36 research outputs found
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
Spin-Polarized Electrons in Monolayer MoS
The optical susceptibility is a local, minimally-invasive and spin-selective
probe of the ground state of a two-dimensional electron gas. We apply this
probe to a gated monolayer of MoS. We demonstrate that the electrons are
spin polarized. Of the four available bands, only two are occupied. These two
bands have the same spin but different valley quantum numbers. We argue that
strong Coulomb interactions are a key aspect of this spontaneous symmetry
breaking. The Bohr radius is so small that even electrons located far apart in
phase space interact, facilitating exchange couplings to align the spins
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
Quantum confined Stark effect in a MoS monolayer van der Waals heterostructure
The optics of dangling-bond-free van der Waals heterostructures containing
transition metal dichalcogenides are dominated by excitons. A crucial property
of a confined exciton is the quantum confined Stark effect (QCSE). Here, such a
heterostructure is used to probe the QCSE by applying a uniform vertical
electric field across a molybdenum disulfide (MoS) monolayer. The
photoluminescence emission energies of the neutral and charged excitons shift
quadratically with the applied electric field provided the electron density
remains constant, demonstrating that the exciton can be polarized. Stark shifts
corresponding to about half the homogeneous linewidth were achieved. Neutral
and charged exciton polarizabilities of (7.8~\pm~1.0)\times
10^{-10}~\tr{D~m~V}^{-1} and (6.4~\pm~0.9)\times 10^{-10}~\tr{D~m~V}^{-1} at
relatively low electron density (8 \times 10^{11}~\tr{cm}^{-2}) have been
extracted, respectively. These values are one order of magnitude lower than the
previously reported values, but in line with theoretical calculations. The
methodology presented here is versatile and can be applied to other
semiconducting layered materials as well
Characterization and cloning of fasciclin I and fasciclin II glycoproteins in the grasshopper
Monoclonal antibodies were previously used to identify two glycoproteins, called fasciclin I and II (70 and 95 kDa, respectively), which are expressed on different subsets of axon fascicles in the grasshopper (Schistocerca americana) embryo. Here the monoclonal antibodies were used to purify these two membrane-associated glycoproteins for further characterization. Fasciclin II appears to be an integral membrane protein, where fasciclin I is an extrinsic membrane protein. The amino acid sequences of the amino terminus and fragments of both proteins were determined. Using synthetic oligonucleotide probes and antibody screening, we isolated genomic and cDNA clones. Partial DNA sequences of these clones indicate that they encode fasciclins I and II
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
From electroburning to sublimation: substrate and environmental effects in the electrical breakdown process of monolayer graphene
We report on the characterization of the electrical breakdown (EB) process for the formation of tunneling nanogaps in single-layer graphene. In particular, we investigated the role of oxygen in the breakdown process by varying the environmental conditions (vacuum and ambient conditions). We show that the density of oxygen molecules in the chamber is a crucial parameter that defines the physical breakdown process: at low density, the graphene lattice is sublimating, whereas at high density, the process involved is oxidation, independent of the substrate material. To estimate the activation energies of the two processes, we use a scheme which consists of applying voltage pulses across the junction during the breakdown. By systematically varying the voltage pulse length, and estimating the junction temperature from a 1D thermal model, we extract activation energies which are consistent with the sublimation of graphene under high vacuum and the electroburning process under air. Our study demonstrates that, in our system, a better control of the gap formation is achieved in the sublimation regime
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