68 research outputs found
STM contrast inversion of the Fe(110) surface
We extend the orbital-dependent electron tunneling model implemented within
the three-dimensional (3D) Wentzel-Kramers-Brillouin (WKB) atom-superposition
approach to simulate spin-polarized scanning tunneling microscopy (SP-STM)
above magnetic surfaces. The tunneling model is based on the electronic
structure data of the magnetic tip and surface obtained from first principles.
Applying our method, we analyze the orbital contributions to the tunneling
current, and study the nature of atomic contrast reversals occurring on
constant-current SP-STM images above the Fe(110) surface. We find an interplay
of orbital-dependent tunneling and spin-polarization effects responsible for
the contrast inversion, and we discuss its dependence on the bias voltage, on
the tip-sample distance, and on the tip orbital composition.Comment: 20 pages manuscript, 5 figure
What is the orientation of the tip in a scanning tunneling microscope?
We introduce a statistical correlation analysis method to obtain information
on the local geometry and orientation of the tip used in scanning tunneling
microscopy (STM) experiments based on large scale simulations. The key quantity
is the relative brightness correlation of constant-current topographs between
experimental and simulated data. This correlation can be analyzed statistically
for a large number of modeled tip orientations and geometries. Assuming a
stable tip during the STM scans and based on the correlation distribution, it
is possible to determine the tip orientations that are most likely present in
an STM experiment, and exclude other orientations. This is especially important
for substrates such as highly oriented pyrolytic graphite (HOPG) since its STM
contrast is strongly tip dependent, which makes interpretation and comparison
of STM images very challenging. We illustrate the applicability of our method
considering the HOPG surface in combination with tungsten tip models of two
different apex geometries and 18144 different orientations. We calculate
constant-current profiles along the direction of the HOPG(0001)
surface in the V bias voltage range, and compare them with
experimental data. We find that a blunt tip model provides better correlation
with the experiment for a wider range of tip orientations and bias voltages
than a sharp tip model. Such a combination of experiments and large scale
simulations opens up the way for obtaining more detailed information on the
structure of the tip apex and more reliable interpretation of STM data in the
view of local tip geometry effects.Comment: Progress in Surface Science, accepted for publication, 25 pages
manuscript, 9 figures, abstract shortene
Orbital dependent electron tunneling within the atom superposition approach: Theory and application to W(110)
We introduce an orbital dependent electron tunneling model and implement it
within the atom superposition approach for simulating scanning tunneling
microscopy (STM) and spectroscopy (STS). Applying our method, we analyze the
convergence and the orbital contributions to the tunneling current and the
corrugation of constant current STM images above the W(110) surface. In
accordance with a previous study [Heinze et al., Phys. Rev. B 58, 16432
(1998)], we find atomic contrast reversal depending on the bias voltage.
Additionally, we analyze this effect depending on the tip-sample distance using
different tip models, and find two qualitatively different behaviors based on
the tip orbital composition. As an explanation, we highlight the role of the
real space shape of the orbitals involved in the tunneling. STM images
calculated by our model agree well with Tersoff-Hamann and Bardeen results. The
computational efficiency of our model is remarkable as the k-point samplings of
the surface and tip Brillouin zones do not affect the computation time, in
contrast to the Bardeen method.Comment: 28 pages manuscript, 7 figures, 1 tabl
Theoretical study of the role of the tip in enhancing the sensitivity of differential conductance tunneling spectroscopy on magnetic surfaces
Based on a simple model for spin-polarized scanning tunneling spectroscopy
(SP-STS) we study how tip magnetization and electronic structure affects the
differential conductance (dI/dV) tunneling spectrum of an Fe(001) surface. We
take into account energy dependence of the vacuum decay of electron states, and
tip electronic structure either using an ideal model or based on ab initio
electronic structure calculation. In the STS approach, topographic and magnetic
contributions to dI/dV can clearly be distinguished and analyzed separately.
Our results suggest that the sensitivity of STS on a magnetic sample can be
tuned and even enhanced by choosing the appropriate magnetic tip and bias
setpoint, and the effect is governed by the effective spin-polarization.Comment: 22 pages manuscript, 4 figures;
http://link.aps.org/doi/10.1103/PhysRevB.83.21441
Simulation of spin-polarized scanning tunneling microscopy on complex magnetic surfaces: Case of a Cr monolayer on Ag(111)
We propose an atom-superposition-based method for simulating spin-polarized
scanning tunneling microscopy (SP-STM) from first principles. Our approach
provides bias dependent STM images in high spatial resolution, with the
capability of using either constant current or constant height modes of STM. In
addition, topographic and magnetic contributions can clearly be distinguished,
which are directly comparable to results of SP-STM experiments in the
differential magnetic mode. Advantages of the proposed method are that it is
computationally cheap, it is easy to parallelize, and it can employ the results
of any ab initio electronic structure code. Its capabilities are illustrated
for the prototype frustrated hexagonal antiferromagnetic system, Cr monolayer
on Ag(111) in a noncollinear magnetic N\'eel state. We show
evidence that the magnetic contrast is sensitive to the tip electronic
structure, and this contrast can be reversed depending on the bias voltage.Comment: 28 pages manuscript, 1 table, 5 figure
Prediction of the bias voltage dependent magnetic contrast in spin-polarized scanning tunneling microscopy
This work is concerned with the theoretical description of the contrast,
i.e., the apparent height difference between two lateral surface positions on
constant current spin-polarized scanning tunneling microscopy (SP-STM) images.
We propose a method to predict the bias voltage dependent magnetic contrast
from single point tunneling current or differential conductance measurements,
without the need of scanning large areas of the surface. Depending on the
number of single point measurements, the bias positions of magnetic contrast
reversals and of the maximally achievable magnetic contrast can be determined.
We validate this proposal by simulating SP-STM images on a complex magnetic
surface employing a recently developed approach based on atomic superposition.
Furthermore, we show evidence that the tip electronic structure and magnetic
orientation have a major effect on the magnetic contrast. Our theoretical
prediction is expected to inspire experimentalists to considerably reduce
measurement efforts for determining the bias dependent magnetic contrast on
magnetic surfaces.Comment: 28 pages manuscript, 1 table, 6 figure
Contrast stability and "stripe" formation in Scanning Tunnelling Microscopy imaging of highly oriented pyrolytic graphite: The role of STM-tip orientations
Highly oriented pyrolytic graphite (HOPG) is an important substrate in many
technological applications and is routinely used as a standard in Scanning
Tunnelling Microscopy (STM) calibration, which makes the accurate
interpretation of the HOPG STM contrast of great fundamental and applicative
importance. We demonstrate by STM simulations based on electronic structure
obtained from first principles that the relative local orientation of the
STM-tip apex with respect to the HOPG substrate has a considerable effect on
the HOPG STM contrast. Importantly for experimental STM analysis of HOPG, the
simulations indicate that local tip-rotations maintaining a major contribution
of the tip-apex state to the STM current affect only the
secondary features of the HOPG STM contrast resulting in "stripe" formation and
leaving the primary contrast unaltered. Conversely, tip-rotations leading to
enhanced contributions from tip-apex electronic states can cause a
triangular-hexagonal change in the primary contrast. We also report a
comparison of two STM simulation models with experiments in terms of
bias-voltage-dependent STM topography brightness correlations, and discuss our
findings for the HOPG(0001) surface in combination with tungsten tip models of
different sharpnesses and terminations.Comment: 20 pages manuscript, 7 Figures, 2 Tables, accepted for publication in
J. Phys. Condens. Matte
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