95 research outputs found
Calculations of the Local Density of States for some Simple Systems
A recently proposed convolution technique for the calculation of local
density of states is described more thouroughly and new results of its
application are presented. For separable systems the exposed method allows to
construct the ldos for a higher dimensionality out of lower dimensional parts.
Some practical and theoretical aspects of this approach are also discussed.Comment: 5 pages, 3 figure
Soliton effects in dangling-bond wires on Si(001)
Dangling bond wires on Si(001) are prototypical one dimensional wires, which
are expected to show polaronic and solitonic effects. We present electronic
structure calculations, using the tight binding model, of solitons in
dangling-bond wires, and demonstrate that these defects are stable in
even-length wires, although approximately 0.1 eV higher in energy than a
perfect wire. We also note that in contrast to conjugated polymer systems,
there are two types of soliton and that the type of soliton has strong effects
on the energetics of the bandgap edges, with formation of intra-gap states
between 0.1 eV and 0.2 eV from the band edges. These intra-gap states are
localised on the atoms comprising the soliton.Comment: 6 pages, 3 figures, 3 tables, submitted to Phys. Rev.
van der Waals interaction in nanotube bundles : consequences on vibrational modes
We have developed a pair-potential approach for the evaluation of van der
Waals interaction between carbon nanotubes in bundles.
Starting from a continuum model, we show that the intertube modes range from
to . Using a non-orthogonal tight-binding approximation
for describing the covalent intra-tube bonding in addition, we confirme a
slight chiral dependance of the breathing mode frequency and we found that this
breathing mode frequency increase by 10 % if the nanotube lie inside a
bundle as compared to the isolated tube.Comment: 5 pages, 2 figure
Dynamical properties of Au from tight-binding molecular-dynamics simulations
We studied the dynamical properties of Au using our previously developed
tight-binding method. Phonon-dispersion and density-of-states curves at T=0 K
were determined by computing the dynamical-matrix using a supercell approach.
In addition, we performed molecular-dynamics simulations at various
temperatures to obtain the temperature dependence of the lattice constant and
of the atomic mean-square-displacement, as well as the phonon density-of-states
and phonon-dispersion curves at finite temperature. We further tested the
transferability of the model to different atomic environments by simulating
liquid gold. Whenever possible we compared these results to experimental
values.Comment: 7 pages, 9 encapsulated Postscript figures, submitted to Physical
Review
Efficient index handling of multidimensional periodic boundary conditions
An efficient method is described to handle mesh indexes in multidimensional
problems like numerical integration of partial differential equations, lattice
model simulations, and determination of atomic neighbor lists. By creating an
extended mesh, beyond the periodic unit cell, the stride in memory between
equivalent pairs of mesh points is independent of their position within the
cell. This allows to contract the mesh indexes of all dimensions into a single
index, avoiding modulo and other implicit index operations.Comment: 2 pages, 0 figure
Tight-binding modelling of the electronic band structure of layered superconducting perovskites
A detailed tight-binding analysis of the electron band structure of the CuO_2
plane of layered cuprates is performed within a sigma-band Hamiltonian
including four orbitals - Cu3d_x^2-y^2, Cu4s, O2p_x, and O2p_y. Both the
experimental and theoretical hints in favor of Fermi level located in a Cu or O
band, respectively, are considered. For these two alternatives analytical
expressions are obtained for the LCAO electron wave functions suitable for the
treatment of electron superexchange. Simple formulae for the Fermi surface and
electron dispersions are derived by applying the Loewdin down-fold procedure to
set up the effective copper and oxygen Hamiltonians. They are used to fit the
experimental ARUPS Fermi surface of Pb_0.42Bi_1.73Sr_1.94Ca_1.3Cu_1.92O_8+x and
both the ARPES and LDA Fermi surface of Nd_2-xCe_xCuO_4-delta. The value of
presenting the hopping amplitudes as surface integrals of ab initio atomic wave
functions is demonstrated as well. The same approach is applied to the RuO_2
plane of the ruthenate Sr_2RuO_4. The LCAO Hamiltonians including the three
in-plane pi-orbitals Ru4d_xy, O_a 2p_y, O_b 2p_x and the four transversal
pi-orbitals Ru4d_zx, Ru4d_yz, O_a 2p_z, O_b 2p_z, are separately considered. It
is shown that the equation for the constant energy curves and the Fermi
contours has the same canonical form as the one for the layered cuprates.Comment: 21 pages, 10 figures, published in J. Phys.: Condens. Matter
(complete and corrected References section
Development of a tight-binding potential for bcc-Zr. Application to the study of vibrational properties
We present a tight-binding potential based on the moment expansion of the
density of states, which includes up to the fifth moment. The potential is
fitted to bcc and hcp Zr and it is applied to the computation of vibrational
properties of bcc-Zr. In particular, we compute the isothermal elastic
constants in the temperature range 1200K < T < 2000K by means of standard Monte
Carlo simulation techniques. The agreement with experimental results is
satisfactory, especially in the case of the stability of the lattice with
respect to the shear associated with C'. However, the temperature decrease of
the Cauchy pressure is not reproduced. The T=0K phonon frequencies of bcc-Zr
are also computed. The potential predicts several instabilities of the bcc
structure, and a crossing of the longitudinal and transverse modes in the (001)
direction. This is in agreement with recent ab initio calculations in Sc, Ti,
Hf, and La.Comment: 14 pages, 6 tables, 4 figures, revtex; the kinetic term of the
isothermal elastic constants has been corrected (Eq. (4.1), Table VI and
Figure 4
Core reconstruction in pseudopotential calculations
A new method is presented for obtaining all-electron results from a
pseudopotential calculation. This is achieved by carrying out a localised
calculation in the region of an atomic nucleus using the embedding potential
method of Inglesfield [J.Phys. C {\bf 14}, 3795 (1981)]. In this method the
core region is \emph{reconstructed}, and none of the simplifying approximations
(such as spherical symmetry of the charge density/potential or frozen core
electrons) that previous solutions to this problem have required are made. The
embedding method requires an accurate real space Green function, and an
analysis of the errors introduced in constructing this from a set of numerical
eigenstates is given. Results are presented for an all-electron reconstruction
of bulk aluminium, for both the charge density and the density of states.Comment: 14 pages, 5 figure
Modeling the series of (n x 2) Si-rich reconstructions of beta-SiC(001): a prospective atomic wire?
We perform ab initio plane wave supercell density functional calculations on
three candidate models of the (3 x 2) reconstruction of the beta-SiC(001)
surface. We find that the two-adlayer asymmetric-dimer model (TAADM) is
unambiguously favored for all reasonable values of Si chemical potential. We
then use structures derived from the TAADM parent to model the silicon lines
that are observed when the (3 x 2) reconstruction is annealed (the (n x 2)
series of reconstructions), using a tight-binding method. We find that as we
increase n, and so separate the lines, a structural transition occurs in which
the top addimer of the line flattens. We also find that associated with the
separation of the lines is a large decrease in the HOMO-LUMO gap, and that the
HOMO state becomes quasi-one-dimensional. These properties are qualititatively
and quantitatively different from the electronic properties of the original (3
x 2) reconstruction.Comment: 22 pages, including 6 EPS figure
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