134 research outputs found
Raman spectroscopy on etched graphene nanoribbons
We investigate etched single-layer graphene nanoribbons with different widths
ranging from 30 to 130 nm by confocal Raman spectroscopy. We show that the
D-line intensity only depends on the edge-region of the nanoribbon and that
consequently the fabrication process does not introduce bulk defects. In
contrast, the G- and the 2D-lines scale linearly with the irradiated area and
therefore with the width of the ribbons. We further give indications that the
D- to G-line ratio can be used to gain information about the crystallographic
orientation of the underlying graphene. Finally, we perform polarization angle
dependent measurements to analyze the nanoribbon edge-regions
Coulomb oscillations in three-layer graphene nanostructures
We present transport measurements on a tunable three-layer graphene single
electron transistor (SET). The device consists of an etched three-layer
graphene flake with two narrow constrictions separating the island from source
and drain contacts. Three lateral graphene gates are used to electrostatically
tune the device. An individual three-layer graphene constriction has been
investigated separately showing a transport gap near the charge neutrality
point. The graphene tunneling barriers show a strongly nonmonotonic coupling as
function of gate voltage indicating the presence of localized states in the
constrictions. We show Coulomb oscillations and Coulomb diamond measurements
proving the functionality of the graphene SET. A charging energy of meV is extracted.Comment: 10 pages, 6 figure
Local gating of a graphene Hall bar by graphene side gates
We have investigated the magnetotransport properties of a single-layer
graphene Hall bar with additional graphene side gates. The side gating in the
absence of a magnetic field can be modeled by considering two parallel
conducting channels within the Hall bar. This results in an average penetration
depth of the side gate created field of approx. 90 nm. The side gates are also
effective in the quantum Hall regime, and allow to modify the longitudinal and
Hall resistances
Transition to Landau Levels in Graphene Quantum Dots
We investigate the electronic eigenstates of graphene quantum dots of
realistic size (i.e., up to 80 nm diameter) in the presence of a perpendicular
magnetic field B. Numerical tight-binding calculations and Coulomb-blockade
measurements performed near the Dirac point exhibit the transition from the
linear density of states at B=0 to the Landau level regime at high fields.
Details of this transition sensitively depend on the underlying graphene
lattice structure, bulk defects, and localization effects at the edges. Key to
the understanding of the parametric evolution of the levels is the strength of
the chiral-symmetry breaking K-K' scattering. We show that the parametric
variation of the level variance provides a quantitative measure for this
scattering mechanism. We perform measurements of the parametric motion of
Coulomb blockade peaks as a function of magnetic field and find good agreement.
We thereby demonstrate that the magnetic-field dependence of graphene energy
levels may serve as a sensitive indicator for the properties of graphene
quantum dots and, in further consequence, for the validity of the
Dirac-picture.Comment: 10 pages, 11 figures, higher quality images available on reques
Transport through a strongly coupled graphene quantum dot in perpendicular magnetic field
We present transport measurements on a strongly coupled graphene quantum dot
in a perpendicular magnetic field. The device consists of an etched
single-layer graphene flake with two narrow constrictions separating a 140 nm
diameter island from source and drain graphene contacts. Lateral graphene gates
are used to electrostatically tune the device. Measurements of Coulomb
resonances, including constriction resonances and Coulomb diamonds prove the
functionality of the graphene quantum dot with a charging energy of around 4.5
meV. We show the evolution of Coulomb resonances as a function of perpendicular
magnetic field, which provides indications of the formation of the graphene
specific 0th Landau level. Finally, we demonstrate that the complex pattern
superimposing the quantum dot energy spectra is due to the formation of
additional localized states with increasing magnetic field.Comment: 6 pages, 4 figure
Transport through graphene double dots
We present Coulomb blockade measurements in a graphene double dot system. The
coupling of the dots to the leads and between the dots can be tuned by graphene
in-plane gates. The coupling is a non-monotonic function of the gate voltage.
Using a purely capacitive model, we extract all relevant energy scales of the
double dot system
Observation of excited states in a graphene quantum dot
We demonstrate that excited states in single-layer graphene quantum dots can
be detected via direct transport experiments. Coulomb diamond measurements show
distinct features of sequential tunneling through an excited state. Moreover,
the onset of inelastic cotunneling in the diamond region could be detected. For
low magnetic fields, the positions of the single-particle energy levels
fluctuate on the scale of a flux quantum penetrating the dot area. For higher
magnetic fields, the transition to the formation of Landau levels is observed.
Estimates based on the linear energy-momentum relation of graphene give carrier
numbers of the order of 10 for our device.Comment: 3 pages, 3 figure
Quantum dots and spin qubits in graphene
This is a review on graphene quantum dots and their use as a host for spin
qubits. We discuss the advantages but also the challenges to use graphene
quantum dots for spin qubits as compared to the more standard materials like
GaAs. We start with an overview of this young and fascinating field and will
then discuss gate-tunable quantum dots in detail. We calculate the bound states
for three different quantum dot architectures where a bulk gap allows for
confinement via electrostatic fields: (i) graphene nanoribbons with armchair
boundary, (ii) a disc in single-layer graphene, and (iii) a disc in bilayer
graphene. In order for graphene quantum dots to be useful in the context of
spin qubits, one needs to find reliable ways to break the valley-degeneracy.
This is achieved here, either by a specific termination of graphene in (i) or
in (ii) and (iii) by a magnetic field, without the need of a specific boundary.
We further discuss how to manipulate spin in these quantum dots and explain the
mechanism of spin decoherence and relaxation caused by spin-orbit interaction
in combination with electron-phonon coupling, and by hyperfine interaction with
the nuclear spin system.Comment: 23 pages, 10 figures, topical review prepared for Nanotechnolog
Coherent Electron-Phonon Coupling in Tailored Quantum Systems
The coupling between a two-level system and its environment leads to
decoherence. Within the context of coherent manipulation of electronic or
quasiparticle states in nanostructures, it is crucial to understand the sources
of decoherence. Here, we study the effect of electron-phonon coupling in a
graphene and an InAs nanowire double quantum dot. Our measurements reveal
oscillations of the double quantum dot current periodic in energy detuning
between the two levels. These periodic peaks are more pronounced in the
nanowire than in graphene, and disappear when the temperature is increased. We
attribute the oscillations to an interference effect between two alternative
inelastic decay paths involving acoustic phonons present in these materials.
This interpretation predicts the oscillations to wash out when temperature is
increased, as observed experimentally.Comment: 11 pages, 4 figure
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