251 research outputs found
Native defects in hybrid C/BN nanostructures
First-principles calculations of substitutional defects and vacancies are
performed for zigzag-edged hybrid C/BN nanosheets and nanotubes which recently
have been proposed to exhibit half-metallic properties. The formation energies
show that defects form preferentially at the interfaces between graphene and BN
domains rather than in the middle of these domains, and that substitutional
defects dominate over vacancies. Chemical control can be used to favor
localization of defects at C- B interfaces (nitrogen-rich environment) or C-N
interfaces (nitrogen-poor environment). Although large defect concentrations
have been considered here (106 cm-1), half-metallic properties can subsist when
defects are localized at the C-B interface and for negatively charged defects
localized at the C- N interface, hence the promising magnetic properties
theoretically predicted for these zigzag-edged nanointerfaces might not be
destroyed by point defects if these are conveniently engineered during
synthesis.Comment: 6 pages, 5 figure
Origin of half-semimetallicity induced at interfaces of C-BN heterostructures
First-principles density functional calculations are performed in C-BN
heterojunctions. It is shown that the magnetism of the edge states in zigzag
shaped graphene strips and polarity effects in BN strips team up to give a spin
asymmetric screening that induces half-semimetallicity at the interface, with a
gap of at least a few tenths of eV for one spin orientation and a tiny gap of
hundredths of eV for the other. The dependence with ribbon widths is discussed,
showing that a range of ribbon widths is required to obtain
half-semimetallicity. These results open new routes for tuning electronic
properties at nanointerfaces and exploring new physical effects similar to
those observed at oxide interfaces, in lower dimensions.Comment: 4 pages, 4 figure
Short range repulsive interatomic interactions in energetic processes in solids
The repulsive interaction between two atoms at short distances is studied in
order to explore the range of validity of standard first-principles simulation
techniques and improve the available short-range potentials for the description
of energetic collision cascades in solids. Pseudopotentials represent the
weakest approximation, given their lack of explicit Pauli repulsion in the
core-core interactions. The energy (distance) scale realistically accessible is
studied by comparison with all-electron reference calculations in some binary
systems. Reference calculations are performed with no approximations related to
either core (frozen core, augmentation spheres) or basis set. This is important
since the validity of such approximations, even in all-electron calculations,
rely on the small core perturbation usual in low-energy studies. The expected
importance of semicore states is quantified. We propose a scheme for improving
the electronic screening given by pseudopotentials for very short distances.
The results of this study are applied to the assessment and improvement of
existing repulsive empirical potentials.Comment: 10 pages, 7 figure
First-principles study of structural, elastic, and bonding properties of pyrochlores
Density Functional Theory calculations have been performed to obtain lattice
parameters, elastic constants, and electronic properties of ideal pyrochlores
with the composition ABO (where A=La,Y and B=Ti,Sn,Hf, Zr). Some
thermal properties are also inferred from the elastic properties. A decrease of
the sound velocity (and thus, of the Debye temperature) with the atomic mass of
the B ion is observed. Static and dynamical atomic charges are obtained to
quantify the degree of covalency/ionicity. A large anomalous contribution to
the dynamical charge is observed for Hf, Zr, and specially for Ti. It is
attributed to the hybridization between occupied states of oxygen and
unoccupied d states of the B cation. The analysis based on Mulliken population
and deformation charge integrated in the Voronoi polyhedra indicates that the
ionicity of these pyrochlores increases in the order Sn--Ti--Hf--Zr. The charge
deformation contour plots support this assignment.Comment: Modified contact details, and acknowledgment
ab inito local vibrational modes of light impurities in silicon
We have developed a formulation of density functional perturbation theory for
the calculation of vibrational frequencies in molecules and solids, which uses
numerical atomic orbitals as a basis set for the electronic states. The
(harmonic) dynamical matrix is extracted directly from the first order change
in the density matrix with respect to infinitesimal atomic displacements from
the equilibrium configuration. We have applied this method to study the
vibrational properties of a number of hydrogen-related complexes and light
impurities in silicon. The diagonalization of the dynamical matrix provides the
vibrational modes and frequencies, including the local vibrational modes (LVMs)
associated with the defects. In addition to tests on simple molecules, results
for interstitial hydrogen, hydrogen dimers, vacancy-hydrogen and
self-interstitial-hydrogen complexes, the boron-hydrogen pair, substitutional
C, and several O-related defects in c-Si are presented. The average error
relative to experiment for the aprox.60 predicted LVMs is about 2% with most
highly harmonic modes being extremely close and the more anharmonic ones within
5-6% of the measured values.Comment: 18 pages, 1 figur
Band selection and disentanglement using maximally-localized Wannier functions: the cases of Co impurities in bulk copper and the Cu (111) surface
We have adapted the maximally-localized Wannier function approach of [I.
Souza, N. Marzari and D. Vanderbilt, Phys. Rev. B 65, 035109 (2002)] to the
density functional theory based Siesta method [J. M. Soler et al., J. Phys.:
Cond. Mat. 14, 2745 (2002)] and applied it to the study of Co substitutional
impurities in bulk copper as well as to the Cu (111) surface. In the Co
impurity case, we have reduced the problem to the Co d-electrons and the Cu
sp-band, permitting us to obtain an Anderson-like Hamiltonian from well defined
density functional parameters in a fully orthonormal basis set. In order to
test the quality of the Wannier approach to surfaces, we have studied the
electronic structure of the Cu (111) surface by again transforming the density
functional problem into the Wannier representation. An excellent description of
the Shockley surface state is attained, permitting us to be confident in the
application of this method to future studies of magnetic adsorbates in the
presence of an extended surface state
Energetics of intrinsic point defects in ZrSiO
Using first principles calculations we have studied the formation energies,
electron and hole affinities, and electronic levels of intrinsic point defects
in zircon. The atomic structures of charged interstitials, vacancies, Frenkel
pairs and anti-site defects are obtained. The limit of high concentration of
point defects, relevant for the use of this material in nuclear waste
immobilization, was studied with a variable lattice relaxation that can
simulate the swelling induced by radiation damage. The limit of low
concentration of defects is simulated with larger cells and fixed lattice
parameters. Using known band offset values at the interface of zircon with
silicon, we analyze the foreseeable effect of the defects on the electronic
properties of zircon used as gate in metal-oxide-semiconductor devices.Comment: preprint 16 pages, 4 figures, and 5 table
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