2,031 research outputs found
On the nature of prominence emission observed by SDO/AIA
The Prominence-Corona Transition Region (PCTR) plays a key role in the
thermal and pressure equilibrium of solar prominences. Our knowledge of this
interface is limited and several major issues remain open, including the
thermal structure and, in particular, the maximum temperature of the detectable
plasma. The high signal-to-noise ratio of images obtained by the Atmospheric
Imaging Assembly (AIA) on NASA's Solar Dynamics Observatory clearly show that
prominences are often seen in emission in the 171 and 131 bands. We investigate
the temperature sensitivity of these AIA bands for prominence observation, in
order to infer the temperature content in an effort to explain the emission.
Using the CHIANTI atomic database and previously determined prominence
differential emission measure distributions, we build synthetic spectra to
establish the main emission-line contributors in the AIA bands. We find that
the Fe IX line always dominates the 171 band, even in the absence of plasma at
> 10^6 K temperatures, while the 131 band is dominated by Fe VIII. We conclude
that the PCTR has sufficient plasma emitting at > 4 10^5 K to be detected by
AIA.Comment: accepted Ap
Subtraction of test mass angular noise in the LISA Technology Package interferometer
We present recent sensitivity measurements of the LISA Technology Package
interferometer with articulated mirrors as test masses, actuated by
piezo-electric transducers. The required longitudinal displacement resolution
of 9 pm/sqrt[Hz] above 3 mHz has been demonstrated with an angular noise that
corresponds to the expected in on-orbit operation. The excess noise
contribution of this test mass jitter onto the sensitive displacement readout
was completely subtracted by fitting the angular interferometric data streams
to the longitudinal displacement measurement. Thus, this cross-coupling
constitutes no limitation to the required performance of the LISA Technology
Package interferometry.Comment: Applied Physics B - Lasers and Optics (2008
Small scale lateral superlattices in two-dimensional electron gases prepared by diblock copolymer masks
A poly(styrene-block-methylmethacrylate) diblock copolymer in the hexagonal
cylindrical phase has been used as a mask for preparing a periodic gate on top
of a Ga[Al]As-heterostructure. A superlattice period of 43 nm could be imposed
onto the two-dimensional electron gas. Transport measurements show a
characteristic positive magnetoresistance around zero magnetic field which we
interpret as a signature of electron motion guided by the superlattice
potential.Comment: 3 pages, 3 figure
Electronic properties of quantum dots formed by magnetic double barriers in quantum wires
The transport through a quantum wire exposed to two magnetic spikes in series
is modeled. We demonstrate that quantum dots can be formed this way which
couple to the leads via magnetic barriers. Conceptually, all quantum dot states
are accessible by transport experiments. The simulations show Breit-Wigner
resonances in the closed regime, while Fano resonances appear as soon as one
open transmission channel is present. The system allows to tune the dot's
confinement potential from sub-parabolic to superparabolic by experimentally
accessible parameters.Comment: 5 pages, 5 figure
Transport properties of quantum dots with hard walls
Quantum dots are fabricated in a Ga[Al]As-heterostructure by local oxidation
with an atomic force microscope. This technique, in combination with top gate
voltages, allows us to generate steep walls at the confining edges and small
lateral depletion lengths. The confinement is characterized by low-temperature
magnetotransport measurements, from which the dots' energy spectrum is
reconstructed. We find that in small dots, the addition spectrum can
qualitatively be described within a Fock-Darwin model. For a quantitative
analysis, however, a hard-wall confinement has to be considered. In large dots,
the energy level spectrum deviates even qualitatively from a Fock-Darwin model.
The maximum wall steepness achieved is of the order of 0.4 meV/nm.Comment: 9 pages, 5 figure
In-plane gate single-electron transistor in Ga[Al]As fabricated by scanning probe lithography
A single-electron transistor has been realized in a Ga[Al]As heterostructure
by oxidizing lines in the GaAs cap layer with an atomic force microscope. The
oxide lines define the boundaries of the quantum dot, the in-plane gate
electrodes, and the contacts of the dot to source and drain. Both the number of
electrons in the dot as well as its coupling to the leads can be tuned with an
additional, homogeneous top gate electrode. Pronounced Coulomb blockade
oscillations are observed as a function of voltages applied to different gates.
We find that, for positive top-gate voltages, the lithographic pattern is
transferred with high accuracy to the electron gas. Furthermore, the dot shape
does not change significantly when in-plane voltages are tuned.Comment: 4 pages, 3 figure
Transport properties of quantum dots with hard walls
Quantum dots are fabricated in a Ga[Al]As-heterostructure by local oxidation
with an atomic force microscope. This technique, in combination with top gate
voltages, allows us to generate steep walls at the confining edges and small
lateral depletion lengths. The confinement is characterized by low-temperature
magnetotransport measurements, from which the dots' energy spectrum is
reconstructed. We find that in small dots, the addition spectrum can
qualitatively be described within a Fock-Darwin model. For a quantitative
analysis, however, a hard-wall confinement has to be considered. In large dots,
the energy level spectrum deviates even qualitatively from a Fock-Darwin model.
The maximum wall steepness achieved is of the order of 0.4 meV/nm.Comment: 9 pages, 5 figure
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