267 research outputs found
Dangling-bond charge qubit on a silicon surface
Two closely spaced dangling bonds positioned on a silicon surface and sharing
an excess electron are revealed to be a strong candidate for a charge qubit.
Based on our study of the coherent dynamics of this qubit, its extremely high
tunneling rate ~ 10^14 1/s greatly exceeds the expected decoherence rates for a
silicon-based system, thereby overcoming a critical obstacle of charge qubit
quantum computing. We investigate possible configurations of dangling bond
qubits for quantum computing devices. A first-order analysis of coherent
dynamics of dangling bonds shows promise in this respect.Comment: 17 pages, 3 EPS figures, 1 tabl
Silicon-based molecular electronics
Molecular electronics on silicon has distinct advantages over its metallic
counterpart. We describe a theoretical formalism for transport through
semiconductor-molecule heterostructures, combining a semi-empirical treatment
of the bulk silicon bandstructure with a first-principles description of the
molecular chemistry and its bonding with silicon. Using this method, we
demonstrate that the presence of a semiconducting band-edge can lead to a novel
molecular resonant tunneling diode (RTD) that shows negative differential
resistance (NDR) when the molecular levels are driven by an STM potential into
the semiconducting band-gap. The peaks appear for positive bias on a p-doped
and negative for an n-doped substrate. Charging in these devices is compromised
by the RTD action, allowing possible identification of several molecular
highest occupied (HOMO) and lowest unoccupied (LUMO) levels. Recent experiments
by Hersam et al. [1] support our theoretical predictions.Comment: Author list is reverse alphabetical. All authors contributed equally.
Email: rakshit/liangg/ ghosha/[email protected]
The role of electronic correlation in the Si(100) reconstruction: a quantum Monte Carlo study
Recent low-temperature scanning tunneling experiments have challenged the
generally accepted picture of buckled silicon dimers as the ground state
reconstruction of the Si(100) surface. Together with the symmetric dimer model
of the surface suggested by quantum chemistry calculations on small clusters,
these findings question our general understanding of electronic correlations at
surfaces and its proper description within density functional theory. We
present quantum Monte Carlo calculations on large cluster models of the
symmetric and buckled surface, and conclude that buckling remains energetically
more favorable even when the present-day best treatment of electronic
correlation is employed.Comment: 5 pages, Revtex, 10 figure
Back-Reaction In Lightcone QED
We consider the back-reaction of quantum electrodynamics upon an electric
field E(x_+) = - A'_-(x_+) which is parallel to x^3 and depends only on the
lightcone coordinate x_+ = (x^0 + x^3)/\sqrt{2}. Novel features are that the
mode functions have simple expressions for arbitrary A_-(x_+), and that one
cannot ignore the usual lightcone ambiguity at zero + momentum. Each mode of
definite canonical momenta k_+ experiences pair creation at the instant when
its kinetic momentum p_+=k_+ - e A_-(x_+) vanishes, at which point operators
from the surface at x_- =-\infty play a crucial role. Our formalism permits a
more explicit and complete derivation of the rate of particle production than
is usually given. We show that the system can be understood as the infinite
boost limit of the analogous problem of an electric field which is homogeneous
on surfaces of constant x^0.Comment: 37 pages, 2 figures, LaTeX 2 epsilo
Adsorption of benzene on Si(100) from first principles
Adsorption of benzene on the Si(100) surface is studied from first
principles. We find that the most stable configuration is a
tetra--bonded structure characterized by one C-C double bond and four
C-Si bonds. A similar structure, obtained by rotating the benzene molecule by
90 degrees, lies slightly higher in energy. However, rather narrow wells on the
potential energy surface characterize these adsorption configurations. A
benzene molecule impinging on the Si surface is most likely to be adsorbed in
one of three different di--bonded, metastable structures, characterized
by two C-Si bonds, and eventually converts into the lowest-energy
configurations. These results are consistent with recent experiments.Comment: 4 pages, RevTex, 2 PostScript gzipped figure
Split-off dimer defects on the Si(001)2x1 surface
Dimer vacancy (DV) defect complexes in the Si(001)2x1 surface were
investigated using high-resolution scanning tunneling microscopy and first
principles calculations. We find that under low bias filled-state tunneling
conditions, isolated 'split-off' dimers in these defect complexes are imaged as
pairs of protrusions while the surrounding Si surface dimers appear as the
usual 'bean-shaped' protrusions. We attribute this to the formation of pi-bonds
between the two atoms of the split-off dimer and second layer atoms, and
present charge density plots to support this assignment. We observe a local
brightness enhancement due to strain for different DV complexes and provide the
first experimental confirmation of an earlier prediction that the 1+2-DV
induces less surface strain than other DV complexes. Finally, we present a
previously unreported triangular shaped split-off dimer defect complex that
exists at SB-type step edges, and propose a structure for this defect involving
a bound Si monomer.Comment: 8 pages, 7 figures, submitted to Phys. Rev.
Density-functional study of hydrogen chemisorption on vicinal Si(001) surfaces
Relaxed atomic geometries and chemisorption energies have been calculated for
the dissociative adsorption of molecular hydrogen on vicinal Si(001) surfaces.
We employ density-functional theory, together with a pseudopotential for Si,
and apply the generalized gradient approximation by Perdew and Wang to the
exchange-correlation functional. We find the double-atomic-height rebonded D_B
step, which is known to be stable on the clean surface, to remain stable on
partially hydrogen-covered surfaces. The H atoms preferentially bind to the Si
atoms at the rebonded step edge, with a chemisorption energy difference with
respect to the terrace sites of >sim 0.1 eV. A surface with rebonded single
atomic height S_A and S_B steps gives very similar results. The interaction
between H-Si-Si-H mono-hydride units is shown to be unimportant for the
calculation of the step-edge hydrogen-occupation. Our results confirm the
interpretation and results of the recent H_2 adsorption experiments on vicinal
Si surfaces by Raschke and Hoefer described in the preceding paper.Comment: 13 pages, 8 figures, submitted to Phys. Rev. B. Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Simulations of denuded-zone formation during growth on surfaces with anisotropic diffusion
We have investigated the formation of denuded zones during epitaxial growth on surfaces exhibiting anisotropic diffusion of adparticles, such as Si(001)-2x1, using Monte Carlo simulations and a continuum model. In both the simulations, which were mainly for low-temperature cases (small critical clusters), and the continuum model, appropriate for high-temperature cases (large critical clusters), it was found that the ratio of denuded-zone widths Wf and Ws in the fast- and slow-diffusion directions scales with the ratio Df/Ds of the diffusion constants in the two directions with a power of 1/2, i.e., Wf/Ws ≈ (Df/Ds)1/2, independent of various conditions including the degree of diffusion anisotropy. This supplies the foundation of a method for extracting the diffusion anisotropy from the denuded zone anisotropy which is experimentally measurable. Further, we find that unequal probabilities of a diffusing particle sticking to different types of step edges [e.g., S A and SB steps on Si(001)] does not affect the relation Wf/Ws ≈ (Df/Ds)1/2 seriously unless the smaller of the two sticking probabilities is less than about 0.1. Finally, we examined the relation between the number of steps and the number of sites visited in anisotropic random walks, finding it is better described by a crossover from one-dimensional to two-dimensional behavior than by scaling behavior with a single exponent. This result has bearing on scaling arguments relating denuded-zone widths to diffusion constants for anisotropic diffusion.open7
Coherent electron-phonon coupling and polaron-like transport in molecular wires
We present a technique to calculate the transport properties through
one-dimensional models of molecular wires. The calculations include inelastic
electron scattering due to electron-lattice interaction. The coupling between
the electron and the lattice is crucial to determine the transport properties
in one-dimensional systems subject to Peierls transition since it drives the
transition itself. The electron-phonon coupling is treated as a quantum
coherent process, in the sense that no random dephasing due to electron-phonon
interactions is introduced in the scattering wave functions. We show that
charge carrier injection, even in the tunneling regime, induces lattice
distortions localized around the tunneling electron. The transport in the
molecular wire is due to polaron-like propagation. We show typical examples of
the lattice distortions induced by charge injection into the wire. In the
tunneling regime, the electron transmission is strongly enhanced in comparison
with the case of elastic scattering through the undistorted molecular wire. We
also show that although lattice fluctuations modify the electron transmission
through the wire, the modifications are qualitatively different from those
obtained by the quantum electron-phonon inelastic scattering technique. Our
results should hold in principle for other one-dimensional atomic-scale wires
subject to Peierls transitions.Comment: 21 pages, 8 figures, accepted for publication in Phys. Rev. B (to
appear march 2001
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