203 research outputs found
Electric field dependent structural and vibrational properties of the Si(100)-H(2 \times 1) surface and its implications for STM induced hydrogen desorption
We report a first principles study of the structure and the vibrational
properties of the Si(100)-H(2 \times 1) surface in an electric field. The
calculated vibrational parameters are used to model the vibrational modes in
the presence of the electric field corresponding to a realistic STM tip-surface
geometry. We find that local one-phonon excitations have short lifetimes (10 ps
at room temperature) due to incoherent lateral diffusion, while diffusion of
local multi-phonon excitations are suppressed due to anharmonic frequency
shifts and have much longer lifetimes (10 ns at room temperature). We calculate
the implications for current induced desorption of H using a recently developed
first principles model of electron inelastic scattering. The calculations show
that inelastic scattering events with energy transfer , where
n>1, play an important role in the desorption process.Comment: 10 pages, RevTeX, epsf files. submitted to surface scienc
Construction of transferable spherically-averaged electron potentials
A new scheme for constructing approximate effective electron potentials
within density-functional theory is proposed. The scheme consists of
calculating the effective potential for a series of reference systems, and then
using these potentials to construct the potential of a general system. To make
contact to the reference system the neutral-sphere radius of each atom is used.
The scheme can simplify calculations with partial wave methods in the
atomic-sphere or muffin-tin approximation, since potential parameters can be
precalculated and then for a general system obtained through simple
interpolation formulas. We have applied the scheme to construct electron
potentials of phonons, surfaces, and different crystal structures of silicon
and aluminum atoms, and found excellent agreement with the self-consistent
effective potential. By using an approximate total electron density obtained
from a superposition of atom-based densities, the energy zero of the
corresponding effective potential can be found and the energy shifts in the
mean potential between inequivalent atoms can therefore be directly estimated.
This approach is shown to work well for surfaces and phonons of silicon.Comment: 8 pages (3 uuencoded Postscript figures appended), LaTeX,
CAMP-090594-
Temperature suppression of STM-induced desorption of hydrogen on Si(100) surfaces
The temperature dependence of hydrogen (H) desorption from Si(100)
H-terminated surfaces by a scanning tunneling microscope (STM) is reported for
negative sample bias. It is found that the STM induced H desorption rate ()
decreases several orders of magnitude when the substrate temperature is
increased from 300 K to 610 K. This is most noticeable at a bias voltage of -7
V where decreases by a factor of ~200 for a temperature change of 80 K,
whilst it only decreases by a factor of ~3 at -5 V upon the same temperature
change. The experimental data can be explained by desorption due to vibrational
heating by inelastic scattering via a hole resonance. This theory predicts a
weak suppression of desorption with increasing temperature due to a decreasing
vibrational lifetime, and a strong bias dependent suppression due to a
temperature dependent lifetime of the hole resonance.Comment: 5 pages, RevTeX, epsf files. Accepted for surface science letter
Effect of the attachment of ferromagnetic contacts on the conductivity and giant magnetoresistance of graphene nanoribbons
Carbon-based nanostructures and graphene, in particular, evoke a lot of
interest as new promising materials for nanoelectronics and spintronics. One of
the most important issue in this context is the impact of external electrodes
on electronic properties of graphene nanoribbons (GNR). The present theoretical
method is based on the tight-binding model and a modified recursive procedure
for Green's functions. The results show that within the ballistic transport
regime, the so called end-contacted geometry (of minimal GNR/electrode
interface area), is usually more advantageous for practical applications than
its side-contacted counterpart (with a larger coverage area), as far as the
electrical conductivity is concerned. As regards the giant magnetoresistance
coefficient, however, the situation is exactly opposite, since spin- splitting
effects are more pronounced in the lower conductive side-contacted setups.Comment: 8 pages, 4 figure
A mixed ultrasoft/normconserved pseudopotential scheme
A variant of the Vanderbilt ultrasoft pseudopotential scheme, where the
normconservation is released for only one or a few angular channels, is
presented. Within this scheme some difficulties of the truly ultrasoft
pseudopotentials are overcome without sacrificing the pseudopotential softness.
i) Ghost states are easily avoided without including semicore shells. ii) The
ultrasoft pseudo-charge-augmentation functions can be made more soft. iii) The
number of nonlocal operators is reduced. The scheme will be most useful for
transition metals, and the feasibility and accuracy of the scheme is
demonstrated for the 4d transition metal rhodium.Comment: 4 pages, 2 figure
First principles theory of inelastic currents in a scanning tunneling microscope
A first principles theory of inelastic tunneling between a model probe tip
and an atom adsorbed on a surface is presented, extending the elastic tunneling
theory of Tersoff and Hamann. The inelastic current is proportional to the
change in the local density of states at the center of the tip due to the
addition of the adsorbate. We use the theory to investigate the vibrational
heating of an adsorbate below an STM tip. We calculate the desorption rate of H
from Si(100)-H(21) as function of the sample bias and tunnel current,
and find excellent agreement with recent experimental data.Comment: 5 pages, RevTeX, epsf file
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