23 research outputs found
Versatile scanning tunneling microscopy with 120ps time resolution
We describe a fully ultra-high vacuum compatible scanning tunneling
microscope (STM) optimized for radio-frequency signals. It includes in-situ
exchangeable tips adapted to high frequency cabling and a standard sample
holder, which offer access to the whole range of samples typically investigated
by STM. We demonstrate a time resolution of 120 ps using the nonlinear
I(V)-characteristic of the surface of highly oriented pyrolithic graphite. We
provide atomically resolved images in pulse mode related to a spatially varying
nonlinearity of the local density of states of the sample, thus, demonstrating
the possible spatial resolution of the instrument in pulse mode. Analysis of
the noise reveals that changes in the tunneling junction of 50 pA are
dynamically detectable at 120 ps time resolution.Comment: 4 pages, 4 figure
Graphene quantum dots probed by scanning tunneling spectroscopy and transport spectroscopy after local anodic oxidation
Graphene quantum dots are considered as promising alternatives to quantum
dots in III-V semiconductors, e.g., for the use as spin qubits due to their
consistency made of light atoms including spin-free nuclei which both imply
relatively long spin decoherene times. However, this potential has not been
realized in experiments so far, most likely, due to a missing control of the
edge configurations of the quantum dots. Thus, a more fundamental investigation
of Graphene quantum dots appears to be necessary including a full control of
the wave function properties most favorably during transport spectroscopy
measurements. Here, we review the recent success in mapping wave functions of
graphene quantum dots supported by metals, in particular Ir(111), and show how
the goal of probing such wave functions on insulating supports during transport
spectroscopy might be achieved.Comment: 14 pages, review articl
One-dimensional Si chains embedded in Pt(111)and protected by a hexagonal boron-nitride monolayer
Using scanning tunneling microscopy, we show that Si deposition on Pt(111) at
300K leads to a network of one-dimensional Si chains. On the bare Pt(111)
surface, the chains, embedded into the Pt surface, are orientated along the
-direction. They disappear within a few hours in ultrahigh vacuum due to
the presence of residual gas. Exposing the chains to different gases
deliberately reveals that CO is largely responsible for the disappearance of
the chains. The chains can be stabilized by a monolayer of hexagonal boron
nitride, which is deposited prior to the Si deposition. The resulting Si chains
are rotated by 30{\deg} with respect to the chains on the bare Pt(111) surface
and survive even an exposure to air for 10 minutes.Comment: 8 pages, 4 Figure
Mask aligner for ultrahigh vacuum with capacitive distance control
We present a mask aligner driven by three piezo motors which guides and
aligns a SiN shadow mask under capacitive control towards a sample surface. The
three capacitors for read out are located at the backside of the thin mask such
that the mask can be placed in m distance from the sample surface, while
keeping it parallel to the surface. Samples and masks can be exchanged in-situ
and the mask can additionally be displaced parallel to the surface. We
demonstrate an edge sharpness of the deposited structures below 100 nm, which
is likely limited by the diffusion of the deposited Au on Si(111).Comment: 5 pages, 3 figure
Evaluating the effectiveness of structural changes in the enterprise
Данная статья посвящена описанию шести ключевых шагов к эффективному управлению организационными изменениями. В статье рассматривается вопрос оценки и анализа организационных изменений в организации. Также в статье представлены восемь основных шагов для эффективного процесса управления изменениями.This article describes six key steps to effectively manage organizational change. The article deals with the issue of evaluating and analyzing organizational changes in an organization. The article also presents eight basic steps for an effective change management process
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Strong and Weak 3D Topological Insulators Probed by Surface Science Methods
The contributions of surface science methods to discover and improve 3D topological insulator materials are reviewed herein, illustrated with examples from the authors’ own work. In particular, it is demonstrated that spin-polarized angular-resolved photoelectron spectroscopy is instrumental to evidence the spin-helical surface Dirac cone, to tune its Dirac point energy toward the Fermi level, and to discover novel types of topological insulators such as dual ones or switchable ones in phase change materials. Moreover, procedures are introduced to spatially map potential fluctuations by scanning tunneling spectroscopy and to identify topological edge states in weak topological insulators. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
Tuning the pseudospin polarization of graphene by a pseudo-magnetic field
One of the intriguing characteristics of honeycomb lattices is the appearance
of a pseudo-magnetic field as a result of mechanical deformation. In the case
of graphene, the Landau quantization resulting from this pseudo-magnetic field
has been measured using scanning tunneling microscopy. Here we show that a
signature of the pseudo-magnetic field is a local sublattice symmetry breaking
observable as a redistribution of the local density of states. This can be
interpreted as a polarization of graphene's pseudospin due to a strain induced
pseudo-magnetic field, in analogy to the alignment of a real spin in a magnetic
field. We reveal this sublattice symmetry breaking by tunably straining
graphene using the tip of a scanning tunneling microscope. The tip locally
lifts the graphene membrane from a SiO support, as visible by an increased
slope of the curves. The amount of lifting is consistent with molecular
dynamics calculations, which reveal a deformed graphene area under the tip in
the shape of a Gaussian. The pseudo-magnetic field induced by the deformation
becomes visible as a sublattice symmetry breaking which scales with the lifting
height of the strained deformation and therefore with the pseudo-magnetic field
strength. Its magnitude is quantitatively reproduced by analytic and
tight-binding models, revealing fields of 1000 T. These results might be the
starting point for an effective THz valley filter, as a basic element of
valleytronics.Comment: Revised manuscript: streamlined the abstract and introduction, added
methods to supplement, Nano Letters, 201
Sub-nm wide electron channels protected by topology
Helical locking of spin and momentum and prohibited backscattering are the
key properties of topologically protected states. They are expected to enable
novel types of information processing such as spintronics by providing pure
spin currents, or fault tolerant quantum computation by using the Majorana
fermions at interfaces of topological states with superconductors. So far, the
required helical conduction channels used to realize Majorana fermions are
generated through application of an axial magnetic field to conventional
semiconductor nanowires. Avoiding the magnetic field enhances the possibilities
for circuit design significantly. Here, we show that sub-nanometer wide
electron channels with natural helicity are present at surface step-edges of
the recently discovered topological insulator Bi14Rh3I9. Scanning tunneling
spectroscopy reveals the electron channels to be continuous in both energy and
space within a large band gap of 200 meV, thereby, evidencing its non-trivial
topology. The absence of these channels in the closely related, but
topologically trivial insulator Bi13Pt3I7 corroborates the channels'
topological nature. The backscatter-free electron channels are a direct
consequence of Bi14Rh3I9's structure, a stack of 2D topologically insulating,
graphene-like planes separated by trivial insulators. We demonstrate that the
surface of Bi14Rh3I9 can be engraved using an atomic force microscope, allowing
networks of protected channels to be patterned with nm precision.Comment: 17 pages, 4 figures, and supplementary material, Nature Physics in
pres
Untersuchung magnetischer Nanostrukturen mittels Rastertunnelspektroskopie und Kerr-Magnetometrie am Beispiel von Fe, Co, Co-Fe und Fe-Mn Nanostrukturen
Nach einer kurzen Einführung in die Entwicklung der magnetischen Anwendungen, werden in Kapitel 2 und 3 die physikalischen Grundlagen der Messmethoden, insbesondere die Rastertunnelspektroskopie und Kerr-Magnetometrie, sowie der experimentelle Aufbau erläutert.
Kapitel 4 beschäftigt sich mit den magnetischen Eigenschaften von quasi ein-dimensionalen ferromagnetischen Nanostreifen und Monolagen, die durch Selbstorganisation auf einem Wolfram(110)-Einkristall mit vizinaler und glatter Oberfläche präpariert werden. Hierbei wird die Temperaturabhängigkeit der magnetischen Größen, wie Remanenz, Sättigungsmagnetisierung und Suszeptibilität, sowie die Auswirkung einer Abdeckung des Systems auf die Domänenwandenergie und Anisotropie untersucht. Zusätzlich wird die Kopplung von parallelen Nanostreifen in Abhängigkeit des Streifenabstandes betrachtet.
In Kapitel 5
werden das Wachstum und die Morphologie von Co-Monolagen auf W(110) untersucht. Der Übergang von pseudomorphem zu dicht gepacktem Wachstum in der Monolage wird mit Hilfe der Rastertunnelspektroskopie sichtbar gemacht, ebenso wie unterschiedliche Stapelfolgen in Tripellagen Co-Systemen. Atomar aufgelöste Rastertunnelmikroskopie erlaubt die genauen Atompositionen der Oberfläche zu bestimmen und mit theoretischen Wachstumsmodellen zu vergleichen.
Auf die Untersuchung zwei-dimensionaler binärer Co-Fe und Fe-Mn Legierungen auf W(110) wird in Kapitel 6 eingegangen. Mit einer Präparationstemperatur von T=520 K ist es möglich, atomar geordnete Co-Fe Legierungsmonolagen wachsen zu lassen. Ein direkter Zusammenhang zwischen der Magnetisierung und der lokalen Zustandsdichte in Abhängigkeit der Legierungszusammensetzung wird gezeigt.After a short introduction into the development of the magnetic applications, the physical background of the experiments, especially Kerr-magnetometry and scanning tunneling spectroscopy, will be described in chapter 2 and 3.
Chapter 4 deals with the magnetic properties of quasi one-dimensional ferromagnetic nanostripes and monolayers. All samples are grown on a tungsten(110) single-crystal with a flat and a vicinal surface. The temperature dependence of the remanence, saturation magnetization and susceptibility is investigated as well as the influence of a coverage on the domain wall energy and anisotropy. Additionally, the magnetic coupling of a nanostripe-array depending on the stripe distance will be discussed.
In chapter 5 the growth and morphology of Co/W(110)-monolayers will be investigated. The transition from pseudomorphic to close packed growth in
the monolayer and different stacking sequences in triple layers are visualized by scanning tunneling spectroscopy. The exact atomic positions, determined by atomically resolved images, are compared to theoretical growth models.
The investigation of two-dimensional binary Co-Fe and Fe-Mn alloys on W(110) will be described in chapter 6. It is possible to grow atomically ordered Co-Fe alloy monolayers using an annealing temperature of T=520 K. A correlation between the magnetization and the local density of states depending on the alloy composition will be shown