Raman spectra of BN nanotubes: Ab initio and bond-polarizability model calculations

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 ▲]

Pseudopath semiclassical approximation to transport through open quantum billiards: Dyson equation for diffractive scattering

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 ▲]

Multi-electron dynamics for neutralization of highly charged ions near surfaces

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 ▲]

The phonon dispersion of graphite revisited

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 ▲]

Liouville master equation for multielectron dynamics: Neutralization of highly charged ions near a LiF surface

in Physical Review. A (2003), 67(1),

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. The input in terms of elementary one- and two-electron transfer rates is determined from classical trajectory Monte Carlo calculations as well as quantum-mechanical Auger calculations. 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. For grazing incidence, our simulation delineates the pathways to complete neutralization. In accordance with recent experimental observations, most ions are reflected as neutral or even as singly charged negative particles, irrespective of the charge state of the incoming ions. [less ▲]

Semiclassical theory for transmission through open billiards: Convergence towards quantum transport

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 ▲]

Band structure of boron doped carbon nanotubes

in AIP Conference Proceedings (2003), 685

We present ab initio and self-consistent tight-binding calculations on the band structure of single wall semiconducting carbon nanotubes with high degrees (up to 25 %) of boron substitution. Besides a ... [more ▼]

We present ab initio and self-consistent tight-binding calculations on the band structure of single wall semiconducting carbon nanotubes with high degrees (up to 25 %) of boron substitution. Besides a lowering of the Fermi energy into the valence band, a regular, periodic distribution of the p-dopants leads to the formation of a dispersive "acceptor"-like band in the band gap of the undoped tube. This comes from the superposition of acceptor levels at the boron atoms with the delocalized carbon pi-orbitals. Irregular (random) boron-doping leads to a high concentration of hybrids of acceptor and unoccupied carbon states above the Fermi edge. [less ▲]

Vibrational properties of boron nitride nanotubes: Effects of finite length and bundling

in IEEE Transactions on Nanotechnology (2003), 2

We present ab initio calculations of phonons in single-wall. boron nitride nanotubes. Raman and infrared active modes of isolated and infinitely long tubes are evaluated according to the non-symmorphic ... [more ▼]

We present ab initio calculations of phonons in single-wall. boron nitride nanotubes. Raman and infrared active modes of isolated and infinitely long tubes are evaluated according to the non-symmorphic rod groups of BN nanotubes. For tubes of finite length, the selection rules are less restrictive and give rise to additional modes which may be observed in Raman and IR spectroscopy with an intensity depending on the tube length. Bundling of tubes is shown to have little effect on the phonon frequencies. However, arranging tubes in a large periodic array (larger than the wave-length of incoming light) gives rise to a strong frequency shift (LO-TO splitting) of some modes due to the establishing of a macroscopic electric field. Modes of A(1) symmetry experience a shift for laser light along the tube axis and E-1 modes are split for light incidence in the perpendicular direction. [less ▲]

Ab initio calculations of the lattice dynamics of boron nitride nanotubes

in Physical Review. B (2003), 68(4),

We present an extensive first-principles study of the phonons in boron nitride nanotubes using density functional perturbation theory in the local density approximation. Based on the nonsymmorphic rod ... [more ▼]

We present an extensive first-principles study of the phonons in boron nitride nanotubes using density functional perturbation theory in the local density approximation. Based on the nonsymmorphic rod-group symmetry of the tubes, the Raman- and infrared-active modes at the Gamma point of the one-dimensional Brillouin zone are evaluated. For zigzag and chiral nanotubes, the set of infrared-active modes is a subset of the Raman- active modes. In particular, the radial breathing mode is not only Raman but also infrared active. However, for armchair tubes, the sets of infrared-and Raman- active modes are disjoint. This may serve to spectroscopically distinguish between macroscopic samples of zigzag-chiral and armchair nanotubes. We present the frequencies of the active modes of zigzag, chiral, and armchair tubes as a function of the tube diameter and compare the results with the frequencies obtained by the zone-folding method, i.e., the rolling of a single hexagonal BN sheet into a tube. Except for the high-frequency tangential modes, the zone-folding results are in very good agreement with the ab initio calculations. The radial breathing mode frequency can be derived by folding a sheet of finite width. Finally, we show that the effects of bundling on the phonon frequencies are small. This demonstrates that the obtained results for isolated BN tubes may serve as a basis for an accurate assignment of phonon modes in spectroscopic measurements. [less ▲]

Scattering of highly charged ions at microcapillaries

Scientific Conference (2001)

Transmission of highly charged ions through microcapillaries is studied theoretically by a classical trajectory simulation based on the classical-over-the-barrier model. The interaction of highly charged ... [more ▼]

Transmission of highly charged ions through microcapillaries is studied theoretically by a classical trajectory simulation based on the classical-over-the-barrier model. The interaction of highly charged ions with the internal surface of the capillary is treated within the framework of dielectric response theory. We find the resulting charge state distribution of transmitted projectiles in good agreement with measurements. We analyze the angular distributions and the distribution of the mean occupation numbers of n shells of highly charged ions. We also present theoretical results of the energy loss of the highly charged ions undergoing distant collisions at grazing incidence angles with the internal surface of the microcapillary target. [less ▲]