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See detailAdsorption of Ar on graphite using London dispersion forces corrected Kohn-Sham density functional theory
Tkatchenko, Alexandre UL; von Lilienfeld, O. Anatole

in PHYSICAL REVIEW B (2006), 73(15),

Using Kohn-Sham (KS) density functional theory, the adsorption of Ar on graphite has been computed with various conventional exchange-correlation functionals. While the local density approximation yields ... [more ▼]

Using Kohn-Sham (KS) density functional theory, the adsorption of Ar on graphite has been computed with various conventional exchange-correlation functionals. While the local density approximation yields a reasonable estimate of equilibrium distance and energy, three generalized gradient approximated functionals fail. Extending the KS Hamiltonian by an empirical nonlocal and atom-centered potential enables quantitative predictions. The adsorption on the on-top, hollow, and bridge sites has been investigated, and it is found that the London dispersion corrected calculations prefer the hollow site which is in agreement with other studies. Furthermore, the adsorption effect of several submonolayer coverages and of the graphitic bulk has been studied. [less ▲]

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See detailMechanisms of pattern formation in grazing-incidence ion bombardment of Pt(111)
Hansen, Henri; Redinger, Alex UL; Messlinger, Sebastian et al

in PHYSICAL REVIEW B (2006), 73(23),

Ripple patterns forming on Pt(111) due to 5 keV Ar(+) grazing-incidence ion bombardment were investigated by scanning tunneling microscopy in a broad temperature range from 100 to 720 K and for ion ... [more ▼]

Ripple patterns forming on Pt(111) due to 5 keV Ar(+) grazing-incidence ion bombardment were investigated by scanning tunneling microscopy in a broad temperature range from 100 to 720 K and for ion fluences up to 3x10(20) ions/m(2). A detailed morphological analysis together with molecular dynamics simulations of single ion impacts allow us to develop atomic scale models for the formation of these patterns. The large difference in step edge versus terrace damage is shown to be crucial for ripple formation under grazing incidence. The importance of distinct diffusion processes-step adatom generation at kinks and adatom lattice gas formation-for temperature dependent transitions in the surface morphology is highlighted. Surprisingly, ion bombardment effects like thermal spike induced adatom production and planar subsurface channeling are important for pattern ordering. [less ▲]

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See detailEmergence of diffusion in finite quantum systems
Esposito, Massimiliano UL; Gaspard, P.

in Physical Review B (2005), 71(21),

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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 ▲]

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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 ▲]

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See detailAb initio calculations of the lattice dynamics of boron nitride nanotubes
Wirtz, Ludger UL; Rubio, A.; de la Concha, R. A. et al

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 ▲]

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See detailModular recursive Green's function method for ballistic quantum transport
Rotter, S.; Tang, J. Z.; Wirtz, Ludger UL et al

in Physical Review B (2000), 62(3), 1950-1960

A modification of the standard recursive Green's function method for quantum transport through microstructures is presented which is based on the decomposition into separable substructures. The Green's ... [more ▼]

A modification of the standard recursive Green's function method for quantum transport through microstructures is presented which is based on the decomposition into separable substructures. The Green's functions for these modules are joined by discretized Dyson equations. Nonseparable structures can thereby be calculated with the help of a few recursions with high accuracy. We apply this method to the calculation of ballistic quantum transport through a circular and stadium-shaped quantum dot for high mode numbers to test semi-classical predictions in detail. Among other results we find the breakdown of the semiclassical approximation for long path lengths which is due to the spreading of wave packets in the cavity. [less ▲]

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See detailGauge-invariant theory for semiclassical magnetotransport through ballistic microstructures
Wirtz, Ludger UL; Tang, J. Z.; Burgdorfer, J.

in Physical Review B (1999), 59(4), 2956-2967

Within the semiclassical theory of magnetotransport through ballistic cavities, fluctuations in the transmission amplitude and in the conductance originate from the Aharonov-Bohm phase of directed areas ... [more ▼]

Within the semiclassical theory of magnetotransport through ballistic cavities, fluctuations in the transmission amplitude and in the conductance originate from the Aharonov-Bohm phase of directed areas. We formulate the semiclassical transmission amplitude in gauge-invariant form. The gauge invariant phases can be visualized in terms of areas enclosed by classical paths, which consist of the real path connecting the entrance point to the exit point and a virtual path leading back to the entrance point. We implement this method on different levels of a semiclassical description of magnetotransport with applications to magnetoconductance fluctuations and correlations. The validity of the semiclassical theories is analyzed. [S0163-1829(99)10903-2]. [less ▲]

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See detailGeometry-dependent scattering through ballistic microstructures: Semiclassical theory beyond the stationary-phase approximation
Wirtz, Ludger UL; Tang, J. Z.; Burgdorfer, J.

in Physical Review B (1997), 56(12), 7589-7597

The conductance of a ballistic microstructure shows strong fluctuations as a function of the Fermi wave number. We present a semiclassical-theory that describes these fluctuations in terms of bundles of ... [more ▼]

The conductance of a ballistic microstructure shows strong fluctuations as a function of the Fermi wave number. We present a semiclassical-theory that describes these fluctuations in terms of bundles of short trajectories. These trajectories provide the dominant contribution to electron transport through a weakly open microstructure. For the coupling between the quantum wires and the cavity, contributions beyond the stationary phase approximation are taken into account giving rise to diffraction effects. A comparison with full quantum calculations for st rectangular billiard is made. The peak positions of the power spectrum agree very well between the quantum and semiclassical theories. Numerical evidence is found for the breakdown of the semiclassical approximation for long paths. A simple explanation in terms of the dispersion of the semiclassical wave packet in the interior of the cavity is proposed. [less ▲]

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