2,034 research outputs found
Contact-less characterizations of encapsulated graphene p-n junctions
Accessing intrinsic properties of a graphene device can be hindered by the
influence of contact electrodes. Here, we capacitively couple graphene devices
to superconducting resonant circuits and observe clear changes in the
resonance- frequency and -widths originating from the internal charge dynamics
of graphene. This allows us to extract the density of states and charge
relaxation resistance in graphene p-n junctions without the need of electrical
contacts. The presented characterizations pave a fast, sensitive and
non-invasive measurement of graphene nanocircuits.Comment: 4 figures, supplementary information on reques
Magnetoresistence engineering and singlet/triplet switching in InAs nanowire quantum dots with ferromagnetic sidegates
We present magnetoresistance (MR) experiments on an InAs nanowire quantum dot
device with two ferromagnetic sidegates (FSGs) in a split-gate geometry. The
wire segment can be electrically tuned to a single dot or to a double dot
regime using the FSGs and a backgate. In both regimes we find a strong MR and a
sharp MR switching of up to 25\% at the field at which the magnetizations of
the FSGs are inverted by the external field. The sign and amplitude of the MR
and the MR switching can both be tuned electrically by the FSGs. In a double
dot regime close to pinch-off we find {\it two} sharp transitions in the
conductance, reminiscent of tunneling MR (TMR) between two ferromagnetic
contacts, with one transition near zero and one at the FSG switching fields.
These surprisingly rich characteristics we explain in several simple resonant
tunneling models. For example, the TMR-like MR can be understood as a
stray-field controlled transition between singlet and a triplet double dot
states. Such local magnetic fields are the key elements in various proposals to
engineer novel states of matter and may be used for testing electron spin-based
Bell inequalities.Comment: 7 pages, 6 figure
Wideband and on-chip excitation for dynamical spin injection into graphene
Graphene is an ideal material for spin transport as very long spin relaxation
times and lengths can be achieved even at room temperature. However, electrical
spin injection is challenging due to the conductivity mismatch problem. Spin
pumping driven by ferromagnetic resonance is a neat way to circumvent this
problem as it produces a pure spin current in the absence of a charge current.
Here, we show spin pumping into single layer graphene in micron scale devices.
A broadband on-chip RF current line is used to bring micron scale permalloy
(NiFe) pads to ferromagnetic resonance with a magnetic field
tunable resonance condition. At resonance, a spin current is emitted into
graphene, which is detected by the inverse spin hall voltage in a close-by
platinum electrode. Clear spin current signals are detected down to a power of
a few milliwatts over a frequency range of 2 GHz to 8 GHz. This compact device
scheme paves the way for more complex device structures and allows the
investigation of novel materials.Comment: 7 pages, 4 figure
Multi-wall carbon nanotubes as quantum dots
We have measured the differential conductance dI/dV of individual multi-wall
carbon nanotubes (MWNT) of different lengths. A cross-over from wire-like (long
tubes) to dot-like (short tubes) behavior is observed. dI/dV is dominated by
random conductance fluctuations (UCF) in long MWNT devices (L=2...7 ),
while Coulomb blockade and energy level quantization are observed in short ones
(L=300 nm). The electron levels of short MWNT dots are nearly four-fold
degenerate (including spin) and their evolution in magnetic field (Zeeman
splitting) agrees with a g-factor of 2. In zero magnetic field the sequential
filling of states evolves with spin S according to S=0 -> 1/2 -> 0... In
addition, a Kondo enhancement of the conductance is observed when the number of
electrons on the tube is odd.Comment: 10 pages, 4 figure
Improving the establishment submodel of a forest patch model to assess the long-term protective effect of mountain forests
Simulation models such as forest patch models can be used to forecast the development of forest structural attributes over time. However, predictions of such models with respect to the impact of forest dynamics on the long-term protective effect of mountain forests may be of limited accuracy where tree regeneration is simulated with little detail. For this reason, we improved the establishment submodel of the ForClim forest patch model by implementing a more detailed representation of tree regeneration. Our refined submodel included canopy shading and ungulate browsing, two important constraints to sapling growth in mountain forests. To compare the old and the new establishment submodel of ForClim, we simulated the successional dynamics of the Stotzigwald protection forest in the Swiss Alps over a 60-year period. This forest provides protection for an important traffic route, but currently contains an alarmingly low density of tree regeneration. The comparison yielded a significantly longer regeneration period for the new model version, bringing the simulations into closer agreement with the known slow stand dynamics of mountain forests. In addition, the new model version was applied to forecast the future ability of the Stotzigwald forest to buffer the valley below from rockfall disturbance. Two scenarios were simulated: (1) canopy shading but no browsing impact, and (2) canopy shading and high browsing impact. The simulated stand structures were then compared to stand structure targets for rockfall protection, in order to assess their long-term protective effects. Under both scenarios, the initial sparse level of tree regeneration affected the long-term protective effect of the forest, which considerably declined during the first 40years. In the complete absence of browsing, the density of small trees increased slightly after 60years, raising hope for an eventual recovery of the protective effect. In the scenario that included browsing, however, the density of small trees remained at very low levels. With our improved establishment submodel, we provide an enhanced tool for studying the impacts of structural dynamics on the long-term protective effect of mountain forests. For certain purposes, it is important that predictive models of forest dynamics adequately represent critical processes for tree regeneration, such as sapling responses to low light levels and high browsing pressur
Local electrical tuning of the nonlocal signals in a Cooper pair splitter
A Cooper pair splitter consists of a central superconducting contact, S, from
which electrons are injected into two parallel, spatially separated quantum
dots (QDs). This geometry and electron interactions can lead to correlated
electrical currents due to the spatial separation of spin-singlet Cooper pairs
from S. We present experiments on such a device with a series of bottom gates,
which allows for spatially resolved tuning of the tunnel couplings between the
QDs and the electrical contacts and between the QDs. Our main findings are
gate-induced transitions between positive conductance correlation in the QDs
due to Cooper pair splitting and negative correlations due to QD dynamics.
Using a semi-classical rate equation model we show that the experimental
findings are consistent with in-situ electrical tuning of the local and
nonlocal quantum transport processes. In particular, we illustrate how the
competition between Cooper pair splitting and local processes can be optimized
in such hybrid nanostructures.Comment: 9 pages, 6 figures, 2 table
Electronic Transport Spectroscopy of Carbon Nanotubes in a Magnetic Field
We report magnetic field spectroscopy measurements in carbon nanotube quantum
dots exhibiting four-fold shell structure in the energy level spectrum. The
magnetic field induces a large splitting between the two orbital states of each
shell, demonstrating their opposite magnetic moment and determining transitions
in the spin and orbital configuration of the quantum dot ground state. We use
inelastic cotunneling spectroscopy to accurately resolve the spin and orbital
contributions to the magnetic moment. A small coupling is found between
orbitals with opposite magnetic moment leading to anticrossing behavior at zero
field.Comment: 7 pages, 4 figure
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
Defining and controlling double quantum dots in single-walled carbon nanotubes
We report the experimental realization of double quantum dots in
single-walled carbon nanotubes. The device consists of a nanotube with source
and drain contact, and three additional top-gate electrodes in between. We show
that, by energizing these top-gates, it is possible to locally gate a nanotube,
to create a barrier, or to tune the chemical potential of a part of the
nanotube. At low temperatures we find (for three different devices) that in
certain ranges of top-gate voltages our device acts as a double quantum dot,
evidenced by the typical honeycomb charge stability pattern.Comment: 9 pages, 3 figure
Magnetic field tuning and quantum interference in a Cooper pair splitter
Cooper pair splitting (CPS) is a process in which the electrons of naturally
occurring spin-singlet pairs in a superconductor are spatially separated using
two quantum dots. Here we investigate the evolution of the conductance
correlations in an InAs CPS device in the presence of an external magnetic
field. In our experiments the gate dependence of the signal that depends on
both quantum dots continuously evolves from a slightly asymmetric Lorentzian to
a strongly asymmetric Fano-type resonance with increasing field. These
experiments can be understood in a simple three - site model, which shows that
the nonlocal CPS leads to symmetric line shapes, while the local transport
processes can exhibit an asymmetric shape due to quantum interference. These
findings demonstrate that the electrons from a Cooper pair splitter can
propagate coherently after their emission from the superconductor and how a
magnetic field can be used to optimize the performance of a CPS device. In
addition, the model calculations suggest that the estimate of the CPS
efficiency in the experiments is a lower bound for the actual efficiency.Comment: 5 pages + 4 pages supplementary informatio
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