9,558 research outputs found
Relaxation of hole spins in quantum dots via two-phonon processes
We investigate theoretically spin relaxation in heavy hole quantum dots in
low external magnetic fields. We demonstrate that two-phonon processes and
spin-orbit interaction are experimentally relevant and provide an explanation
for the recently observed saturation of the spin relaxation rate in heavy hole
quantum dots with vanishing magnetic fields. We propose further experiments to
identify the relevant spin relaxation mechanisms in low magnetic fields.Comment: 5 pages, 2 figure
Episodic disk accretion in the halo of the 'old' Pre-Main Sequence cluster Eta Chamaeleontis
We present multi-epoch medium-resolution observations of two M4.5 candidate
members in the halo of the ~8 Myr Eta Chamaeleontis open cluster. Over six
months of observations both stars exhibited variations in their H-alpha line
profiles on timescales of days to months, with at least one episode of
substantial activity attributable to accretion from a circumstellar disk. We
derive an accretion rate ~10^-8.7 Msun/yr for this event, with a rate of
~10^-10.6 Msun/yr in quiescence. Episodic accretion like that observed here
means existing surveys of accreting Weak-lined T-Tauri Stars in young clusters
are likely incomplete and that gas dissipation timescales calculated from the
fraction of accreting objects are underestimates.Comment: 5 pages, 5 figures, 1 table. Accepted for publication in MNRAS
Letter
The Histone 3'-Terminal Stem-Loop-Binding Protein Enhances Translation through a Functional and Physical Interaction with Eukaryotic Initiation Factor 4G (eIF4G) and eIF3
Metazoan cell cycle-regulated histone mRNAs are unique cellular mRNAs in that they terminate in a highly conserved stem-loop structure instead of a poly(A) tail. Not only is the stem-loop structure necessary for 3'-end formation but it regulates the stability and translational efficiency of histone mRNAs. The histone stem-loop structure is recognized by the stem-loop-binding protein (SLBP), which is required for the regulation of mRNA processing and turnover. In this study, we show that SLBP is required for the translation of mRNAs containing the histone stem-loop structure. Moreover, we show that the translation of mRNAs ending in the histone stem-loop is stimulated in Saccharomyces cerevisiae cells expressing mammalian SLBP. The translational function of SLBP genetically required eukaryotic initiation factor 4E (eIF4E), eIF4G, and eIF3, and expressed SLBP coisolated with S. cerevisiae initiation factor complexes that bound the 5' cap in a manner dependent on eIF4G and eIF3. Furthermore, eIF4G coimmunoprecipitated with endogenous SLBP in mammalian cell extracts and recombinant SLBP and eIF4G coisolated. These data indicate that SLBP stimulates the translation of histone mRNAs through a functional interaction with both the mRNA stem-loop and the 5' cap that is mediated by eIF4G and eIF3
Theory of Electron-Phonon Dynamics in Insulating Nanoparticles
We discuss the rich vibrational dynamics of nanometer-scale semiconducting
and insulating crystals as probed by localized electronic impurity states, with
an emphasis on nanoparticles that are only weakly coupled to their environment.
Two principal regimes of electron-phonon dynamics are distinguished, and a
brief survey of vibrational-mode broadening mechanisms is presented. Recent
work on the effects of mechanical interaction with the environment is
discussed.Comment: Revte
Asteroseismology of 16000 Kepler Red Giants: Global Oscillation Parameters, Masses, and Radii
The Kepler mission has provided exquisite data to perform an ensemble
asteroseismic analysis on evolved stars. In this work we systematically
characterize solar-like oscillations and granulation for 16,094 oscillating red
giants, using end-of-mission long-cadence data. We produced a homogeneous
catalog of the frequency of maximum power (typical uncertainty
=1.6\%), the mean large frequency separation
(=0.6\%), oscillation amplitude (=4.7\%),
granulation power (=8.6\%), power excess width (=8.8\%), seismically-derived stellar mass (=7.8\%),
radius (=2.9\%), and thus surface gravity (=0.01 dex). Thanks to the large red giant sample, we confirm that
red-giant-branch (RGB) and helium-core-burning (HeB) stars collectively differ
in the distribution of oscillation amplitude, granulation power, and width of
power excess, which is mainly due to the mass difference. The distribution of
oscillation amplitudes shows an extremely sharp upper edge at fixed , which might hold clues to understand the excitation and damping
mechanisms of the oscillation modes. We find both oscillation amplitude and
granulation power depend on metallicity, causing a spread of 15\% in
oscillation amplitudes and a spread of 25\% in granulation power from
[Fe/H]=-0.7 to 0.5 dex. Our asteroseismic stellar properties can be used as
reliable distance indicators and age proxies for mapping and dating galactic
stellar populations observed by Kepler. They will also provide an excellent
opportunity to test asteroseismology using Gaia parallaxes, and lift
degeneracies in deriving atmospheric parameters in large spectroscopic surveys
such as APOGEE and LAMOST.Comment: Accepted for publication in ApJS. Both table 1 and 2 are available
for download as ancillary file
An optimized tuned mass damper/harvester device
Much work has been conducted on vibration absorbers, such as tuned mass dampers (TMD), where significant energy is extracted from a structure. Traditionally, this energy is dissipated through the devices as heat. In this paper, the concept of recovering some of this energy electrically and reuse it for structural control or health monitoring is investigated. The energy-dissipating damper of a TMD is replaced with an electromagnetic device in order to transform mechanical vibration into electrical energy. That gives the possibility of controlled damping force whilst generating useful electrical energy. Both analytical and experimental results from an adaptive and a semi-active tuned mass damper/harvester are presented. The obtained results suggest that sufficient energy might be harvested for the device to tune itself to optimise vibration suppression
Simultaneous in vivo positron emission tomography and magnetic resonance imaging
Positron emission tomography (PET) and magnetic resonance imaging (MRI) are widely used in vivo imaging technologies with both clinical and biomedical research applications. The strengths of MRI include high-resolution, high-contrast morphologic imaging of soft tissues; the ability to image physiologic parameters such as diffusion and changes in oxygenation level resulting from neuronal stimulation; and the measurement of metabolites using chemical shift imaging. PET images the distribution of biologically targeted radiotracers with high sensitivity, but images generally lack anatomic context and are of lower spatial resolution. Integration of these technologies permits the acquisition of temporally correlated data showing the distribution of PET radiotracers and MRI contrast agents or MR-detectable metabolites, with registration to the underlying anatomy. An MRI-compatible PET scanner has been built for biomedical research applications that allows data from both modalities to be acquired simultaneously. Experiments demonstrate no effect of the MRI system on the spatial resolution of the PET system and <10% reduction in the fraction of radioactive decay events detected by the PET scanner inside the MRI. The signal-to-noise ratio and uniformity of the MR images, with the exception of one particular pulse sequence, were little affected by the presence of the PET scanner. In vivo simultaneous PET and MRI studies were performed in mice. Proof-of-principle in vivo MR spectroscopy and functional MRI experiments were also demonstrated with the combined scanner
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