394 research outputs found
A low-temperature dynamic mode scanning force microscope operating in high magnetic fields
A scanning force microscope was implemented operating at temperatures below
4.2K and in magnetic fields up to 8T. Piezoelectric quartz tuning forks were
employed for non optical tip-sample distance control in the dynamic operation
mode. Fast response was achieved by using a phase-locked loop for driving the
mechanical oscillator. Possible applications of this setup for various scanning
probe techniques are discussed.Comment: 5 pages, 5 figures, submitted to "Review of Scientific Instruments
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
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
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
High-frequency gate manipulation of a bilayer graphene quantum dot
We report transport data obtained for a double-gated bilayer graphene quantum
dot. In Coulomb blockade measurements, the gate dielectric Cytop(TM) is found
to provide remarkable electronic stability even at cryogenic temperatures.
Moreover, we demonstrate gate manipulation with square shaped voltage pulses at
frequencies up to 100 MHz and show that the signal amplitude is not affected by
the presence of the capacitively coupled back gate
Spin States in Graphene Quantum Dots
We investigate ground and excited state transport through small (d = 70 nm)
graphene quantum dots. The successive spin filling of orbital states is
detected by measuring the ground state energy as a function of a magnetic
field. For a magnetic field in-plane of the quantum dot the Zemann splitting of
spin states is measured. The results are compatible with a g-factor of 2 and we
detect a spin-filling sequence for a series of states which is reasonable given
the strength of exchange interaction effects expected for graphene
Electron-Hole Crossover in Graphene Quantum Dots
We investigate the addition spectrum of a graphene quantum dot in the
vicinity of the electron-hole crossover as a function of perpendicular magnetic
field. Coulomb blockade resonances of the 50 nm wide dot are visible at all
gate voltages across the transport gap ranging from hole to electron transport.
The magnetic field dependence of more than 50 states displays the unique
complex evolution of the diamagnetic spectrum of a graphene dot from the
low-field regime to the Landau regime with the n=0 Landau level situated in the
center of the transport gap marking the electron-hole crossover. The average
peak spacing in the energy region around the crossover decreases with
increasing magnetic field. In the vicinity of the charge neutrality point we
observe a well resolved and rich excited state spectrum.Comment: 4 pages, 3 figure
Characterization of a microwave frequency resonator via a nearby quantum dot
We present measurements of a hybrid system consisting of a microwave
transmission-line resonator and a lateral quantum dot defined on a GaAs
heterostructure. The two subsystems are separately characterized and their
interaction is studied by monitoring the electrical conductance through the
quantum dot. The presence of a strong microwave field in the resonator is found
to reduce the resonant conductance through the quantum dot, and is attributed
to electron heating and modulation of the dot potential. We use this
interaction to demonstrate a measurement of the resonator transmission spectrum
using the quantum dot.Comment: 3 pages, 3 figure
Quantum dot admittance probed at microwave frequencies with an on-chip resonator
We present microwave frequency measurements of the dynamic admittance of a
quantum dot tunnel coupled to a two-dimensional electron gas. The measurements
are made via a high-quality 6.75 GHz on-chip resonator capacitively coupled to
the dot. The resonator frequency is found to shift both down and up close to
conductance resonance of the dot corresponding to a change of sign of the
reactance of the system from capacitive to inductive. The observations are
consistent with a scattering matrix model. The sign of the reactance depends on
the detuning of the dot from conductance resonance and on the magnitude of the
tunnel rate to the lead with respect to the resonator frequency. Inductive
response is observed on a conductance resonance, when tunnel coupling and
temperature are sufficiently small compared to the resonator frequency.Comment: 8 pages, 4 figure
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