317 research outputs found
Probing a Kondo correlated quantum dot with spin spectroscopy
We investigate Kondo effect and spin blockade observed on a many-electron
quantum dot and study the magnetic field dependence. At lower fields a
pronounced Kondo effect is found which is replaced by spin blockade at higher
fields. In an intermediate regime both effects are visible. We make use of this
combined effect to gain information about the internal spin configuration of
our quantum dot. We find that the data cannot be explained assuming regular
filling of electronic orbitals. Instead spin polarized filling seems to be
probable.Comment: 4 pages, 5 figure
Dephasing in (Ga,Mn)As nanowires and rings
To understand quantum mechanical transport in ferromagnetic semiconductor the
knowledge of basic material properties like phase coherence length and
corresponding dephasing mechanism are indispensable ingredients. The lack of
observable quantum phenomena prevented experimental access to these quantities
so far. Here we report about the observations of universal conductance
fluctuations in ferromagnetic (Ga,Mn)As. The analysis of the length and
temperature dependence of the fluctuations reveals a T^{-1} dependence of the
dephasing time.Comment: 5 pages, 4 figure
Non-invasive detection of charge-rearrangement in a quantum dot in high magnetic fields
We demonstrate electron redistribution caused by magnetic field on a single
quantum dot measured by means of a quantum point contact as non-invasive
detector. Our device which is fabricated by local anodic oxidation allows to
control independently the quantum point contact and all tunnelling barriers of
the quantum dot. Thus we are able to measure both the change of the quantum dot
charge and also changes of the electron configuration at constant number of
electrons on the quantum dot. We use these features to exploit the quantum dot
in a high magnetic field where transport through the quantum dot displays the
effects of Landau shells and spin blockade. We confirm the internal
rearrangement of electrons as function of the magnetic field for a fixed number
of electrons on the quantum dot.Comment: 4 pages, 5 figure
Measurements of higher order noise correlations in a quantum dot with a finite bandwidth detector
We present measurements of the fourth and fifth cumulants of the distribution
of transmitted charge in a tunable quantum dot. We investigate how the measured
statistics is influenced by the finite bandwidth of the detector and by the
finite measurement time. By including the detector when modeling the system, we
use the theory of full counting statistics to calculate the noise levels for
the combined system. The predictions of the finite-bandwidth model are in good
agreement with measured data
Time-resolved charge detection with cross-correlation techniques
We present time-resolved charge sensing measurements on a GaAs double quantum
dot with two proximal quantum point contact (QPC) detectors. The QPC currents
are analyzed with cross-correlation techniques, which enables us to measure dot
charging and discharging rates for significantly smaller signal-to-noise ratios
than required for charge detection with a single QPC. This allows to reduce the
current level in the detector and therefore the invasiveness of the detection
process and may help to increase the available measurement bandwidth in
noise-limited setups.Comment: 6 pages, 4 figure
Spatially resolved manipulation of single electrons in quantum dots using a scanned probe
The scanning metallic tip of a scanning force microscope was coupled
capacitively to electrons confined in a lithographically defined gate-tunable
quantum dot at a temperature of 300 mK. Single electrons were made to hop on or
off the dot by moving the tip or by changing the tip bias voltage owing to the
Coulomb-blockade effect. Spatial images of conductance resonances map the
interaction potential between the tip and individual electronic quantum dot
states. Under certain conditions this interaction is found to contain a
tip-voltage induced and a tip-voltage independent contribution.Comment: 4 pages, 4 figure
Single electron charging of impurity sites visualized by scanning gate experiments on a quantum point contact
A quantum point contact (QPC) patterned on a two-dimensional electron gas is
investigated with a scanning gate setup operated at a temperature of 300 mK.
The conductance of the point contact is recorded while the local potential is
modified by scanning the tip. Single electron charging of impurities induced by
the local potential is observed as a stepwise conductance change of the
constriction. By selectively changing the state of some of these impurities, it
is possible to observe changes in transmission resonances of the QPC. The
location of such impurities is determined, and their density is estimated to be
below 50 per \mu m^2, corresponding to less than 1 % of the doping
concentration
Phase coherent transport in (Ga,Mn)As
Quantum interference effects and resulting quantum corrections of the
conductivity have been intensively studied in disordered conductors over the
last decades. The knowledge of phase coherence lengths and underlying dephasing
mechanisms are crucial to understand quantum corrections to the resistivity in
the different material systems. Due to the internal magnetic field and the
associated breaking of time-reversal symmetry quantum interference effects in
ferromagnetic materials have been scarcely explored. Below we describe the
investigation of phase coherent transport phenomena in the newly discovered
ferromagnetic semiconductor (Ga,Mn)As. We explore universal conductance
fluctuations in mesoscopic (Ga,Mn)As wires and rings, the Aharonov-Bohm effect
in nanoscale rings and weak localization in arrays of wires, made of the
ferromagnetic semiconductor material. The experiments allow to probe the phase
coherence length L_phi and the spin flip length L_SO as well as the temperature
dependence of dephasing.Comment: 22 pages, 10 figure
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