604 research outputs found
Local Semiconducting Transition in Armchair Carbon Nanotubes: The Effect of Periodic Bi-site Perturbation on Electronic and Transport Properties of Carbon Nanotubes
In carbon nanotubes, the most abundant defects, caused for example by
irradiation or chemisorption treatments, are small perturbing clusters, i.e.
bi-site defects, extending over both A and B sites. The relative positions of
these perturbing clusters play a crucial role in determining the electronic
properties of carbon nanotubes. Using bandstructure and electronic transport
calculations, we find out that in the case of armchair metallic nanotubes a
band gap opens up when the clusters fulfill a certain periodicity condition.
This phenomenon might be used in future nanoelectronic devices in which certain
regions of single metallic nanotubes could be turned to semiconducting ones.
Although in this work we study specifically the effect of hydrogen adatom
clusters, the phenomenon is general for different types of defects. Moreover,
we study the influence of the length and randomness of the defected region on
the electron transport through it.Comment: 5 Pages, 5 Figure
Wigner molecules in polygonal quantum dots: A density functional study
We investigate the properties of many-electron systems in two-dimensional
polygonal (triangle, square, pentagon, hexagon) potential wells by using the
density functional theory. The development of the ground state electronic
structure as a function of the dot size is of particular interest. First we
show that in the case of two electrons, the Wigner molecule formation agrees
with the previous exact diagonalization studies. Then we present in detail how
the spin symmetry breaks in polygonal geometries as the spin density functional
theory is applied. In several cases with more than two electrons, we find a
transition to the crystallized state, yielding coincidence with the number of
density maxima and the electron number. We show that this transition density,
which agrees reasonably well with previous estimations, is rather insensitive
to both the shape of the dot and the electron number.Comment: 8 pages, 11 figure
Electonic transport properties of nitrate-doped carbon nanotube networks
The conductivity of carbon nanotube (CNT) networks can be improved markedly
by doping with nitric acid. In the present work, CNTs and junctions of CNTs
functionalized with NO molecules are investigated to understand the
microscopic mechanism of nitric acid doping. According to our density
functional theory band structure calculations, there is charge transfer from
the CNT to adsorbed molecules indicating p-type doping. The average doping
efficiency of the NO molecules is higher if the NO molecules form
complexes with water molecules. In addition to electron transport along
individual CNTs, we have also studied electron transport between different
types (metallic, semiconducting) of CNTs. Reflecting the differences in the
electronic structures of semiconducting and metallic CNTs, we have found that
besides turning semiconducting CNTs metallic, doping further increases electron
transport most efficiently along semiconducting CNTs as well as through a
junction between them.Comment: 13 pages, 12 figure
Positron Surface States on Clean and Oxidized Al and in Surface Vacancies
This Letter reports on the first discrete-lattice calculation of positron surface states on the surfaces of Al. The authors reproduce the observed values and anisotropy of the binding energies on clean surfaces, and predict the surface-state lifetimes. The temperature-independent lateral diffusion constant is calculated. Monovacancies on surfaces are predicted not to trap positrons. The effect of ordered chemisorbed monolayers of oxygen is investigated: Oxidation makes the surface state unstable with respect to positronium emission.Peer reviewe
Embedded-atom calculations of Auger and x-ray photoemission shifts for metallic elements
Change in self-consistent-field energy density-functional calculations are reported for Auger and core-level binding-energy shifts in sp-bonded metals. The basic model, atom in jellium vacancy, gives good agreement with experiment, especially in the Auger case. The chemical and relaxation contributions to the shifts are discussed, and the extra-atomic response is analyzed in detail, both in position and energy space. The adequacy of the "excited-atom" approach to the energy shifts is discussed.Peer reviewe
Theory of positrons in solids and on solid surfaces
Various experimental methods based on positron annihilation have evolved into important tools for researching the structure and properties of condensed matter. In particular, positron techniques are useful for the investigation of defects in solids and for the investigation of solid surfaces. Experimental methods need a comprehensive theory for a deep, quantitative understanding of the results. In the case of positron annihilation, the relevant theory includes models needed to describe the positron states as well as the different interaction processes in matter. In this review the present status of the theory of positrons in solids and on solid surfaces is given. The review consists of three main parts describing (a) the interaction processes, (b) the theory and methods for calculating positron states, and (c) selected recent results of positron studies of condensed matter.Peer reviewe
Photoabsorption of atoms inside C60
The photoabsorption spectrum of the C60 molecule and those of Xe and Ba atoms inside the C60 molecule are calculated using a jelliumlike model for the confining cage. The dynamic electronic response to an external electric field is obtained through time-dependent density-functional theory. The photoabsorption cross section for C60 shows strong collective resonances corresponding to plasmonlike excitations. The resonant 4d photoemission of the free atom is suppressed by the carbon cage, resulting in a weakly oscillating spectrum.Peer reviewe
Positron and electron energy levels in rare-gas solids
The positron and electron band structures are calculated for the rare-gas solids Ne, Ar, Kr, and Xe on the basis of density-functional theory. The effects due to different approximations for the positron correlation and electron exchange-correlation potential are studied. The positron band structures obtained are compared with the measured band gaps in the 〈111〉 direction and with the measured positron work functions. A semiempirical positron correlation potential is presentePeer reviewe
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