displays an instability under a distortion following the A(1)' 'phonon at K. [less ▲]

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See detailDiffractive paths for weak localization in quantum billiards
Brezinova, Iva; Stampfer, Christoph; Wirtz, Ludger UL et al

in Physical Review. B (2008), 77(16),

We study the weak-localization effect in quantum transport through a clean ballistic cavity with regular classical dynamics. We address the question which paths account for the suppression of conductance ... [more ▼]

We study the weak-localization effect in quantum transport through a clean ballistic cavity with regular classical dynamics. We address the question which paths account for the suppression of conductance through a system where disorder and chaos are absent. By exploiting both quantum and semiclassical methods, we unambiguously identify paths that are diffractively backscattered into the cavity (when approaching the lead mouths from the cavity interior) to play a key role. Diffractive scattering couples transmitted and reflected paths and is thus essential to reproduce the weak-localization peak in reflection and the corresponding antipeak in transmission. A comparison of semiclassical calculations featuring these diffractive paths yields good agreement with full quantum calculations and experimental data. Our theory provides system-specific predictions for the quantum regime of few open lead modes and can be expected to be relevant also for mixed as well as chaotic systems. [less ▲]

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See detailUltrafast electron-phonon decoupling in graphite
Ishioka, Kunie; Hase, Muneaki; Kitajima, Masahiro et al

in Physical Review. B (2008), 77(12),

We report the ultrafast dynamics of the 47.4 THz coherent phonons of graphite interacting with a photoinduced nonequilibrium electron-hole plasma. Unlike conventional materials, upon photoexcitation the ... [more ▼]

We report the ultrafast dynamics of the 47.4 THz coherent phonons of graphite interacting with a photoinduced nonequilibrium electron-hole plasma. Unlike conventional materials, upon photoexcitation the phonon frequency of graphite upshifts, and within a few picoseconds relaxes to the stationary value. Our first-principles density functional calculations demonstrate that the phonon stiffening stems from the light-induced decoupling of the nonadiabatic electron-phonon interaction by creating a nonequilibrium electron-hole plasma. Time-resolved vibrational spectroscopy provides a window on the ultrafast nonquilibrium electron dynamics. [less ▲]

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See detailTight-binding description of the quasiparticle dispersion of graphite and few-layer graphene
Grueneis, A.; Attaccalite, C.; Wirtz, Ludger UL 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 ▲]

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See detailInfluence of Inelastic Processes on Fast-Atom-Surface Diffraction
Aigner, F.; Simonovic, N.; Solleder, B. et al

in Journal of Physics: Conference Series (2008), 133

Diffraction of fast helium atoms at alkali-halide surfaces under grazing angles of incidence shows intriguing diffraction patterns. The persistence of quantum coherence is remarkably strong, even though ... [more ▼]

Diffraction of fast helium atoms at alkali-halide surfaces under grazing angles of incidence shows intriguing diffraction patterns. The persistence of quantum coherence is remarkably strong, even though high surface temperatures and high (keV) kinetic energies of the incident atoms would strongly suggest the dominance of dissipative and decohering processes. The main source of decoherence is the excitation or absorption of surface vibrations upon impact. The momentum transfer between the surface and the incident helium atom depends on the amplitude of the thermal vibrations of the surface atoms and the energy of the incident particle. We present an ab initio simulation of the quantum diffraction of fast helium beams at a LiF (100) surface in the < 110 > direction, and compare with recent experimental diffraction data. [less ▲]

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See detailComment on "Huge Excitonic Effects in Layered Hexagonal Boron Nitride"
Wirtz, Ludger UL; Marini, Andrea; Gruning, Myrta et al

in Physical Review Letters (2008), 100(18),

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See detailSuppression of Decoherence in Fast-Atom Diffraction at Surfaces
Aigner, F.; Simonovic, N.; Solleder, B. et al

in Physical Review Letters (2008), 101(25),

Scattering of fast neutral atoms with keV kinetic energies at alkali-halide surfaces under grazing angles displays intriguing diffraction patterns. The surprisingly strong persistence of quantum coherence ... [more ▼]

Scattering of fast neutral atoms with keV kinetic energies at alkali-halide surfaces under grazing angles displays intriguing diffraction patterns. The surprisingly strong persistence of quantum coherence despite the impulsive interaction with an environment at solid state density and elevated temperatures raises fundamental questions such as to the suppression of decoherence and of the quantum-to-classical crossover. We present an ab initio simulation of the quantum diffraction of fast helium beams at a LiF (100) surface in the < 110 > direction and compare with recent experimental diffraction data. From the quantitative reconstruction of diffraction images the vertical LiF-surface reconstruction, or buckling, can be determined. [less ▲]

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