1,292 research outputs found
Electrical Spin Injection in Multi-Wall carbon NanoTubes with transparent ferromagnetic contacts
We report on electrical spin injection measurements on MWNTs . We use a
ferromagnetic alloy PdNi with x 0.7 which allows to
obtain devices with resistances as low as 5.6 at 300 . The yield
of device resistances below 100 , at 300 , is around 50%. We
measure at 2 a hysteretic magneto-resistance due to the magnetization
reversal of the ferromagnetic leads. The relative difference between the
resistance in the antiparallel (AP) orientation and the parallel (P)
orientation is about 2%.Comment: submitted to APL version without figures version with figures
available on http://www.unibas.ch/phys-meso
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
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
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
Resonant tunneling through a C60 molecular junction in liquid environment
We present electronic transport measurements through thiolated C
molecules in liquid environment. The molecules were placed within a
mechanically controllable break junction using a single anchoring group per
molecule. When varying the electrode separation of the C-modified
junctions, we observed a peak in the conductance traces. The shape of the
curves is strongly influenced by the environment of the junction as shown by
measurements in two distinct solvents. In the framework of a simple resonant
tunneling model, we can extract the electronic tunneling rates governing the
transport properties of the junctions.Comment: 13 pages, 4 figures. To appear in Nanotechnolog
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
Scaling of 1/f noise in tunable break-junctions
We have studied the voltage noise of gold nano-contacts in
electromigrated and mechanically controlled break-junctions having resistance
values that can be tuned from 10 (many channels) to 10 k
(single atom contact). The noise is caused by resistance fluctuations as
evidenced by the dependence of the power spectral density
on the applied DC voltage . As a function of the normalized noise
shows a pronounced cross-over from for low-ohmic
junctions to for high-ohmic ones. The measured powers of 3
and 1.5 are in agreement with -noise generated in the bulk and reflect the
transition from diffusive to ballistic transport
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