2,320 research outputs found
Gate-tunable split Kondo effect in a carbon nanotube quantum dot
We show a detailed investigation of the split Kondo effect in a carbon
nanotube quantum dot with multiple gate electrodes. It is found that the
splitting decreases for increasing magnetic field, to result in a recovered
zero-bias Kondo resonance at finite magnetic field. Surprisingly, in the same
charge state, but under different gate-configurations, the splitting does not
disappear for any value of the magnetic field, but we observe an avoided
crossing of two high-conductance lines. We think that our observations can be
understood in terms of a two-impurity Kondo effect with two spins coupled
antiferromagnetically. The exchange coupling between the two spins can be
influenced by a local gate, and the non-recovery of the Kondo resonance for
certain gate configurations is explained by the existence of a small
antisymmetric contribution to the exchange interaction between the two spins.Comment: 12 pages, 4 figures, published versio
Fork stamping of pristine carbon nanotubes onto ferromagnetic contacts for spin-valve devices
We present a fabrication scheme called 'fork stamping' optimized for the dry
transfer of individual pristine carbon nanotubes (CNTs) onto ferromagnetic
contact electrodes fabricated by standard lithography. We demonstrate the
detailed recipes for a residue-free device fabrication and in-situ current
annealing on suspended CNT spin-valve devices with ferromagnetic Permalloy (Py)
contacts and report preliminary transport characterization and
magnetoresistance experiments at cryogenic temperatures. This scheme can
directly be used to implement more complex device structures, including
multiple gates or superconducting contacts.Comment: 7 pages, 4 figures, submitted to IWEPNM 2015 conference proceedings
(physica status solidi (b)
Measurements of a Quantum Dot with an Impedance-Matching On-Chip LC Resonator at GHz Frequencies
We report the realization of a bonded-bridge on-chip superconducting coil and
its use in impedance-matching a highly ohmic quantum dot (QD) to a
measurement setup. The coil, modeled as a lumped-element resonator, is
more compact and has a wider bandwidth than resonators based on coplanar
transmission lines (e.g. impedance transformers and stub tuners) at
potentially better signal-to-noise ratios. In particular for measurements of
radiation emitted by the device, such as shot noise, the 50 larger
bandwidth reduces the time to acquire the spectral density. The resonance
frequency, close to 3.25 GHz, is three times higher than that of the one
previously reported wire-bonded coil. As a proof of principle, we fabricated an
circuit that achieves impedance-matching to a load
and validate it with a load defined by a carbon nanotube QD of which we measure
the shot noise in the Coulomb blockade regime.Comment: 7 pages, 6 figure
Large oscillating non-local voltage in multi-terminal single wall carbon nanotube devices
We report on the observation of a non-local voltage in a ballistic
one-dimensional conductor, realized by a single-wall carbon nanotube with four
contacts. The contacts divide the tube into three quantum dots which we control
by the back-gate voltage . We measure a large \emph{oscillating} non-local
voltage as a function of with zero mean. Though a classical
resistor model can account for a non-local voltage including change of sign, it
fails to describe the magnitude properly. The large amplitude of is
due to quantum interference effects and can be understood within the
scattering-approach of electron transport
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
Shot noise of a quantum dot measured with GHz stub impedance matching
The demand for a fast high-frequency read-out of high impedance devices, such
as quantum dots, necessitates impedance matching. Here we use a resonant
impedance matching circuit (a stub tuner) realized by on-chip superconducting
transmission lines to measure the electronic shot noise of a carbon nanotube
quantum dot at a frequency close to 3 GHz in an efficient way. As compared to
wide-band detection without impedance matching, the signal to noise ratio can
be enhanced by as much as a factor of 800 for a device with an impedance of 100
k. The advantage of the stub resonator concept is the ease with which
the response of the circuit can be predicted, designed and fabricated. We
further demonstrate that all relevant matching circuit parameters can reliably
be deduced from power reflectance measurements and then used to predict the
power transmission function from the device through the circuit. The shot noise
of the carbon nanotube quantum dot in the Coulomb blockade regime shows an
oscillating suppression below the Schottky value of , as well an
enhancement in specific regions.Comment: 6 pages, 4 figures, supplementar
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
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
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