100 research outputs found
Open-boundary Ehrenfest molecular dynamics: towards a model of current unduced heating in nanowires
We present a time-dependent method based on the single-particle electron density matrix that allows the electronic and ionic degrees of freedom to be modelled within the Ehrenfest approximation in the presence of open boundaries. We describe a practical implementation using tight binding, and use it to investigate steady-state conduction through a single-atom device and to perform molecular dynamics. We find that in the Ehrenfest approximation an electric current allows both ionic heating and cooling to take place, depending on the bias
Interaction between electronic structure and strain in Bi nanolines on Si(001)
Heteroepitaxial strain can be a controlling factor in the lateral dimensions
of 1-D nanostructures. Bi nanolines on Si(001) have an atomic structure which
involves a large sub-surface reconstruction, resulting in a strong elastic
coupling to the surrounding silicon. We present variable-bias STM and first
principles electronic structure calculations of the Bi nanolines, which
investigates this interaction. We show that the strain associated with the
nanolines affects the atomic and electronic structure of at least two
neighbouring Si dimers, and identify the mechanism behind this. We also present
partial charge densities (projected by energy) for the nanoline with clean and
hydrogenated surroundings and contrast it to the clean Si(001) surface.Comment: 10 pages, 3 figures, submitted to Surface Scienc
A first principles study of sub-monolayer Ge on Si(001)
Experimental observations of heteroepitaxial growth of Ge on Si(001) show a
(2xn) reconstruction for sub-monolayer coverages, with dimer rows crossed by
missing-dimer trenches. We present first-principles density-functional
calculations designed to elucidate the energetics and relaxed geometries
associated with this reconstruction. We also address the problem of how the
formation energies of reconstructions having different stoichiometries should
be compared. The calculations reveal a strong dependence of the formation
energy of the missing-dimer trenches on spacing n, and demonstrate that this
dependence stems almost entirely from elastic relaxation. The results provide a
natural explanation for the experimentally observed spacings in the region of n
\~ 8.Comment: 13 pages, 4 figures, submitted to Surface Scienc
Expansion algorithm for the density matrix
A purification algorithm for expanding the single-particle density matrix in
terms of the Hamiltonian operator is proposed. The scheme works with a
predefined occupation and requires less than half the number of matrix-matrix
multiplications compared to existing methods at low (90%)
occupancy. The expansion can be used with a fixed chemical potential in which
case it is an asymmetric generalization of and a substantial improvement over
grand canonical McWeeny purification. It is shown that the computational
complexity, measured as number of matrix multiplications, essentially is
independent of system size even for metallic materials with a vanishing band
gap.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
Efficient Recursion Method for Inverting Overlap Matrix
A new O(N) algorithm based on a recursion method, in which the computational
effort is proportional to the number of atoms N, is presented for calculating
the inverse of an overlap matrix which is needed in electronic structure
calculations with the the non-orthogonal localized basis set. This efficient
inverting method can be incorporated in several O(N) methods for
diagonalization of a generalized secular equation. By studying convergence
properties of the 1-norm of an error matrix for diamond and fcc Al, this method
is compared to three other O(N) methods (the divide method, Taylor expansion
method, and Hotelling's method) with regard to computational accuracy and
efficiency within the density functional theory. The test calculations show
that the new method is about one-hundred times faster than the divide method in
computational time to achieve the same convergence for both diamond and fcc Al,
while the Taylor expansion method and Hotelling's method suffer from numerical
instabilities in most cases.Comment: 17 pages and 4 figure
Block bond-order potential as a convergent moments-based method
The theory of a novel bond-order potential, which is based on the block
Lanczos algorithm, is presented within an orthogonal tight-binding
representation. The block scheme handles automatically the very different
character of sigma and pi bonds by introducing block elements, which produces
rapid convergence of the energies and forces within insulators, semiconductors,
metals, and molecules. The method gives the first convergent results for
vacancies in semiconductors using a moments-based method with a low number of
moments. Our use of the Lanczos basis simplifies the calculations of the band
energy and forces, which allows the application of the method to the molecular
dynamics simulations of large systems. As an illustration of this convergent
O(N) method we apply the block bond-order potential to the large scale
simulation of the deformation of a carbon nanotube.Comment: revtex, 43 pages, 11 figures, submitted to Phys. Rev.
STM characterization of the Si-P heterodimer
We use scanning tunneling microscopy (STM) and Auger electron spectroscopy to
study the behavior of adsorbed phosphine (PH) on Si(001), as a function
of annealing temperature, paying particular attention to the formation of the
Si-P heterodimer. Dosing the Si(001) surface with 0.002 Langmuirs of
PH results in the adsorption of PH (x=2,3) onto the surface and
some etching of Si to form individual Si ad-dimers. Annealing to 350C
results in the incorporation of P into the surface layer to form Si-P
heterodimers and the formation of short 1-dimensional Si dimer chains and
monohydrides. In filled state STM images, isolated Si-P heterodimers appear as
zig-zag features on the surface due to the static dimer buckling induced by the
heterodimer. In the presence of a moderate coverage of monohydrides this static
buckling is lifted, rending the Si-P heterodimers invisible in filled state
images. However, we find that we can image the heterodimer at all H coverages
using empty state imaging. The ability to identify single P atoms incorporated
into Si(001) will be invaluable in the development of nanoscale electronic
devices based on controlled atomic-scale doping of Si.Comment: 6 pages, 4 figures (only 72dpi
Vibrational properties of amorphous silicon from tight-binding O(N) calculation
We present an O(N) algorithm to study the vibrational properties of amorphous
silicon within the framework of tight-binding approach. The dynamical matrix
elements have been evaluated numerically in the harmonic approximation
exploiting the short-range nature of the density matrix to calculate the
vibrational density of states which is then compared with the same obtained
from a standard O() algorithm. For the purpose of illustration, an
1000-atom model is studied to calculate the localization properties of the
vibrational eigenstates using the participation numbers calculation.Comment: 5 pages including 5 ps figures; added a figure and a few references;
accepted in Phys. Rev.
Model Hessian for accelerating first-principles structure optimizations
We present two methods to accelerate first-principles structural relaxations,
both based on the dynamical matrix obtained from a universal model of springs
for bond stretching and bending. Despite its simplicity, the normal modes of
this model Hessian represent excellent internal coordinates for molecules and
solids irrespective of coordination, capturing not only the long-wavelength
acoustic modes of large systems, but also the short-wavelength low-frequency
modes that appear in complex systems. In the first method, the model Hessian is
used to precondition a conjugate gradients minimization, thereby drastically
reducing the effective spectral width and thus obtaining a substantial
improvement of convergence. The same Hessian is used in the second method as a
starting point of a quasi-Newton algorithm (Broyden's method and modifications
thereof), reducing the number of steps needed to find the correct Hessian.
Results for both methods are presented for geometry optimizations of clusters,
slabs, and biomolecules, with speed-up factors between 2 and 8.Comment: 5 pages, 3 figures submites to Phys. Rev.
Form Factors for B -> pi l nu-bar_l and B -> K* gamma Decays on the Lattice
We present a unified method for analysing form factors in B -> pi l nu-bar_l
and B -> K* gamma decays. The analysis provides consistency checks on the q^2
and 1/M extrapolations necessary to obtain the physical decay rates. For the
first time the q^2 dependence of the form factors is obtained at the B scale.
In the B -> pi l nu-bar_l case, we show that pole fits to f^+ may not be
consistent with the q^2 behaviour of f^0, leading to a possible factor of two
uncertainty in the decay rate and hence in the value of |V_{ub}|^2 deduced from
it. For B -> K* gamma, from the combined analysis of form factors T_1 and T_2,
we find the hadronisation ratio R_{K^*} of the exclusive B -> K* gamma to the
inclusive b -> s gamma rates is of order 35% or 15% for constant and pole-type
behaviour of T_2 respectively.Comment: 13 pages, uuencoded compressed postscript file (including five
figures). Also available from
http://wwwhep.phys.soton.ac.uk/hepwww/papers/shep9509
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