2,290 research outputs found
Spectroscopy of nanoscopic semiconductor rings
Making use of self-assembly techniques, we demonstrate the realization of
nanoscopic semiconductor quantum rings in which the electronic states are in
the true quantum limit. We employ two complementary spectroscopic techniques to
investigate both the ground states and the excitations of these rings. Applying
a magnetic field perpendicular to the plane of the rings, we find that when
approximately one flux quantum threads the interior of each ring, a change in
the ground state from angular momentum to takes place.
This ground state transition is revealed both by a drastic modification of the
excitation spectrum and by a change in the magnetic field dispersion of the
single-electron charging energy
Resolving the decades-long transient FIRST J141918.9+394036: an orphan long gamma-ray burst or a young magnetar nebula?
Ofek (2017) identified FIRST J141918.9+394036 (hereafter FIRST J1419+3940) as
a radio source sharing similar properties and host galaxy type to the compact,
persistent radio source associated with the first known repeating fast radio
burst, FRB 121102. Law et al. (2018) showed that FIRST J1419+3940 is a
transient source decaying in brightness over the last few decades. One possible
interpretation is that FIRST J1419+3940 is a nearby analogue to FRB 121102 and
that the radio emission represents a young magnetar nebula (as several
scenarios assume for FRB 121102). Another interpretation is that FIRST
J1419+3940 is the afterglow of an `orphan' long gamma-ray burst (GRB). The
environment is similar to where most such events are produced. To distinguish
between these hypotheses, we conducted radio observations using the European
VLBI Network at 1.6 GHz to spatially resolve the emission and to search for
millisecond-duration radio bursts. We detect FIRST J1419+3940 as a compact
radio source with a flux density of (on 2018
September 18) and a source size of (i.e. given the angular diameter distance of ).
These results confirm that the radio emission is non-thermal and imply an
average expansion velocity of . Contemporaneous
high-time-resolution observations using the 100-m Effelsberg telescope detected
no millisecond-duration bursts of astrophysical origin. The source properties
and lack of short-duration bursts are consistent with a GRB jet expansion,
whereas they disfavor a magnetar birth nebula.Comment: 8 pages, 4 figures, accepted for publication in ApJ
Electronic states tuning of InAs self-assembled quantum dots
3 páginas, 3 figuras.-- PACS: 778.66.Fd, 78.55.Cr, 81.15.Hi, 81.05.Ea, 73.21.-b, 85.35.BeWe demonstrate the dimensional tuning of InAs self-assembled quantum dots (QDs) by changing the growth kinetics during the capping of InAs islands with GaAs. Modifying the growth sequence during the capping of InAs islands, allows us to tune the thickness and lateral dimensions of the QDs while keeping the wetting layer thickness constant. Using the same method but embedding the tuned InAs islands into AlAs layers allows to further blueshift the photoluminescence emission to higher energies while keeping the wetting layer thickness constant. The main process responsible for the QDs size modification is consistent with a kinetically controlled materials redistribution of the InAs islands that minimizes the energy of the epitaxial layers at the start up of the GaAs capping deposition.The authors want to acknowledge the financial support
of QUEST an NSF-Science and Technology center (DMR
No. 91-20007), the Linköping University (POH), the Deutsche Forschungsgemeinschaft (PJW) and the Spanish Ministry
of Education and Science (JMG) for financial support.Peer reviewe
Resolving the decades-long transient FIRST J141918.9+394036: an orphan long gamma-ray burst or a young magnetar nebula?
Ofek (2017) identified FIRST J141918.9+394036 (hereafter FIRST J1419+3940) as
a radio source sharing similar properties and host galaxy type to the compact,
persistent radio source associated with the first known repeating fast radio
burst, FRB 121102. Law et al. (2018) showed that FIRST J1419+3940 is a
transient source decaying in brightness over the last few decades. One possible
interpretation is that FIRST J1419+3940 is a nearby analogue to FRB 121102 and
that the radio emission represents a young magnetar nebula (as several
scenarios assume for FRB 121102). Another interpretation is that FIRST
J1419+3940 is the afterglow of an `orphan' long gamma-ray burst (GRB). The
environment is similar to where most such events are produced. To distinguish
between these hypotheses, we conducted radio observations using the European
VLBI Network at 1.6 GHz to spatially resolve the emission and to search for
millisecond-duration radio bursts. We detect FIRST J1419+3940 as a compact
radio source with a flux density of (on 2018
September 18) and a source size of (i.e. given the angular diameter distance of ).
These results confirm that the radio emission is non-thermal and imply an
average expansion velocity of . Contemporaneous
high-time-resolution observations using the 100-m Effelsberg telescope detected
no millisecond-duration bursts of astrophysical origin. The source properties
and lack of short-duration bursts are consistent with a GRB jet expansion,
whereas they disfavor a magnetar birth nebula.Comment: 8 pages, 4 figures, accepted for publication in ApJ
The Chemorepellent, Netrin-1, Appears to Signal Through a Tyrosine Kinase in Tetrahymena thermophila
Netrin-1 is a pleiotropic peptide signaling molecule. Its most well-known role in vertebrate development is neuronal guidance. Depending upon the cell type and signal concentration gradient, netrin-1 may serve either as a chemoattractant, causing formation of axonal growth cones, or as a chemorepellent, causing growth cone collapse within the axon.
Netrin-1 can bind to at least two types of receptors, and uses a variety of signaling proteins to convey its message. In some vertebrate cell types, the netrin-1 signal is G-protein mediated, while in other cell types, netrin signaling requires a tyrosine kinase or some other combination of kinases in order to signal.
Tetrahymena thermophila are free-living, eukaryotic cells that can respond to chemoattractants and chemorepellents by moving toward attractants and away from repellents. By studying the behavior of these organisms, we have found that netrin-1 acts as a chemorepellent in T. thermophila. Response to netrin-1 is concentration dependent, with an EC100 of approximately 1 micromolar, and an EC50 of approximately 10 pM.
Netrin-1 avoidance may be effectively eliminated by the addition of the broad-spectrum tyrosine kinase inhibitor, genistein, to the behavioral assay. The IC100 of genistein was approximately 75 micrograms/ml, while the IC50 of this compound was near 50 micrograms/ml. G-protein inhibitors, calcium chelators, and a number of other pharmacological inhibitors had no effect on netrin-1 signaling in this organism.
These data show that netrin-1 is a chemorepellent in Tetrahymena thermophila and that netrin signaling appears to implicate a tyrosine kinase in this organism. Further studies will help us to determine whether genistein is specifically acting upon a tyrosine kinase pathway or whether the inhibition is occurring via some other genistein-mediated effect
Low-aberration beamline optics for synchrotron infrared nanospectroscopy
MCTIC - MINISTÉRIO DA CIÊNCIA, TECNOLOGIA, INOVAÇÕES E COMUNICAÇÕESSynchrotron infrared nanospectroscopy is a recently developed technique that enables new possibilities in the broadband chemical analysis of materials in the nanoscale, far beyond the diffraction limit in this frequency domain. Synchrotron infrared ports have exploited mainly the high brightness advantage provided by electron storage rings across the whole infrared range. However, optical aberrations in the beam produced by the source depth of bending magnet emission at large angles prevent infrared nanospectroscopy to reach its maximum capability. In this work we present a low-aberration optical layout specially designed and constructed for a dedicated synchrotron infrared nanospectroscopy beamline. We report excellent agreement between simulated beam profiles (from standard wave propagation and raytracing optics simulations) with experimental measurements. We report an important improvement in the infrared nanospectroscopy experiment related to the improved beamline optics. Finally, we demonstrate the performance of the nanospectroscopy endstation by measuring a hyperspectral image of a polar material and we evaluate the setup sensitivity by measuring ultra-thin polymer films down to 6 nm thick.2691123811249MCTIC - MINISTÉRIO DA CIÊNCIA, TECNOLOGIA, INOVAÇÕES E COMUNICAÇÕESMCTIC - MINISTÉRIO DA CIÊNCIA, TECNOLOGIA, INOVAÇÕES E COMUNICAÇÕE
Pure luminescence transitions from a small InAs/GaAs quantum dot exhibiting a single electron level
2 páginas, 3 figuras.Pure photoluminescence spectra originating from a single InAs/GaAs quantum dot, which is small enough to possess only one single-electron level, are demonstrated. A symmetric fine structure of the exciton and the biexciton is observed.Peer reviewe
Microwave excitations associated with a wavy angular dependence of the spin transfer torque : model and experiments
The spin transfer torque (STT) can lead to steady precession of magnetization
without any external applied field in magnetic spin valve where the magnetic
layer have very different spin diffusion length. This effect is associated with
an unusual angular dependence of the STT, called "wavy" (WAD-STT), predicted in
the frame of diffusive models of spin transfer. In this article, we present a
complete experimental characterization of the magnetization dynamics in the
presence of a WAD-STT. The results are compared to the prediction of the
magnetization dynamics obtained by single domain magnetic simulations
(macrospin approximation). The macrospin simulations well reproduced the main
static and dynamical experimental features (phase diagram, R(I) curves,
dependence of frequency with current and field) and suggest that the dynamical
excitations observed experimentally are associated with a large angle
out-of-plane precession mode. The present work validates the diffusive models
of the spin transfer and underlines the role of the spin accumulation and the
spin relaxation effects on the STT
Charged Excitons in Self-assembled Quantum Dots
11 páginas, 8 figuras.-- 2002 Mrs Fall Meeting, Boston.-- Victor I. Klimov, Jillian M. Buriak, Wayner Danial D. M., Francesco Priolo, Bruce White (Eds.).We have succeeded in generating highly charged excitons in InAs self-assembled quantum
dots by embedding the dots in a field-effect heterostructure. We discover an excitonic
Coulomb blockage: over large regions of gate voltage, the exciton charge remains constant.
We present here a summary of the emission properties of the charged excitons.This work was funded by EPSRC and the DFG (SFB348).Peer reviewe
Manipulating exciton fine-structure in quantum dots with a lateral electric field
The fine structure of the neutral exciton in a single self assembled InGaAs
quantum dot is investigated under the effect of a lateral electric field. Stark
shifts up to 1.5 meV, an increase in linewidth, and a decrease in
photoluminescence intensity were observed due to the electric field. We show
that the lateral electric field strongly affects the exciton fine structure
splitting due to active manipulation of the single particle wave-functions.
Remarkably, the splitting can be tuned over large values and through zero.Comment: 4 pages, 5 figure
- …