405 research outputs found
Theory of the Eigler-swith
We suggest a simple model to describe the reversible field-induced transfer
of a single Xe-atom in a scanning tunneling microscope, --- the Eigler-switch.
The inelasticly tunneling electrons give rise to fluctuating forces on and
damping of the Xe-atom resulting in an effective current dependent temperature.
The rate of transfer is controlled by the well-known Arrhenius law with this
effective temperature. The directionality of atom transfer is discussed, and
the importance of use of non-equlibrium-formalism for the electronic
environment is emphasized. The theory constitutes a formal derivation and
generalization of the so-called Desorption Induced by Multiple Electron
Transitions (DIMET) point of view.Comment: 13 pages (including 2 figures in separate LaTeX-files with
ps-\specials), REVTEX 3.
Surface Screening Charge and Effective Charge
The charge on an atom at a metallic surface in an electric field is defined
as the field-derivative of the force on the atom, and this is consistent with
definitions of effective charge and screening charge. This charge can be found
from the shift in the potential outside the surface when the atoms are moved.
This is used to study forces and screening on surface atoms of Ag(001)
c -- Xe as a function of external field. It is found that at low
positive (outward) fields, the Xe with a negative effective charge of -0.093
is pushed into the surface. At a field of 2.3 V \AA the charge
changes sign, and for fields greater than 4.1 V \AA the Xe experiences
an outward force. Field desorption and the Eigler switch are discussed in terms
of these results.Comment: 4 pages, 1 figure, RevTex (accepted by PRL
Fermi level alignment in molecular nanojunctions and its relation to charge transfer
The alignment of the Fermi level of a metal electrode within the gap of the
hi ghest occupied (HOMO) and lowest unoccupied orbital (LUMO) of a molecule is
a key quantity in molecular electronics, which can vary the electron
transparency of a single molecule junction by orders of magnitude. We present a
quantitative analysis of the relation between this level alignment (which can
be estimated from charging free molecules) and charge transfer for bipyridine
and biphenyl dithiolate (BPDT) molecules attached to gold leads based on
density functional theory calculations. For both systems the charge
distribution is defined by a balance between Pauli repulsion with subsequent
electrostatic screening and the filling of the LUMO, where bipyridine loses
electrons to the leads and BPDT gains electrons. As a direct consequence the
Fermi level of the metal is found close to the LUMO for bipyridine and close to
the HOMO for BPDT
Site determination and thermally assisted tunneling in homogenous nucleation
A combined low-temperature scanning tunneling microscopy and density
functional theory study on the binding and diffusion of copper monomers,
dimers, and trimers adsorbed on Cu(111) is presented. Whereas atoms in trimers
are found in fcc sites only, monomers as well as atoms in dimers can occupy the
stable fcc as well as the metastable hcp site. In fact the dimer fcc-hcp
configuration was found to be only 1.3 meV less favorable with respect to the
fcc-fcc configuration. This enables a confined intra-cell dimer motion, which
at temperatures below 5 K is dominated by thermally assisted tunneling.Comment: 4 pages, 4 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
Quantum Breaking of Elastic String
Breaking of an atomic chain under stress is a collective many-particle
tunneling phenomenon. We study classical dynamics in imaginary time by using
conformal mapping technique, and derive an analytic formula for the probability
of breaking. The result covers a broad temperature interval and interpolates
between two regimes: tunneling and thermal activation. Also, we consider the
breaking induced by an ultrasonic wave propagating in the chain, and propose to
observe it in an STM experiment.Comment: 8 pages, RevTeX 3.0, Landau Institute preprint 261/643
Atomic Scale Memory at a Silicon Surface
The limits of pushing storage density to the atomic scale are explored with a
memory that stores a bit by the presence or absence of one silicon atom. These
atoms are positioned at lattice sites along self-assembled tracks with a pitch
of 5 atom rows. The writing process involves removal of Si atoms with the tip
of a scanning tunneling microscope. The memory can be reformatted by controlled
deposition of silicon. The constraints on speed and reliability are compared
with data storage in magnetic hard disks and DNA.Comment: 13 pages, 5 figures, accepted by Nanotechnolog
Atomic Tunneling from a STM/AFM tip: Dissipative Quantum Effects from Phonons
We study the effects of phonons on the tunneling of an atom between two
surfaces. In contrast to an atom tunneling in the bulk, the phonons couple very
strongly, and qualitatively change the tunneling behavior. This is the first
example of {\it ohmic} coupling from phonons for a two-state system. We propose
an experiment in which an atom tunnels from the tip of an STM, and show how its
behavior would be similar to the Macroscopic Quantum Coherence behavior
predicted for SQUIDS. The ability to tune and calculate many parameters would
lead to detailed tests of the standard theories. (For a general intro to this
work on the on the World-Wide-Web: http://www.lassp.cornell.edu. Click on
``Entertaining Science Done Here'' and ``Quantum Tunneling of Atoms'')Comment: 12 pages, ReVTex3.0, two figures (postscript). This is a
(substantially) revised version of cond-mat/9406043. More info (+ postscript
text) at : http://www.lassp.cornell.edu/ardlouis/publications.htm
Quantum Coherence Oscillations in Antiferromagnetic Chains
Macroscopic quantum coherence oscillations in mesoscopic antiferromagnets may
appear when the anisotropy potential creates a barrier between the
antiferromagnetic states with opposite orientations of the Neel vector. This
phenomenon is studied for the physical situation of the nuclear spin system of
eight Xe atoms arranged on a magnetic surface along a chain. The oscillation
period is calculated as a function of the chain constant. The environmental
decoherence effects at finite temperature are accounted assuming a dipole
coupling between the spin chain and the fluctuating magnetic field of the
surface. The numerical calculations indicate that the oscillations are damped
by a rate , where is the number of spins and is
the relaxation time of a single spin.Comment: 10 pages, Latex, two postscript figures; submitted to Phys. Rev.
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