Search

A study based on density functional theory calculations was performed for the three charge states of interstitial hydrogen in Yttria. The present calculations were carried out by employing the GGA-PBE and the HSE06 exchange-correlation functional. It is observed that the ground state H$^0$ and H$^-$ configurations are similar in behaviour; these prefer to relax in the O vacant site, with limited structural relaxation. For the neutral and the negative charged systems more two different geometrical configurations occur with higher energies, which evidence the existence of metastable sites of H in the Yttria lattice. The H$^+$ equilibrium state is found only when a strong bond between the impurity ion and an anion, O-H bond of $\sim$1 \AA, is formed. The formation energy of the interstitial impurity is studied, for which amphoteric behaviour is found for this defect and for the ground state structures. These results evidence that hydrogen counteracts the prevailing conductivity of the host lattice, thus passivating any existing electrical levels originated from other doping sources. To compare results with non-equilibrium and short time-scale $\mu$SR measurements, the formation energy of the higher energy configurations were also evaluated. For the Bond O geometrical site, H evidences a donor-dopant behaviour, hence enabling the coexistence of donor behaviour with the ground-state amphoteric behaviour. From the density of states results, and for the H$^0$ and H$^-$ charge states, a defect level was observed in the band gap. For the ground state Vacant O configuration and the metastable Interstitial structure the defect level was found to be slightly above the valence band maximum, hence indicating no dopant-donor character for the Yttria material. But, for the Bond O, local-minimum, configuration, the defect level is located above the middle of the band gap, closer the conduction band., Comment: 19 pages (one column), 12 figures This paper has been withdrawn by the author because there are major differences between the two versions that are online (title, authors, abstract, body). It is intended to do a new submission of the paper

The electron energy-loss function of graphite is studied for momentum transfers q beyond the first Brillouin zone. We find that near Bragg reflections the spectra can change drastically for very small variations in q. The effect is investigated by means of first principle calculations in the random phase approximation and confirmed by inelastic x-ray scattering measurements of the dynamic structure factor S(q,\omega). We demonstrate that this effect is governed by crystal local field effects and the stacking of graphite. It is traced back to a strong coupling between excitations at small and large momentum transfers.

We have measured a strictly linear pi-plasmon dispersion along the axis of individualized single wall carbon nanotubes, which is completely different from plasmon dispersions of graphite or bundled single wall carbon nanotubes. Comparative ab initio studies on graphene based systems allow us to reproduce the different dispersions. This suggests that individualized nanotubes provide viable experimental access to collective electronic excitations of graphene, and it validates the use of graphene to understand electronic excitations of carbon nanotubes. In particular, the calculations reveal that local field effects (LFE) cause a mixing of electronic transitions, including the 'Dirac cone', resulting in the observed linear dispersion.

We present a theoretical study of the optical absorption spectrum of small boron-nitride and carbon nanotubes using time-dependent density-functional theory and the random phase approximation. Both for C and BN tubes, the absorption of light polarized perpendicular to the tube-axis is strongly suppressed due to local field effects. Since BN-tubes are wide band-gap insulators, they only absorb in the ultra-violet energy regime, independently of chirality and diameter. In comparison with the spectra of the single C and BN-sheets, the tubes display additional fine-structure which stems from the (quasi-) one-dimensionality of the tubes and sensitively depends on the chirality and tube diameter. This fine structure can provide additional information for the assignment of tube indices in high resolution optical absorption spectroscopy., Comment: 5 pages, 3 figures

We present results for the optical absorption spectra of small-diameter single-wall carbon and boron nitride nanotubes obtained by {\it 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 BN sheets. In particular, we focus on the role of depolarization effects, anisotropies and interactions in the excited states. We show that already the random phase approximation reproduces well the main features of the spectra when crystal local field effects are correctly included, and discuss to which 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., Comment: 13 pages, 9 embedded eps figures. Submitted to Applied Physics A