37,897 research outputs found
Theory of Non-equilibrium Single Electron Dynamics in STM Imaging of Dangling Bonds on a Hydrogenated Silicon Surface
During fabrication and scanning-tunneling-microscope (STM) imaging of
dangling bonds (DBs) on a hydrogenated silicon surface, we consistently
observed halo-like features around isolated DBs for specific imaging
conditions. These surround individual or small groups of DBs, have abnormally
sharp edges, and cannot be explained by conventional STM theory. Here we
investigate the nature of these features by a comprehensive 3-dimensional model
of elastic and inelastic charge transfer in the vicinity of a DB. Our essential
finding is that non-equilibrium current through the localized electronic state
of a DB determines the charging state of the DB. This localized charge distorts
the electronic bands of the silicon sample, which in turn affects the STM
current in that vicinity causing the halo effect. The influence of various
imaging conditions and characteristics of the sample on STM images of DBs is
also investigated.Comment: 33 pages, 9 figure
Imaging Coulomb Islands in a Quantum Hall Interferometer
In the Quantum Hall regime, near integer filling factors, electrons should
only be transmitted through spatially-separated edge states. However, in
mesoscopic systems, electronic transmission turns out to be more complex,
giving rise to a large spectrum of magnetoresistance oscillations. To explain
these observations, recent models put forward that, as edge states come close
to each other, electrons can hop between counterpropagating edge channels, or
tunnel through Coulomb islands. Here, we use scanning gate microscopy to
demonstrate the presence of quantum Hall Coulomb islands, and reveal the
spatial structure of transport inside a quantum Hall interferometer. Electron
islands locations are found by modulating the tunneling between edge states and
confined electron orbits. Tuning the magnetic field, we unveil a continuous
evolution of active electron islands. This allows to decrypt the complexity of
high magnetic field magnetoresistance oscillations, and opens the way to
further local scale manipulations of quantum Hall localized states
Imaging coherent transport in graphene (Part II): Probing weak localization
Graphene has opened new avenues of research in quantum transport, with
potential applications for coherent electronics. Coherent transport depends
sensitively on scattering from microscopic disorder present in graphene
samples: electron waves traveling along different paths interfere, changing the
total conductance. Weak localization is produced by the coherent backscattering
of waves, while universal conductance fluctuations are created by summing over
all paths. In this work, we obtain conductance images of weak localization with
a liquid-He-cooled scanning probe microscope, by using the tip to create a
movable scatterer in a graphene device. This technique allows us to investigate
coherent transport with a probe of size comparable to the electron wavelength.
Images of magnetoconductance \textit{vs.} tip position map the effects of
disorder by moving a single scatterer, revealing how electron interference is
modified by the tip perturbation. The weak localization dip in conductivity at
B=0 is obtained by averaging magnetoconductance traces at different positions
of the tip-created scatterer. The width of the dip yields an
estimate of the electron coherence length at fixed charge density.
This "scanning scatterer" method provides a new way of investigating coherent
transport in graphene by directly perturbing the disorder configuration that
creates these interferometric effects.Comment: 18 pages, 7 figure
Scanning Gate Imaging of quantum point contacts and the origin of the 0.7 Anomaly
The origin of the anomalous transport feature appearing at conductance G
\approx 0.7 x (2e2/h) in quasi-1D ballistic devices - the so-called 0.7 anomaly
- represents a long standing puzzle. Several mechanisms were proposed to
explain it, but a general consensus has not been achieved. Proposed
explanations are based on quantum interference, Kondo effect, Wigner
crystallization, and more. A key open issue is whether point defects that can
occur in these low-dimensional devices are the physical cause behind this
conductance anomaly. Here we adopt a scanning gate microscopy technique to map
individual impurity positions in several quasi-1D constrictions and correlate
these with conductance characteristics. Our data demonstrate that the 0.7
anomaly can be observed irrespective of the presence of localized defects, and
we conclude that the 0.7 anomaly is a fundamental property of low-dimensional
systems
Magnetometry with nitrogen-vacancy defects in diamond
The isolated electronic spin system of the Nitrogen-Vacancy (NV) centre in
diamond offers unique possibilities to be employed as a nanoscale sensor for
detection and imaging of weak magnetic fields. Magnetic imaging with nanometric
resolution and field detection capabilities in the nanotesla range are enabled
by the atomic-size and exceptionally long spin-coherence times of this
naturally occurring defect. The exciting perspectives that ensue from these
characteristics have triggered vivid experimental activities in the emerging
field of "NV magnetometry". It is the purpose of this article to review the
recent progress in high-sensitivity nanoscale NV magnetometry, generate an
overview of the most pertinent results of the last years and highlight
perspectives for future developments. We will present the physical principles
that allow for magnetic field detection with NV centres and discuss first
applications of NV magnetometers that have been demonstrated in the context of
nano magnetism, mesoscopic physics and the life sciences.Comment: Review article, 28 pages, 16 figure
Wigner and Kondo physics in quantum point contacts revealed by scanning gate microscopy
Quantum point contacts exhibit mysterious conductance anomalies in addition
to well known conductance plateaus at multiples of 2e^2/h. These 0.7 and
zero-bias anomalies have been intensively studied, but their microscopic origin
in terms of many-body effects is still highly debated. Here we use the charged
tip of a scanning gate microscope to tune in situ the electrostatic potential
of the point contact. While sweeping the tip distance, we observe repetitive
splittings of the zero-bias anomaly, correlated with simultaneous appearances
of the 0.7 anomaly. We interpret this behaviour in terms of alternating
equilibrium and non-equilibrium Kondo screenings of different spin states
localized in the channel. These alternating Kondo effects point towards the
presence of a Wigner crystal containing several charges with different
parities. Indeed, simulations show that the electron density in the channel is
low enough to reach one-dimensional Wigner crystallization over a size
controlled by the tip position
Silicon Atomic Quantum Dots Enable Beyond-CMOS Electronics
We review our recent efforts in building atom-scale quantum-dot cellular
automata circuits on a silicon surface. Our building block consists of silicon
dangling bond on a H-Si(001) surface, which has been shown to act as a quantum
dot. First the fabrication, experimental imaging, and charging character of the
dangling bond are discussed. We then show how precise assemblies of such dots
can be created to form artificial molecules. Such complex structures can be
used as systems with custom optical properties, circuit elements for
quantum-dot cellular automata, and quantum computing. Considerations on
macro-to-atom connections are discussed.Comment: 28 pages, 19 figure
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