4,711 research outputs found
Multiple layer local oxidation for fabricating semiconductor nanostructures
Coupled semiconductor nanostructures with a high degree of tunability are
fabricated using local oxidation with a scanning force microscope. Direct
oxidation of the GaAs surface of a Ga[Al]As heterostructure containing a
shallow two-dimensional electron gas is combined with the local oxidation of a
thin titanium film evaporated on top. A four-terminal quantum dot and a double
quantum dot system with integrated charge readout are realized. The structures
are tunable via in-plane gates formed by isolated regions in the electron gas
and by mutually isolated regions of the Ti film acting as top gates. Coulomb
blockade experiments demonstrate the high quality of this fabrication process.Comment: 3 pages, 3 figure
In Situ Treatment of a Scanning Gate Microscopy Tip
In scanning gate microscopy, where the tip of a scanning force microscope is
used as a movable gate to study electronic transport in nanostructures, the
shape and magnitude of the tip-induced potential are important for the
resolution and interpretation of the measurements. Contaminations picked up
during topography scans may significantly alter this potential. We present an
in situ high-field treatment of the tip that improves the tip-induced
potential. A quantum dot was used to measure the tip-induced potential.Comment: 3 pages, 1 figure, minor changes to fit published versio
Blood volume changes
Analysis of radionuclide volume determinations made for the crewmembers of selected Gemini and Apollo missions showed that orbital spaceflight has an effect on red cell mass. Because the methods and the protocol developed for earlier flights were used for the crews of the three Skylab missions, direct comparisons are possible. After each Skylab mission, decreases were found in crewmembers' red cell masses. The mean red cell mass decrease of 11 percent or 232 milliliters was approximately equal to the 10 percent mean red cell mass decrease of the Apollo 14 to 17 crewmembers. The red cell mass drop was greatest and the postrecovery reticulocyte response least for crewmembers of the 28-day Skylab 2 mission. Analyses of data from the red cell mass determinations indicate that the red cell mass drops occurred in the first 30 days of flight and that a gradual recovery of the red cell mass deficits began approximately 60 days after launch. The beginning of red cell mass regeneration during the Skylab 4 flight may explain the higher postmission reticulocyte counts
Time-Resolved Detection of Individual Electrons in a Quantum Dot
We present measurements on a quantum dot and a nearby, capacitively coupled,
quantum point contact used as a charge detector. With the dot being weakly
coupled to only a single reservoir, the transfer of individual electrons onto
and off the dot can be observed in real time in the current signal from the
quantum point contact. From these time-dependent traces, the quantum mechanical
coupling between dot and reservoir can be extracted quantitatively. A similar
analysis allows the determination of the occupation probability of the dot
states.Comment: 3 pages, 3 figure
Finite bias charge detection in a quantum dot
We present finite bias measurements on a quantum dot coupled capacitively to
a quantum point contact used as a charge detector. The transconductance signal
measured in the quantum point contact at finite dot bias shows structure which
allows us to determine the time-averaged charge on the dot in the non-blockaded
regime and to estimate the coupling of the dot to the leads.Comment: 6 pages, 4 figure
Local oxidation of Ga[Al]As heterostructures with modulated tip-sample voltages
Nanolithography based on local oxidation with a scanning force microscope has
been performed on an undoped GaAs wafer and a Ga[Al]As heterostructure with an
undoped GaAs cap layer and a shallow two-dimensional electron gas. The oxide
growth and the resulting electronic properties of the patterned structures are
compared for constant and modulated voltage applied to the conductive tip of
the scanning force microscope. All the lithography has been performed in
non-contact mode. Modulating the applied voltage enhances the aspect ratio of
the oxide lines, which significantly strengthens the insulating properties of
the lines on GaAs. In addition, the oxidation process is found to be more
reliable and reproducible. Using this technique, a quantum point contact and a
quantum wire have been defined and the electronic stability, the confinement
potential and the electrical tunability are demonstrated to be similar to the
oxidation with constant voltage.Comment: 7 pages, 7 figures, accepted by J. Appl. Phy
Measurement of the Tip-Induced Potential in Scanning Gate Experiments
We present a detailed experimental study on the electrostatic interaction
between a quantum dot and the metallic tip of a scanning force microscope. Our
method allowed us to quantitatively map the tip-induced potential and to
determine the spatial dependence of the tip's lever arm with high resolution.
We find that two parts of the tip-induced potential can be distinguished, one
that depends on the voltage applied to the tip and one that is independent of
this voltage. The first part is due to the metallic tip while we interpret the
second part as the effect of a charged dielectric particle on the tip. In the
measurements of the lever arm we find fine structure that depends on which
quantum state we study. The results are discussed in view of scanning gate
experiments where the tip is used as a movable gate to study nanostructures.Comment: 7 pages, 5 figures, minor changes to fit published versio
Frequency-selective single photon detection using a double quantum dot
We use a double quantum dot as a frequency-tunable on-chip microwave detector
to investigate the radiation from electron shot-noise in a near-by quantum
point contact. The device is realized by monitoring the inelastic tunneling of
electrons between the quantum dots due to photon absorption. The frequency of
the absorbed radiation is set by the energy separation between the dots, which
is easily tuned with gate voltages. Using time-resolved charge detection
techniques, we can directly relate the detection of a tunneling electron to the
absorption of a single photon
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