Klaus Peter Bohnen

Lattice Dynamics and Electron-Phonon Interaction in (3,3) Carbon Nanotubes

We present a detailed study of the lattice dynamics and electron-phonon coupling for a (3,3) carbon nanotube which belongs to the class of small diameter based nanotubes which have recently been claimed to be superconducting. We treat the electronic and phononic degrees of freedom completely by modern ab initio methods without involving approximations beyond the local density approximation. Using density functional perturbation theory we find a mean-field Peierls transition temperature of approximately 240 K which is an order of magnitude larger than the calculated superconducting transition temperature. Thus in (3,3) tubes the Peierls transition might compete with superconductivity. The Peierls instability is related to the special 2k(F) nesting feature of the Fermi surface. Because of the special topology of the (n,n) tubes we also find a phonon softening at the Gamma point.

Ab initio calculations of multilayer relaxations of stepped Cu surfaces

We present trends in the multilayer relaxations of several vicinals of Cu(100) and Cu( 111) of varying terrace widths and geometries. The electronic structure calculations are based on density-functional theory in the local-density approximation with norm-conserving, nonlocal pseudopotentials in the mixed basis representation. While relaxations continue for several layers, the major effect is concentrated near the step and comer atoms. On all surfaces the step atoms contract inward, in agreement with experimental findings. Additionally, the corner atoms move outward and the atoms in the adjacent chain undergo a large inward relaxation. Correspondingly, the largest contraction (4%) is in the bond length between the step atom and its bulk nearest neighbor (BNN), while that between the corner atom and the BNN is somewhat enlarged. The surface atoms also display changes in registry of up to 1.5%. Our results are in general in good agreement with low-energy electron-diffraction data including the controversial case of Cu(511). Subtle differences are found with results obtained from semiempirical potentials.

Energetics of CO on stepped and kinked Cu surfaces : A comparative theoretical study

Our ab initio calculations of CO adsorption on several low and high miller index surfaces of Cu show that the adsorption energy increases as the coordination of the adsorption site decreases from 11 to 6, in qualitative agreement with experimental observations. On each surface the adsorption energy is also found to decrease with increase in coverage, although the decrement is not uniform. Calculated vibrational properties show an increase in the frequency of the metal-C mode with decrease in coordination whereas no such effect is found in the frequency of the CO stretch mode. Examination of the surface electronic structure shows a strong local effect of CO adsorption on the local density of state of the substrate atoms. We also provide some energetics of CO diffusion on Cu(111) and Cu(211).

Ab initio calculations of adsorbate-induced stress on Ni(100)

We have calculated the surface stress induced on adsorption of

First-principles calculations of the phonon dispersion curves of H on Pt(111)

c(2\ifmmode\times\else\texttimes\fi{}2)

On the bilayer growth mode of Au on Ag(110)

overlayers of O and C using first-principles electronic structure calculations based on density-functional theory within the local-density approximation (LDA) and nonlocal pseudopotentials. The most remarkable result is found in the case of the C overlayer which introduces a large compressive stress, while that on clean Ni(100) and that in the presence of the O overlayer are found to be tensile and smaller in comparison. We find a correlation between the height of the C overlayer and the resulting clock reconstruction of Ni(100) but no specific relationship between surface stress and surface reconstruction. We discuss our results in the context of experimental data, and insights from electronic structure calculations.

Lattice instability and superconductivity in electron doped (3, 3) carbon nanotubes

We have calculated the surface phonon dispersion curves for H on Pt(111), using first-principles, total energy calculations based on a mixed-basis set and norm-conserving pseudopotentials. Linear response theory and the harmonic approximation are invoked. For one monolayer of H in the preferred adsorption site (fcc hollow) vibrational modes polarized parallel and perpendicular to the surface are found, respectively, at 73.5 meV and 142.6 meV, at the Γ point of the surface Brillouin zone. The degeneracy of the parallel mode is lifted at the zone boundaries, yielding energies of 69.6 meV and 86.3 meV at the M point and 79.4 meV and 80.8 meV at the K point. The dispersion curves for H adsorption at the hcp hollow site differ only slightly from the above. In either case, H adsorption has considerable impact on the substrate modes; in particular the surface mode in the gap in the bulk phonon spectrum (around M point) is pushed into the bulk band. For on-top H adsorption, modes polarized parallel and perpendicular to the surface have respective energies of 47.4 meV and 277.2 meV, at the Γ point. The former disperses to 49.1 meV and 59.5 meV at the M point and to 56 meV and 56.7 meV at the K point. The H vibrational mode polarized perpendicular to the surface shows little dispersion, in all three cases considered. Insights are obtained from the hybridization of the H and Pt electronic states.

First-principles calculations of the dispersion of surface phonons on unreconstructed and reconstructed Pt(110)

Abstract The energetics of the growth of Au on the Ag(110) surfave is investigated using first principles total energy calculations. Our results indicate that a recently observed bilayer growth mode is actually energetically unfavorable for this system.

Initial growth mode of Au on Ag(110) studied with first-principles calculations

We investigated the effect of electron doping on the phonon dispersion and electron-phonon coupling of a small diameter (3, 3) carbon nanotube using first principles density functional perturbation theory. Electron doping increases the number of nesting features in the electronic band structure, which is reflected in a wealth of phonon anomalies. We found that the overall electron-phonon coupling is substantially enhanced with respect to the pristine tube, which improves superconductivity. At the same time, the intrinsic Peierls instability remains similar, but the Peierls temperature still remains larger than the superconducting transition temperature.

Model for the c(2×2) structure induced by K on Au(110)

We present result of calculations of the surface phonon dispersion curves for Pt(110) using density functional theory in the local density approximation and norm conserving pseudopotentials in a mixed-basis approach. Linear response theory is invoked and both the unreconstructed, and the missing row (1x2) reconstructed surfaces are considered. We find that the reconstruction is not driven by a phonon instability. Most of the observed phonon modes for the (1x2) structure can be understood in terms of simple folding of the (1x1) Brillouin zone onto that for the (1x2) surface. Largest changes in the phonon frequencies on surface reconstruction occur close to the zone boundary in the (001) direction. Detailed comparison of atomic force constants for the (1x1) and the (1x2) surfaces and their bulk counterparts show that the bulk value is attained after three layers. Our calculations reproduce nicely the Kohn anomaly observed along the (110) direction in the bulk. We do not find a corresponding effect on the surface.