491 research outputs found
Projection of plane-wave calculations into atomic orbitals
The projection of the eigenfunctions obtained in standard plane-wave
first-principle electronic-structure calculations into atomic-orbital basis
sets is proposed as a formal and practical link between the methods based on
plane waves and the ones based on atomic orbitals. Given a candidate atomic
basis, ({\it i}) its quality is evaluated by its projection into the plane-wave
eigenfunctions, ({\it ii}) it is optimized by maximizing that projection, ({\it
iii}) the associated tight-binding Hamiltonian and energy bands are obtained,
and ({\it iv}) population analysis is performed in a natural way. The proposed
method replaces the traditional trial-and-error procedures of finding
appropriate atomic bases and the fitting of bands to obtain tight-binding
Hamiltonians. Test calculations of some zincblende semiconductors are
presented.Comment: RevTex. 4 pages. 3 uuencoded compressed (tared) postscript figs. To
appear in Solid St. Commu
Resistive and rectifying effects of pulling gold atoms at thiol-gold nano-contacts
We investigate, by means of first-principles calculations, structural and
transport properties of junctions made of symmetric dithiolated molecules
placed between Au electrodes. As the electrodes are pulled apart, we find that
it becomes energetically favorable that Au atoms migrate to positions between
the electrode surface and thiol terminations, with junction structures
alternating between symmetric and asymmetric. As a result, the calculated
\emph{IV} curves alternate between rectifying and non-rectifying behaviors as
the electrodes are pulled apart, which is consistent with recent experimental
results
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
Maximally-localized Wannier functions for disordered systems: application to amorphous silicon
We use the maximally-localized Wannier function method to study bonding
properties in amorphous silicon. This study represents, to our knowledge, the
first application of the Wannier-function analysis to a disordered system. Our
results show that, in the presence of disorder, this method is extremely
helpful in providing an unambiguous picture of the bond distribution. In
particular, defect configurations can be studied and characterized with a novel
degree of accuracy that was not available before.Comment: 4 pages, with 3 PostScript figures embedded. Uses RevTex and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#nm_as
Local-fields and disorder effects in free-standing and embedded Si nanocrystallites
The case study of a 32-atoms Si nanocrystallite (NC) embedded in a SiO2
matrix, both crystalline and amorphous, or free-standing with different
conditions of passivation and strain is analyzed through ab-initio approaches.
The Si32/SiO2 heterojunction shows a type I band offset highlighting a
separation between the NC plus the interface and the matrix around. The
consequence of this separation is the possibility to correctly reproduce the
low energy electronic and optical properties of the composed system simply
studying the suspended NC plus interface oxygens with the appropriate strain.
Moreover, through the definition of an optical absorption threshold we found
that, beside the quantum confinement trend, the amorphization introduces an
additional redshift that increases with increasing NC size: i.e. the gap tends
faster to the bulk limit. Finally, the important changes in the calculated
DFT-RPA optical spectra upon inclusion of local fields point towards the need
of a proper treatment of the optical response of the interface region
Magnetoresistance and Magnetic Ordering Fingerprints in Hydrogenated Graphene
Spin-dependent features in the conductivity of graphene, chemically modified
by a random distribution of hydrogen adatoms, are explored theoretically. The
spin effects are taken into account using a mean-field self-consistent Hubbard
model derived from first-principles calculations. A Kubo-Greenwood transport
methodology is used to compute the spin-dependent transport fingerprints of
weakly hydrogenated graphene-based systems with realistic sizes. Conductivity
responses are obtained for paramagnetic, antiferromagnetic, or ferromagnetic
macroscopic states, constructed from the mean-field solutions obtained for
small graphene supercells. Magnetoresistance signals up to are
calculated for hydrogen densities around 0.25%. These theoretical results could
serve as guidance for experimental observation of induced magnetism in
graphene.Comment: 4 pages, 4 figure
Quantum chemical study of the lanthanide bond length contraction on Ln3+-doped Cs2NaYCl6 crystals (Ln = Ce to Lu)
Manganese 3×3 and √3 × √3-R30º structures and structural phase transition on w-GaN(0001̄) studied by scanning tunneling microscopy and first-principles theory
et al.Manganese deposited on the N-polar face of wurtzite gallium nitride [GaN (0001̄)] results in two unique surface reconstructions, depending on the deposition temperature. At lower temperature (less than 105ºC), it is found that a metastable 3×3 structure forms. Mild annealing of this Mn 3×3 structure leads to an irreversible phase transition to a different, much more stable √3×√3-R30º structure which can withstand high-temperature annealing. Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction data are compared with results from first-principles theoretical calculations. Theory finds a lowest-energy model for the 3×3 structure consisting of Mn trimers bonded to the Ga adlayer atoms but not with N atoms. The lowest-energy model for the more stable √3×√3-R30º structure involves Mn atoms substituting for Ga within the Ga adlayer and thus bonding with N atoms. Tersoff-Hamman simulations of the resulting lowest-energy structural models are found to be in very good agreement with the experimental STM images.Research supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317 (STM studies of nanoscale spintronic nitride systems) and by the National Science Foundation under Award No. 0730257 (advancing nanospintronics through
international collaboration). V.F. and M.A.B. would like to acknowledge support from CONICET (PIP0038) and ANPCyT (PICT1857) as well as the Ohio Supercomputing Center for computer time. P.O. was supported by Spanish MICINN (FIS2009-12721-C04-01, FIS2012-37549-C05-02 and CSD2007-00050).Peer reviewe
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
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