280 research outputs found
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
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
Vertically coupled double quantum rings at zero magnetic field
Within local-spin-density functional theory, we have investigated the
`dissociation' of few-electron circular vertical semiconductor double quantum
ring artificial molecules at zero magnetic field as a function of inter-ring
distance. In a first step, the molecules are constituted by two identical
quantum rings. When the rings are quantum mechanically strongly coupled, the
electronic states are substantially delocalized, and the addition energy
spectra of the artificial molecule resemble those of a single quantum ring in
the few-electron limit. When the rings are quantum mechanically weakly coupled,
the electronic states in the molecule are substantially localized in one ring
or the other, although the rings can be electrostatically coupled. The effect
of a slight mismatch introduced in the molecules from nominally identical
quantum wells, or from changes in the inner radius of the constituent rings,
induces localization by offsetting the energy levels in the quantum rings. This
plays a crucial role in the appearance of the addition spectra as a function of
coupling strength particularly in the weak coupling limit.Comment: 18 pages, 8 figures, submitted to Physical Review
Quantum dot occupation and electron dwell time in the cotunneling regime
We present comparative measurements of the charge occupation and conductance
of a GaAs/AlGaAs quantum dot. The dot charge is measured with a capacitively
coupled quantum point contact sensor. In the single-level Coulomb blockade
regime near equilibrium, charge and conductance signals are found to be
proportional to each other. We conclude that in this regime, the two signals
give equivalent information about the quantum dot system. Out of equilibrium,
we study the inelastic-cotunneling regime. We compare the measured differential
dot charge with an estimate assuming a dwell time of transmitted carriers on
the dot given by h/E, where E is the blockade energy of first-order tunneling.
The measured signal is of a similar magnitude as the estimate, compatible with
a picture of cotunneling as transmission through a virtual intermediate state
with a short lifetime
Analysis of the temperature-dependent quantum point contact conductance in view of the metal-insulator transition in two dimensions
The temperature dependence of the conductance of a quantum point contact has
been measured. The conductance as a function of the Fermi energy shows
temperature-independent fixed points, located at roughly multiple integers of
. Around the first fixed point at e/h, the experimental data for
different temperatures can been scaled onto a single curve. For pure thermal
smearing of the conductance steps, a scaling parameter of one is expected. The
measured scaling parameter, however, is significantly larger than 1. The
deviations are interpreted as a signature of the potential landscape of the
quantum point contact, and of the source-drain bias voltage. We relate our
results phenomenologically to the metal-insulator transition in two dimensions.Comment: 5 pages, 3 figure
Order and Frustration in Chiral Liquid Crystals
This paper reviews the complex ordered structures induced by chirality in
liquid crystals. In general, chirality favors a twist in the orientation of
liquid-crystal molecules. In some cases, as in the cholesteric phase, this
favored twist can be achieved without any defects. More often, the favored
twist competes with applied electric or magnetic fields or with geometric
constraints, leading to frustration. In response to this frustration, the
system develops ordered structures with periodic arrays of defects. The
simplest example of such a structure is the lattice of domains and domain walls
in a cholesteric phase under a magnetic field. More complex examples include
defect structures formed in two-dimensional films of chiral liquid crystals.
The same considerations of chirality and defects apply to three-dimensional
structures, such as the twist-grain-boundary and moire phases.Comment: 39 pages, RevTeX, 14 included eps figure
Theory of electronic transport through a triple quantum dot in the presence of magnetic field
Theory of electronic transport through a triangular triple quantum dot
subject to a perpendicular magnetic field is developed using a tight binding
model. We show that magnetic field allows to engineer degeneracies in the
triple quantum dot energy spectrum. The degeneracies lead to zero electronic
transmission and sharp dips in the current whenever a pair of degenerate states
lies between the chemical potential of the two leads. These dips can occur with
a periodicity of one flux quantum if only two levels contribute to the current
or with half flux quantum if the three levels of the triple dot contribute. The
effect of strong bias voltage and different lead-to-dot connections on
Aharonov-Bohm oscillations in the conductance is also discussed
Imaging Fractal Conductance Fluctuations and Scarred Wave Functions in a Quantum Billiard
We present scanning-probe images and magnetic-field plots which reveal
fractal conductance fluctuations in a quantum billiard. The quantum billiard is
drawn and tuned using erasable electrostatic lithography, where the scanning
probe draws patterns of surface charge in the same environment used for
measurements. A periodicity in magnetic field, which is observed in both the
images and plots, suggests the presence of classical orbits. Subsequent
high-pass filtered high-resolution images resemble the predicted probability
density of scarred wave functions, which describe the classical orbits.Comment: 5 pages, 4 figures To be published in PR
Electron flow in split-gated bilayer graphene
We present transport measurements on a bilayer graphene sheet with
homogeneous back gate and split top gate. The electronic transport data
indicates the capability to direct electron flow through graphene
nanostructures purely defined by electrostatic gating. By comparing the
transconductance data recorded for different top gate geometries - continuous
barrier and split-gate - the observed transport features for the split-gate can
be attributed to interference effects inside the narrow opening
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
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