78 research outputs found
Huge negative differential conductance in Au-H2 molecular nanojunctions
Experimental results showing huge negative differential conductance in
gold-hydrogen molecular nanojunctions are presented. The results are analyzed
in terms of two-level system (TLS) models: it is shown that a simple TLS model
cannot produce peaklike structures in the differential conductance curves,
whereas an asymmetrically coupled TLS model gives perfect fit to the data. Our
analysis implies that the excitation of a bound molecule to a large number of
energetically similar loosely bound states is responsible for the peaklike
structures. Recent experimental studies showing related features are discussed
within the framework of our model.Comment: 9 pages, 8 figure
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
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
Alluvial Architecture and Fluvial Cycles in Quaternary Deposits in a Continental Interior Basin, E Hungary
The thickness of the studied Quaternary alluvial complex, located in the eastern part of the Pannonian Basin System, can exceed 500 m. Based on subsurface facies analysis the following large-scale depositional elements were identified: channel-fill deposits, point bar deposits, alluvial fan (sandy sheet-flood) deposits, floodplain and floodbasin deposits, and thinner sandy–silty beds. They are classified into four types of facies associations, showing a characteristic stacking pattern on the logs. Facies zonation and basin-scale facies mapping of the overall Quaternary sedimentary succession shows that in several areas dominated by stacked, multistorey sandy channel fill sediments, pre-existing superimposed channel belts can be presumed. In the central and deepest part of the basin muddy floodbasin (distal floodplain and wetland) sediments dominate. Between these the largest area represents the floodplain where single channel fill sands are interbedded in the alluvial plain muds. In the eastern part of the basin the well-logs highlight the distal part of an alluvial fan where sandy sheet-flood deposits alternate with floodplain sediments.
The recognized facies associations show a vertical pattern, i.e. they form a 40–100 m thick fining-upward fluvial cycle. The most characteristic and even ideal cycle can be observed in the channel belts and in the proximal floodplain zone. Here the basal member of the cycle is made up of multistorey channel fill beds cut into the underlying floodplain deposits. This is overlain by an alternating sandy–muddy succession of channel fill and floodplain deposits forming the intermediate member. The upper member is composed of silty–clayey floodplain deposits with occasional very thin, discrete silty–sandy bodies
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
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
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
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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