2,008 research outputs found
Back action of graphene charge detectors on graphene and carbon nanotube quantum dots
We report on devices based on graphene charge detectors (CDs) capacitively
coupled to graphene and carbon nanotube quantum dots (QDs). We focus on back
action effects of the CD on the probed QD. A strong influence of the bias
voltage applied to the CD on the current through the QD is observed. Depending
on the charge state of the QD the current through the QD can either strongly
increase or completely reverse as a response to the applied voltage on the CD.
To describe the observed behavior we employ two simple models based on single
electron transport in QDs with asymmetrically broadened energy distributions of
the source and the drain leads. The models successfully explain the back action
effects. The extracted distribution broadening shows a linear dependency on the
bias voltage applied to the CD. We discuss possible mechanisms mediating the
energy transfer between the CD and QD and give an explanation for the origin of
the observed asymmetry.Comment: 6 pages, 4 figure
Uniformity of the pseudomagnetic field in strained graphene
We present a study on the uniformity of the pseudomagnetic field in graphene
as a function of the relative orientation between the graphene lattice and
straining directions. For this, we strained a regular micron-sized graphene
hexagon by deforming it symmetrically by displacing three of its edges. By
simulations, we found that the pseudomagnetic field is strongest if the strain
is applied perpendicular to the armchair direction of graphene. For a hexagon
with a side length of 1 m, the pseudomagnetic field has a maximum of
1.2 T for an applied strain of 3.5% and it is uniform (variance %) within
a circle with a diameter of nm. This diameter is on the order of the
typical diameter of the laser spot in a state-of-the-art confocal Raman
spectroscopy setup, which suggests that observing the pseudomagnetic field in
measurements of shifted magneto-phonon resonance is feasible.Comment: 7 pages, 5 figure
Disorder induced Coulomb gaps in graphene constrictions with different aspect ratios
We present electron transport measurements on lithographically defined and
etched graphene nanoconstrictions with different aspect ratios including
different lengths (L) and widths (W). A roughly length-independent disorder
induced effective energy gap can be observed around the charge neutrality
point. This energy gap scales inversely with the width even in regimes where
the length of the constriction is smaller than its width (L<W). In very short
constrictions, we observe both resonances due to localized states or charged
islands and an elevated overall conductance level (0.1-1e2/h), which is
strongly length-dependent in the gap region. This makes very short graphene
constrictions interesting for highly transparent graphene tunneling barriers.Comment: 4 pages, 4 figure
Raman spectroscopy on mechanically exfoliated pristine graphene ribbons
We present Raman spectroscopy measurements of non-etched graphene
nanoribbons, with widths ranging from 15 to 160 nm, where the D-line intensity
is strongly dependent on the polarization direction of the incident light. The
extracted edge disorder correlation length is approximately one order of
magnitude larger than on previously reported graphene ribbons fabricated by
reactive ion etching techniques. This suggests a more regular crystallographic
orientation of the non-etched graphene ribbons here presented. We further
report on the ribbons width dependence of the line-width and frequency of the
long-wavelength optical phonon mode (G-line) and the 2D-line of the studied
graphene ribbons
Spin and charge transport in graphene-based spin transport devices with Co/MgO spin injection and spin detection electrodes
In this review we discuss spin and charge transport properties in
graphene-based single-layer and few-layer spin-valve devices. We give an
overview of challenges and recent advances in the field of device fabrication
and discuss two of our fabrication methods in more detail which result in
distinctly different device performances. In the first class of devices, Co/MgO
electrodes are directly deposited onto graphene which results in rough
MgO-to-Co interfaces and favor the formation of conducting pinholes throughout
the MgO layer. We show that the contact resistance area product (RA) is a
benchmark for spin transport properties as it scales with the measured spin
lifetime in these devices indicating that contact-induced spin dephasing is the
bottleneck for spin transport even in devices with large RA values. In a
second class of devices, Co/MgO electrodes are first patterned onto a silicon
substrate. Subsequently, a graphene-hBN heterostructure is directly transferred
onto these prepatterned electrodes which provides improved interface
properties. This is seen by a strong enhancement of both charge and spin
transport properties yielding charge carrier mobilities exceeding 20000
cm/(Vs) and spin lifetimes up to 3.7 ns at room temperature. We discuss
several shortcomings in the determination of both quantities which complicates
the analysis of both extrinsic and intrinsic spin scattering mechanisms.
Furthermore, we show that contacts can be the origin of a second charge
neutrality point in gate dependent resistance measurements which is influenced
by the quantum capacitance of the underlying graphene layer.Comment: 19 pages, 8 figure
Spatially Resolved Raman Spectroscopy of Single- and Few-Layer Graphene
We present Raman spectroscopy measurements on single- and few-layer graphene
flakes. Using a scanning confocal approach we collect spectral data with
spatial resolution, which allows us to directly compare Raman images with
scanning force micrographs. Single-layer graphene can be distinguished from
double- and few-layer by the width of the D' line: the single peak for
single-layer graphene splits into different peaks for the double-layer. These
findings are explained using the double-resonant Raman model based on ab-initio
calculations of the electronic structure and of the phonon dispersion. We
investigate the D line intensity and find no defects within the flake. A finite
D line response originating from the edges can be attributed either to defects
or to the breakdown of translational symmetry
Time-resolved charge detection in graphene quantum dots
We present real-time detection measurements of electron tunneling in a
graphene quantum dot. By counting single electron charging events on the dot,
the tunneling process in a graphene constriction and the role of localized
states are studied in detail. In the regime of low charge detector bias we see
only a single time-dependent process in the tunneling rate which can be modeled
using a Fermi-broadened energy distribution of the carriers in the lead. We
find a non-monotonic gate dependence of the tunneling coupling attributed to
the formation of localized states in the constriction. Increasing the detector
bias above 2 mV results in an increase of the dot-lead transition rate related
to back-action of the charge detector current on the dot.Comment: 8 pages, 6 figure
Raman imaging of doping domains in graphene on SiO2
We present spatially resolved Raman images of the G and 2D lines of
single-layer graphene flakes. The spatial fluctuations of G and 2D lines are
correlated and are thus shown to be affiliated with local doping domains. We
investigate the position of the 2D line -- the most significant Raman peak to
identify single-layer graphene -- as a function of charging up to |n|~4 10^12
cm^-2. Contrary to the G line which exhibits a strong and symmetric stiffening
with respect to electron and hole-doping, the 2D line shows a weak and slightly
asymmetric stiffening for low doping. Additionally, the line width of the 2D
line is, in contrast to the G line, doping-independent making this quantity a
reliable measure for identifying single-layer graphene
Imaging Localized States in Graphene Nanostructures
Probing techniques with spatial resolution have the potential to lead to a
better understanding of the microscopic physical processes and to novel routes
for manipulating nanostructures. We present scanning-gate images of a graphene
quantum dot which is coupled to source and drain via two constrictions. We
image and locate conductance resonances of the quantum dot in the
Coulomb-blockade regime as well as resonances of localized states in the
constrictions in real space.Comment: 18 pages, 7 figure
Charge Detection in Graphene Quantum Dots
We report measurements on a graphene quantum dot with an integrated graphene
charge detector. The quantum dot device consists of a graphene island (diameter
approx. 200 nm) connected to source and drain contacts via two narrow graphene
constrictions. From Coulomb diamond measurements a charging energy of 4.3 meV
is extracted. The charge detector is based on a 45 nm wide graphene nanoribbon
placed approx. 60 nm from the island. We show that resonances in the nanoribbon
can be used to detect individual charging events on the quantum dot. The
charging induced potential change on the quantum dot causes a step-like change
of the current in the charge detector. The relative change of the current
ranges from 10% up to 60% for detecting individual charging events.Comment: 4 pages, 3 figure
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