63 research outputs found
Giant valley-isospin conductance oscillations in ballistic graphene
At high magnetic fields the conductance of graphene is governed by the
half-integer quantum Hall effect. By local electrostatic gating a \textit{p-n}
junction perpendicular to the graphene edges can be formed, along which quantum
Hall channels co-propagate. It has been predicted by Tworzid\l{}o and
co-workers that if only the lowest Landau level is filled on both sides of the
junction, the conductance is determined by the valley (isospin) polarization at
the edges and by the width of the flake. This effect remained hidden so far due
to scattering between the channels co-propagating along the \textit{p-n}
interface (equilibration). Here we investigate \textit{p-n} junctions in
encapsulated graphene with a movable \textit{p-n} interface with which we are
able to probe the edge-configuration of graphene flakes. We observe large
quantum conductance oscillations on the order of \si{e^2/h} which solely depend
on the \textit{p-n} junction position providing the first signature of
isospin-defined conductance. Our experiments are underlined by quantum
transport calculations.Comment: 5 pages, 4 figure
Fabrication of ballistic suspended graphene with local-gating
Herein we discuss the fabrication of ballistic suspended graphene
nanostructures supplemented with local gating. Using in-situ current annealing,
we show that exceptional high mobilities can be obtained in these devices. A
detailed description is given of the fabrication of bottom and different
top-gate structures, which enable the realization of complex graphene
structures. We have studied the basic building block, the p-n junction in
detail, where a striking oscillating pattern was observed, which can be traced
back to Fabry-Perot oscillations that are localized in the electronic cavities
formed by the local gates. Finally we show some examples how the method can be
extended to incorporate multi-terminal junctions or shaped graphene. The
structures discussed here enable the access to electron-optics experiments in
ballistic graphene
Signatures of single quantum dots in graphene nanoribbons within the quantum Hall regime
We report on the observation of periodic conductance oscillations near quantum Hall plateaus in suspended graphene nanoribbons. They are attributed to single quantum dots that are formed in the narrowest part of the ribbon, in the valleys and hills of a disorder potential. In a wide flake with two gates, a double-dot system`s signature has been observed. Electrostatic confinement is enabled in single-layer graphene due to the gaps that are formed between the Landau levels, suggesting a way to create gate-defined quantum dots that can be accessed with quantum Hall edge states
Snake Trajectories in Ultraclean Graphene p-n Junctions
Snake states are trajectories of charge carriers curving back and forth along
an interface. There are two types of snake states, formed by either inverting
the magnetic field direction or the charge carrier type at an interface.
Whereas the former has been demonstrated in GaAs-AlGaAs heterostructures, the
latter has become conceivable only with the advance of ballistic graphene where
a gapless p-n interface governed by Klein tunneling can be formed. Such snake
states were hidden in previous experiments due to limited sample quality. Here
we report on magneto-conductance oscillations due to snake states in a
ballistic suspended graphene p-n-junction which occur already at a very small
magnetic field of 20mT. The visibility of 30% is enabled by Klein collimation.
Our finding is firmly supported by quantum transport simulations. We
demonstrate the high tunability of the device and operate it in different
magnetic field regimesComment: Accepted for publication in Nature Communication
GHz nanomechanical resonator in an ultraclean suspended graphene p-n junction
We demonstrate high-frequency mechanical resonators in ballistic graphene p-n
junctions. Fully suspended graphene devices with two bottom gates exhibit
ballistic bipolar behavior after current annealing. We determine the graphene
mass density and built-in tension for different current annealing steps by
comparing the measured mechanical resonant response to a simplified membrane
model. We consistently find that after the last annealing step the mass density
compares well with the expected density of pure graphene. In a graphene
membrane with high built-in tension, but still of macroscopic size with
dimensions 3 1 , a record resonance frequency of 1.17 GHz
is observed after the final current annealing step. We further compare the
resonance response measured in the unipolar with the one in the bipolar regime.
Remarkably, the resonant signals are strongly enhanced in the bipolar regime.
This enhancement is caused in part by the Fabry-Perot resonances that appear in
the bipolar regime and possibly also by the photothermoelectric effect that can
be very pronounced in graphene p-n junctions under microwave irradiation.Comment: 16 pages, 4 figures, 1 tabl
Spin-Delocalization in a Helical Open-Shell Hydrocarbon
Neutral open-shell molecules, in which spin density is delocalized through a helical conjugated backbone, hold promise as models for investigating phenomena arising from the interplay of magnetism and chirality. Apart from a handful of examples, however, the chemistry of these compounds remains largely unexplored. Here, we examine the prospect of extending spin-delocalization over a helical backbone in a model compound naphtho[3,2,1- no ]tetraphene, the first helically chiral open-shell hydrocarbon, in which one benzene ring is fused to [5]helicene, forming a phenalenyl subunit. The unpaired electron in this molecule is delocalized over the entire helical core composed of six rings, albeit in a nonuniform fashion, unlike in phenalenyl. In the case of a monosubstituted derivative, the uneven spin-distribution results in a selective σ-dimer formation in solution, as confirmed by 2D NMR spectroscopy. In contrast, the dimerization process is suppressed entirely when four substituents are installed to sterically hinder all reactive positions. The persistent nature of the tetrasubstituted derivative allowed its characterization by EPR, UV–vis, and CD spectroscopies, validating spin-delocalization through a chiral backbone, in accord with DFT calculations. The nonuniform spin-distribution, which dictates the selectivity of the σ-dimer formation, is rationalized by evaluating the aromaticity of the resonance structures that contribute to spin-delocalization
Scanning NV magnetometry of focused-electron-beam-deposited cobalt nanomagnets
Focused-electron-beam-induced deposition is a promising technique for
patterning nanomagnets for spin qubit control in a single step. We fabricate
cobalt nanomagnets in such a process, obtaining cobalt contents and saturation
magnetizations comparable to or higher than those typically obtained using
electron-beam lithography. We characterize the nanomagnets using transmission
electron microscopy and image their stray magnetic field using scanning NV
magnetometry, finding good agreement with micromagnetic simulations. The
magnetometry reveals the presence of magnetic domains and halo side-deposits,
which are common for this fabrication technique. Finally, we estimate dephasing
times for electron spin qubits in the presence of disordered stray fields due
to these side-deposits
Edge channel confinement in a bilayer graphene -- quantum dot
We combine electrostatic and magnetic confinement to define a quantum dot in
bilayer graphene. The employed geometry couples -doped reservoirs to a
-doped dot. At magnetic field values around T, Coulomb blockade is
observed. This demonstrates that the coupling of the co-propagating modes at
the - interface is weak enough to form a tunnel barrier, facilitating
transport of single charge carriers onto the dot. This result may be of use for
quantum Hall interferometry experiments
GHz nanomechanical resonator in an ultraclean suspended graphene p-n junction
We demonstrate high-frequency mechanical resonators in ballistic graphene p-n junctions. Fully suspended graphene devices with two bottom gates exhibit ballistic bipolar behavior after current annealing. We determine the graphene mass density and built-in tension for different current annealing steps by comparing the measured mechanical resonant response to a simplified membrane model. In a graphene membrane with high built-in tension, but still of macroscopic size with dimensions 3 x 1 m(2), a record resonance frequency of 1.17 GHz is observed after the final current annealing step. We further compare the resonance response measured in the unipolar with the one in the bipolar regime. Remarkably, the resonant signals are strongly enhanced in the bipolar regime
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