References of "Wirtz, Ludger 50003339"
     in
Bookmark and Share    
Full Text
Peer Reviewed
See detailLow energy quasiparticle dispersion of graphite by angle-resolved photoemission spectroscopy
Grueneis, A.; Pichler, T.; Shiozawa, H. 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: 99 (1 UL)
Full Text
Peer Reviewed
See detailSpatially resolved raman spectroscopy of single- and few-layer graphene
Graf, D.; Molitor, F.; Ensslin, K. et al

in Nano Letters (2007), 7(2), 238-242

We present Raman spectroscopy measurements on single- and few-layer graphene flakes. By using a scanning confocal approach, we collect spectral data with spatial resolution, which allows us to directly ... [more ▼]

We present Raman spectroscopy measurements on single- and few-layer graphene flakes. By using a scanning confocal approach, we collect spectral data with spatial resolution, which allows us to directly compare Raman images with scanning force micrographs. Single-layer graphene can be distinguished from double- and few-layer by the width of the D' line: the single peak for single-layer graphene splits into different peaks for the double-layer. These findings are explained using the double-resonant Raman model based on ab initio calculations of the electronic structure and of the phonon dispersion. We investigate the D line intensity and find no defects within the flake. A finite D line response originating from the edges can be attributed either to defects or to the breakdown of translational symmetry. [less ▲]

Detailed reference viewed: 148 (1 UL)
Full Text
Peer Reviewed
See detailRaman mapping of a single-layer to double-layer graphene transition
Graf, D.; Molitor, F.; Ensslin, K. et al

in European Physical Journal. Special Topics (2007), 148

We report on confocal Raman spectroscopy on a few-layer graphene flake. Adjacent single- and double-layer graphene sections allow mapping the transition in vibrational and electronic properties to a ... [more ▼]

We report on confocal Raman spectroscopy on a few-layer graphene flake. Adjacent single- and double-layer graphene sections allow mapping the transition in vibrational and electronic properties to a second stacked graphene sheet and with it a weak interlayer coupling. Most prominently the width of the D' peak doubles upon going from a single to a double layer, which can be explained within the double-resonant Raman model. The intensities of the G and G' lines decrease at the crossover to a single layer. Contrary to the G' line the G peak position shifts to higher wave numbers, however, not uniformly over the entire section: its frequency fluctuates spatially. The Raman map of the D line intensity shows a non-zero contribution at the boundaries of the flake and the individual sections, which can be attributed either to defects and disorder or to the breakdown of translational symmetry, whereas within the flake no D line signal is detected. [less ▲]

Detailed reference viewed: 135 (0 UL)
Full Text
Peer Reviewed
See detailAbsorption of BN nanotubes under the influence of a perpendicular electric field
Attaccalite, Claudio; Wirtz, Ludger UL; Marini, Andrea et al

in PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS (2007), 244(11), 4288-4292

We calculate the optical absorption spectra of prototype (6,6) and (9,9) BN nanotubes in the presence of a perpendicular electric field. This model mimics a gated BN nanotube device. Even though the band ... [more ▼]

We calculate the optical absorption spectra of prototype (6,6) and (9,9) BN nanotubes in the presence of a perpendicular electric field. This model mimics a gated BN nanotube device. Even though the band-gap of the tubes decreases strongly as a function of the electric field strength, the absorption spectrum of the pure tubes remains remarkably constant up to high field-strength. We show that, in contrast, the levels which are responsible for defect-mediated photo-luminescence are shifted by the electric field. We address the use of BN nanotubes for optoelectronic applications. 0 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. [less ▲]

Detailed reference viewed: 294 (0 UL)
Peer Reviewed
See detailRaman spectroscopy on single- and few-layer graphene
Graf, Davy; Molitor, Francoise; Ensslin, Klaus et al

in AIP Conference Proceedings (2007), 893

We report on Raman measurements of single- and few-layer graphene flakes. Raman mapping in combination with scanning force microscopy allows us to locally relate the thickness of the graphite flake with ... [more ▼]

We report on Raman measurements of single- and few-layer graphene flakes. Raman mapping in combination with scanning force microscopy allows us to locally relate the thickness of the graphite flake with the spectral properties. It turns out that the width of the D' line is highly sensitive to the transition from single- to double-layer graphene. The defect-induced D line is found to be most prominent at steps between sections of different height and along the edge of the graphite flake. [less ▲]

Detailed reference viewed: 120 (0 UL)
Full Text
Peer Reviewed
See detailRaman imaging of graphene
Graf, D.; Molitor, F.; Ensslin, K. et al

in Solid State Communications (2007), 143(1-2), 44-46

A Raman spectrum of a solid contains information about its vibrational and electronic properties. Collecting spectral data with spatial resolution and encoding it in a 2D plot generates images with ... [more ▼]

A Raman spectrum of a solid contains information about its vibrational and electronic properties. Collecting spectral data with spatial resolution and encoding it in a 2D plot generates images with information complementary to optical and scanning force imaging. In the case of few-layer graphene the frequency of the G line and especially the width of the D ' line turn out to be sensitive to single layers. The thickness of the few-layer graphene flake is reflected in the intensity of the G line and in the reduced intensity of the dominant peak of the underlying silicon oxide. (c) 2007 Elsevier Ltd. All rights reserved. [less ▲]

Detailed reference viewed: 56 (1 UL)
Full Text
Peer Reviewed
See detailRaman spectroscopy of single-wall boron nitride nanotubes
Arenal, R.; Ferrari, A. C.; Reich, S. et al

in Nano Letters (2006), 6(8), 1812-1816

Single-wall boron nitride nanotubes samples synthesized by laser vaporization of a hexagonal BN target under a nitrogen atmosphere are studied by UV and visible Raman spectroscopy. We show that resonant ... [more ▼]

Single-wall boron nitride nanotubes samples synthesized by laser vaporization of a hexagonal BN target under a nitrogen atmosphere are studied by UV and visible Raman spectroscopy. We show that resonant conditions are necessary for investigating phonon modes of BNNTs. Raman excitation in the UV (229 nm) provides preresonant conditions, allowing the identification of the A(1) tangential mode at 1370 cm(-1). This is 5 cm(-1) higher than the E-2g mode in bulk h-BN. Ab initio calculations show that the lower frequency of bulk h-BN with respect to large diameter nanotubes and the single sheet of h-BN is related to a softening of the sp2 bonds in the bulk due to interlayer interaction. [less ▲]

Detailed reference viewed: 112 (2 UL)
Full Text
Peer Reviewed
See detailCollisions of slow highly charged ions with surfaces
Burgdoerfer, J.; Lemell, C.; Schiessl, K. et al

in Fainstein, P.D.; Lima, M.A.P.; Miraglia, J.E. (Eds.) et al Photonic, Electronic and Atomic Collisions, Proceedings of the XXIV International Conference (2006)

Progress in the study of collisions of multiply charged ions with surfaces is reviewed with the help of a few recent examples. They range from fundamental quasi-one electron processes to highly complex ... [more ▼]

Progress in the study of collisions of multiply charged ions with surfaces is reviewed with the help of a few recent examples. They range from fundamental quasi-one electron processes to highly complex ablation and material modification processes. Open questions and possible future directions will be discussed. [less ▲]

Detailed reference viewed: 139 (1 UL)
Full Text
Peer Reviewed
See detailExcitons in boron nitride nanotubes: Dimensionality effects
Wirtz, Ludger UL; Marini, A.; Rubio, A.

in Physical Review Letters (2006), 96(12), 126104

We show that the optical absorption spectra of boron nitride (BN) nanotubes are dominated by strongly bound excitons. Our first-principles calculations indicate that the binding energy for the first and ... [more ▼]

We show that the optical absorption spectra of boron nitride (BN) nanotubes are dominated by strongly bound excitons. Our first-principles calculations indicate that the binding energy for the first and dominant excitonic peak depends sensitively on the dimensionality of the system, varying from 0.7 eV in bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the hypothetical (2,2) tube. The strongly localized nature of this exciton dictates the fast convergence of its binding energy with increasing tube diameter towards the sheet value. The absolute position of the first excitonic peak is almost independent of the tube radius and system dimensionality. This provides an explanation for the observed "optical gap" constancy for different tubes and bulk hexagonal BN. [less ▲]

Detailed reference viewed: 114 (1 UL)
Full Text
Peer Reviewed
See detailOptical absorption of hexagonal boron nitride and BN nanotubes
Wirtz, Ludger UL; Marini, A.; Rubio, A.

in AIP Conference Proceedings (2005), 786

present calculations for the optical absorption spectra of hexagonal boron nitride (hBN) and BN nanotubes, using many-body perturbation theory. Solution of the Bethe-Salpeter equation for hBN leads to ... [more ▼]

present calculations for the optical absorption spectra of hexagonal boron nitride (hBN) and BN nanotubes, using many-body perturbation theory. Solution of the Bethe-Salpeter equation for hBN leads to optical absorption and loss spectra where the positions and shapes of the peaks are strongly dominated by excitonic effects. The binding energy of the first exciton is about 0.71 eV. Comparison of the calculations with recently measured optical absorption and EELS demonstrates that DFT underestimates the "true" band gap of BN by 2.25 eV. This band gap difference can be partially (hot not completely) reproduced by a calculation of the quasi-particle band-structure on the level of the GW-approximation. We show, how the lower dimensionality of BN nanotubes leads to a much stronger excitonic binding energy and at the same time to a larger quasi-particle gap. This leaves the position of the first absorption peak almost unchanged. However, the difference in the series of excitonic peaks allows the spectroscopic distinction between BN nanotubes and bulk BN. [less ▲]

Detailed reference viewed: 130 (2 UL)
Full Text
Peer Reviewed
See detailRaman spectra of BN nanotubes: Ab initio and bond-polarizability model calculations
Wirtz, Ludger UL; Lazzeri, M.; Mauri, F. et al

in Physical Review. B, Condensed Matter and Materials Physics (2005), 71(24),

We present ab initio calculations of the nonresonant Raman spectra of zigzag and armchair BN nanotubes. In comparison, we implement a generalized bond-polarizability model where the parameters are ... [more ▼]

We present ab initio calculations of the nonresonant Raman spectra of zigzag and armchair BN nanotubes. In comparison, we implement a generalized bond-polarizability model where the parameters are extracted from first-principles calculations of the polarizability tensor of a BN sheet. For light polarization along the tube axis, the agreement between model and ab initio spectra is almost perfect. For perpendicular polarization, depolarization effects have to be included in the model in order to reproduce the ab initio Raman intensities. [less ▲]

Detailed reference viewed: 110 (0 UL)
Full Text
Peer Reviewed
See detailPseudopath semiclassical approximation to transport through open quantum billiards: Dyson equation for diffractive scattering
Stampfer, C.; Rotter, S.; Burgdorfer, J. et al

in PHYSICAL REVIEW E (2005), 72(3),

We present a semiclassical theory for transport through open billiards of arbitrary convex shape that includes diffractively scattered paths at the lead openings. Starting from a Dyson equation for the ... [more ▼]

We present a semiclassical theory for transport through open billiards of arbitrary convex shape that includes diffractively scattered paths at the lead openings. Starting from a Dyson equation for the semiclassical Green's function we develop a diagrammatic expansion that allows a systematic summation over classical and pseudopaths, the latter consisting of classical paths joined by diffractive scatterings ("kinks"). This renders the inclusion of an exponentially proliferating number of pseudopath combinations numerically tractable for both regular and chaotic billiards. For a circular billiard and the Bunimovich stadium the path sum leads to a good agreement with the quantum path length power spectrum up to long path length. Furthermore, we find excellent numerical agreement with experimental studies of quantum scattering in microwave billiards where pseudopaths provide a significant contribution. [less ▲]

Detailed reference viewed: 105 (0 UL)
Peer Reviewed
See detailElectron emission from surfaces induced by HCI and lasers
Lemell, C.; Tong, X. M.; Tokesi, K. et al

in Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms (2005), 235

Electron emission from surfaces due to interaction with charged particles and electromagnetic fields is a topic of research for over 100 years. In the past decade new developments in the production of ... [more ▼]

Electron emission from surfaces due to interaction with charged particles and electromagnetic fields is a topic of research for over 100 years. In the past decade new developments in the production of ultrashort laser pulses (tau < 10 fs at a wavelength of lambda approximate to 750 nm) and of slow highly charged ions (HCI) have rendered these interactions to be remarkably similar. The description of electron emission in HO-surface interactions can therefore be transcribed to model and understand photoemission from surfaces interacting with ultrashort laser pulses. Examples will be presented for two model systems. (c) 2005 Elsevier B.V. All rights reserved. [less ▲]

Detailed reference viewed: 144 (1 UL)
Full Text
Peer Reviewed
See detailMulti-electron dynamics for neutralization of highly charged ions near surfaces
Burgdorfer, J.; Wirtz, Ludger UL; Reinhold, C. O. et al

in Vacuum (2004), 73(1), 3-7

We present a simulation of the neutralization of highly charged ions in front of a lithium fluoride surface including the close-collision regime above the surface. The present approach employs a Monte ... [more ▼]

We present a simulation of the neutralization of highly charged ions in front of a lithium fluoride surface including the close-collision regime above the surface. The present approach employs a Monte-Carlo solution of the Liouville master equation for the joint probability density of the ionic motion and the electronic population of the projectile and the target surface. It includes single as well as double particle-hole (de)excitation processes and incorporates electron correlation effects through the conditional dynamics of population strings. For slow projectiles and normal incidence, the ionic motion depends sensitively on the interplay between image acceleration towards the surface and repulsion by an ensemble of positive hole charges in the surface ("trampoline effect"). For Ne10+ we find that image acceleration is dominant and no collective backscattering high above the surface takes place. (C) 2004 Elsevier Ltd. All rights reserved. [less ▲]

Detailed reference viewed: 89 (0 UL)
Full Text
Peer Reviewed
See detailOptical absorption and electron energy loss spectra of carbon and boron nitride nanotubes: a first-principles approach
Marinopoulos, A. G.; Wirtz, Ludger UL; Marini, A. et al

in Applied Physics A : Materials Science & Processing (2004), 78(8), 1157-1167

We present results for the optical absorption spectra of small-diameter single-walled carbon and boron nitride nanotubes obtained by ab initio calculations in the framework of time-dependent density ... [more ▼]

We present results for the optical absorption spectra of small-diameter single-walled carbon and boron nitride nanotubes obtained by ab initio calculations in the framework of time-dependent density-functional theory. We compare the results with those obtained for the corresponding layered structures, i.e. the graphene and hexagonal boron nitride sheets. In particular, we focus on the role of depolarization effects, anisotropies, and interactions in the excited states. We show that the random phase approximation reproduces well the main features of the spectra when crystal local field effects are correctly included, and discuss to what extent the calculations can be further simplified by extrapolating results obtained for the layered systems to results expected for the tubes. The present results are relevant for the interpretation of data obtained by recent experimental tools for nanotube characterization, such as optical and fluorescence spectroscopies, as well as polarized resonant Raman scattering spectroscopy. We also address electron energy loss spectra in the small-q momentum-transfer limit. In this case, the interlayer and intertube interactions play an enhanced role with respect to optical spectroscopy. [less ▲]

Detailed reference viewed: 88 (0 UL)
Full Text
Peer Reviewed
See detailFormation and electronic properties of BC3 single-wall nanotubes upon boron substitution of carbon nanotubes
Fuentes, G. G.; Borowiak-Palen, E.; Knupfer, M. et al

in Physical Review B (2004), 69(24),

We report a detailed experimental and theoretical study on the electronic and optical properties of highly boron-substituted (up to 15 at.%) single-wall carbon nanotubes. Core-level electron energy-loss ... [more ▼]

We report a detailed experimental and theoretical study on the electronic and optical properties of highly boron-substituted (up to 15 at.%) single-wall carbon nanotubes. Core-level electron energy-loss spectroscopy reveals that the boron incorporates into the lattice structure of the tubes, transferring similar to1/2 hole per boron atom into the carbon derived unoccupied density of states. The charge transfer and the calculated Fermi-energy shift in the doped nanotubes evidence that a simple rigid-band model can be ruled out and that additional effects such as charge localization and doping induced band-structure changes play an important role at this high doping levels. In optical absorption a new peak appears at 0.4 eV which is independent of the doping level. Compared to the results from a series of ab initio calculations our results support the selective doping of semiconducting nanotubes and the formation of BC3 nanotubes instead of a homogeneous random boron substitution. [less ▲]

Detailed reference viewed: 44 (0 UL)
Full Text
Peer Reviewed
See detailThe phonon dispersion of graphite revisited
Wirtz, Ludger UL; Rubio, A.

in Solid State Communications (2004), 131(3-4), 141-152

We review calculations and measurements of the phonon dispersion relation of graphite. First-principles calculations using density-functional theory are generally in good agreement with the experimental ... [more ▼]

We review calculations and measurements of the phonon dispersion relation of graphite. First-principles calculations using density-functional theory are generally in good agreement with the experimental data since the long-range character of the dynamical matrix is properly taken into account. Calculations with a plane-wave basis demonstrate that for the in-plane optical modes, the generalized-gradient approximation (GGA) yields frequencies lower by 2% than the local-density approximation (LDA) and is thus in better agreement with experiment. The long-range character of the dynamical matrix limits the validity of force-constant approaches that take only interaction with few neighboring atoms into account. However, by fitting the force-constants to the ab initio dispersion relation, we show that the popular 4th-nearest-neighbor force-constant approach yields an excellent fit for the low frequency modes and a moderately good fit (with a maximum deviation of 6%) for the high-frequency modes. If, in addition, the non-diagonal force-constant for the second-nearest neighbor interaction is taken into account, all the qualitative features of the high-frequency dispersion can be reproduced and the maximum deviation reduces to 4%. We present the new parameters as a reliable basis for empirical model calculations of phonons in graphitic nanostructures, in particular carbon nanotubes. (C) 2004 Elsevier Ltd. All rights reserved. [less ▲]

Detailed reference viewed: 84 (0 UL)
Full Text
Peer Reviewed
See detailPhonon and plasmon excitation in inelastic electron tunneling spectroscopy of graphite
Vitali, L.; Schneider, M. A.; Kern, K. et al

in Physical Review B (2004), 69(12),

The inelastic electron tunneling spectrum (IETS) of highly oriented pyrolitic graphite has been measured with scanning tunneling spectroscopy (STS) at 6 K. The observed spectral features are in very good ... [more ▼]

The inelastic electron tunneling spectrum (IETS) of highly oriented pyrolitic graphite has been measured with scanning tunneling spectroscopy (STS) at 6 K. The observed spectral features are in very good agreement with the vibrational density of states of graphite calculated from first principles. We discuss the enhancement of certain phonon modes by phonon-assisted tunneling in STS based on the restrictions imposed by the electronic structure of graphite. We also demonstrate the local excitation of surface plasmons in IETS, which are detected at an energy of 40 meV. [less ▲]

Detailed reference viewed: 122 (1 UL)
Full Text
Peer Reviewed
See detailPotential-energy surfaces for charge exchange between singly charged ions and a LiF surface
Wirtz, Ludger UL; Burgdorfer, J.; Dallos, M. et al

in Physical Review. A (2003), 68(3),

We analyze the adiabatic potential-energy surfaces relevant for neutralization of singly charged ions in slow vertical incidence onto a lithium fluoride surface. The surface is represented by a cluster of ... [more ▼]

We analyze the adiabatic potential-energy surfaces relevant for neutralization of singly charged ions in slow vertical incidence onto a lithium fluoride surface. The surface is represented by a cluster of varying size augmented by point charges of alternating sign in order to include the proper Madelung potential of the ionic crystal. Our calculation proceeds on the multiconfiguration self-consistent-field and multireference configuration-interaction levels. Size-consistency corrections based on the Davidson correction and multireference averaged quadratic coupled cluster methods are included as well. We emphasize the importance of a proper treatment of electron correlation signifying the polarization of the surrounding cluster environment in ab initio calculations of charge transfer at surfaces. From the topology of the surfaces, in particular the existence or absence of avoided crossings (or, more generally, conical intersections), qualitative predictions for the neutralization process can be made. The comparative analysis of potential curves for H+, C+, S+, and Ne+ projectiles provides an explanation for the recently observed threshold behavior for potential sputtering. [less ▲]

Detailed reference viewed: 83 (0 UL)
Full Text
Peer Reviewed
See detailSemiclassical theory for transmission through open billiards: Convergence towards quantum transport
Wirtz, Ludger UL; Stampfer, C.; Rotter, S. et al

in PHYSICAL REVIEW E (2003), 67(1),

We present a semiclassical theory for transmission through open quantum billiards which converges towards quantum transport. The transmission amplitude can be expressed as a sum over all classical paths ... [more ▼]

We present a semiclassical theory for transmission through open quantum billiards which converges towards quantum transport. The transmission amplitude can be expressed as a sum over all classical paths and pseudopaths which consist of classical path segments joined by "kinks," i.e., diffractive scattering at lead mouths. For a rectangular billiard we show numerically that the sum over all such paths with a given number of kinks K converges to the quantum transmission amplitude as K-->infinity. Unitarity of the semiclassical theory is restored as K approaches infinity. Moreover, we find excellent agreement with the quantum path-length power spectrum up to very long path length. [less ▲]

Detailed reference viewed: 75 (0 UL)