Analysis of band-gap formation in squashed arm-chair CNT

The electronic properties of squashed arm-chair carbon nanotubes are modeled using constraint free density functional tight binding molecular dynamics simulations. Independent from CNT diameter, squashing path can be divided into {\it three} regimes. In the first regime, the nanotube deforms with negligible force. In the second one, there is significantly more resistance to squashing with the force being $\sim 40-100$ nN/per CNT unit cell. In the last regime, the CNT looses its hexagonal structure resulting in force drop-off followed by substantial force enhancement upon squashing. We compute the change in band-gap as a function of squashing and our main results are: (i) A band-gap initially opens due to interaction between atoms at the top and bottom sides of CNT. The $\pi-$orbital approximation is successful in modeling the band-gap opening at this stage. (ii) In the second regime of squashing, large $\pi-\sigma$ interaction at the edges becomes important, which can lead to band-gap oscillation. (iii) Contrary to a common perception, nanotubes with broken mirror symmetry can have {\it zero} band-gap. (iv) All armchair nanotubes become metallic in the third regime of squashing. Finally, we discuss both differences and similarities obtained from the tight binding and density functional approaches.Comment: 16 pages and 6 figures, To appear in PR

Resonant electron heating and molecular phonon cooling in single C60_{60} junctions

We study heating and heat dissipation of a single \c60 molecule in the junction of a scanning tunneling microscope (STM) by measuring the electron current required to thermally decompose the fullerene cage. The power for decomposition varies with electron energy and reflects the molecular resonance structure. When the STM tip contacts the fullerene the molecule can sustain much larger currents. Transport simulations explain these effects by molecular heating due to resonant electron-phonon coupling and molecular cooling by vibrational decay into the tip upon contact formation.Comment: Accepted in Phys. Rev. Let

Structural properties of amorphous hydrogenated carbon. IV. A molecular-dynamics investigation and comparison to experiments

Hydrogenated amorphous carbon structures, a-C:H, with densities of 1.8 and 2.0 g/cm3, have been generated by semiempirical density-functions (DF) molecular-dynamics (MD) rapid cooling of a liquid phase of 128 carbon and 64 hydrogen atoms within periodically arranged cubic supercells. The electronic bonding properties of the model structures are analyzed within a local-orbital description. The structural properties are compared to relevant statistical and diffraction data obtained by neutron scattering and NMR in order to achieve a fundamental understanding of structure-related properties on the molecular level of chemical bonding

Theoretical investigation of carbon defects and diffusion in α-quartz

The geometries, formation energies, and diffusion barriers of carbon point defects in silica (α-quartz) have been calculated using a charge-self-consistent density-functional based nonorthogonal tight-binding method. It is found that bonded interstitial carbon configurations have significantly lower formation energies (on the order of 5 eV) than substitutionals. The activation energy of atomic C diffusion via trapping and detrapping in interstitial positions is about 2.7 eV. Extraction of a CO molecule requires an activation energy <3.1 eV but the CO molecule can diffuse with an activation energy <0.4 eV. Retrapping in oxygen vacancies is hindered—unlike for O2—by a barrier of about 2 eV

Theory of Threading Edge and Screw Dislocations in GaN

The atomic structures, electrical properties, and line energies for threading screw and threading edge dislocations of wurtzite GaN are calculated within the local-density approximation. Both dislocations are electrically inactive with a band gap free from deep levels. These results are understood to arise from relaxed core structures which are similar to (1010) surfaces

Structure of the silicon vacancy in 6H-SiC after annealing identified as the carbon vacancy–carbon antisite pair

We investigated radiation-induced defects in neutron-irradiated and subsequently annealed 6H-silicon carbide (SiC) with electron paramagnetic resonance (EPR), the magnetic circular dichroism of the absorption (MCDA), and MCDA-detected EPR (MCDA-EPR). In samples annealed beyond the annealing temperature of the isolated silicon vacancy we observed photoinduced EPR spectra of spin S=1 centers that occur in orientations expected for nearest neighbor pair defects. EPR spectra of the defect on the three inequivalent lattice sites were resolved and attributed to optical transitions between photon energies of 999 and 1075 meV by MCDA-EPR. The resolved hyperfine structure indicates the presence of one single carbon nucleus and several silicon ligand nuclei. These experimental findings are interpreted with help of total energy and spin density data obtained from the standard local-spin density approximation of the density-functional theory, using relaxed defect geometries obtained from the self-consistent charge density-functional theory based tight binding scheme. We have checked several defect models of which only the photoexcited spin triplet state of the carbon antisite–carbon vacancy pair (CSi-VC) in the doubly positive charge state can explain all experimental findings. We propose that the (CSi-VC) defect is formed from the isolated silicon vacancy as an annealing product by the movement of a carbon neighbor into the vacancy

Many-body current formula and current conservation for non-equilibrium fully interacting nanojunctions

We consider the electron transport properties through fully interacting nanoscale junctions beyond the linear-response regime. We calculate the current flowing through an interacting region connected to two interacting leads, with interaction crossing at the left and right contacts, by using a non-equilibrium Green's functions (NEGF) technique. The total current at one interface (the left one for example) is made of several terms which can be regrouped into two sets. The first set corresponds to a very generalised Landauer-like current formula with physical quantities defined only in the interacting central region and with renormalised lead self-energies. The second set characterises inelastic scattering events occurring in the left lead. We show how this term can be negligible or even vanish due to the pseudo-equilibrium statistical properties of the lead in the thermodynamic limit. The expressions for the different Green's functions needed for practical calculations of the current are also provided. We determine the constraints imposed by the physical condition of current conservation. The corresponding equation imposed on the different self-energy quantities arising from the current conservation is derived. We discuss in detail its physical interpretation and its relation with previously derived expressions. Finally several important key features are discussed in relation to the implementation of our formalism for calculations of quantum transport in realistic systems

Vibrational signatures for low-energy intermediate-sized Si clusters

We report low-energy locally stable structures for the clusters Si20 and Si21. The structures were obtained by performing geometry optimizations within the local density approximation. Our calculated binding energies for these clusters are larger than any previously reported for this size regime. To aid in the experimental identification of the structures, we have computed the full vibrational spectra of the clusters, along with the Raman and IR activities of the various modes using a recently developed first-principles technique. These represent, to our knowledge, the first calculations of Raman and IR spectra for Si clusters of this size

Band structure and optical properties of germanium sheet polymers

The band structure of H-terminated Ge sheet polymers is calculated using density-functional theory in the local density approximation and compared to the optical properties of epitaxial polygermyne layers as determined from reflection, photoluminescence, and photoluminescence excitation measurements. A direct band gap of 1.7 eV is predicted and a near resonant excitation of the photoluminescence is observed experimentally close to this energy

(Meta-)stable reconstructions of the diamond(111) surface: interplay between diamond- and graphite-like bonding

Off-lattice Grand Canonical Monte Carlo simulations of the clean diamond (111) surface, based on the effective many-body Brenner potential, yield the $(2\times1)$ Pandey reconstruction in agreement with \emph{ab-initio} calculations and predict the existence of new meta-stable states, very near in energy, with all surface atoms in three-fold graphite-like bonding. We believe that the long-standing debate on the structural and electronic properties of this surface could be solved by considering this type of carbon-specific configurations.Comment: 4 pages + 4 figures, Phys. Rev. B Rapid Comm., in press (15Apr00). For many additional details (animations, xyz files) see electronic supplement to this paper at http://www.sci.kun.nl/tvs/carbon/meta.htm