744 research outputs found
Current oscillations in Vanadium Dioxide: evidence for electrically triggered percolation avalanches
In this work, we experimentally and theoretically explore voltage controlled
oscillations occurring in micro-beams of vanadium dioxide. These oscillations
are a result of the reversible insulator to metal phase transition in vanadium
dioxide. Examining the structure of the observed oscillations in detail, we
propose a modified percolative-avalanche model which allows for
voltage-triggering. This model captures the periodicity and waveshape of the
oscillations as well as several other key features. Importantly, our modeling
shows that while temperature plays a critical role in the vanadium dioxide
phase transition, electrically induced heating cannot act as the primary
instigator of the oscillations in this configuration. This realization leads us
to identify electric field as the most likely candidate for driving the phase
transition
Rhodium-oxide-coated indium tin oxide for enhancement of hole injection in organic light emitting diodes
The authors report the enhancement of hole injection using an RhOx layer between indium tin oxide anodes and 4, 4??? -bis[N-(1-naphtyl)- N -phenyl-amino]biphenyl in organic light-emitting diodes (OLEDs). The operation voltage of OLEDs at 700 cdm2 decreased from 13 to 10 V as the Rh layer changed to RhOx by surface treatment using O2 plasma. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased by 0.2 eV as the Rh layer transformed into RhOx. Thus, the hole injection energy barrier was lowered, reducing the turn-on voltage and increasing the quantum efficiency of OLEDs.open281
Nanoscale imaging of the electronic and structural transitions in vanadium dioxide
We investigate the electronic and structural changes at the nanoscale in
vanadium dioxide (VO2) in the vicinity of its thermally driven phase
transition. Both electronic and structural changes exhibit phase coexistence
leading to percolation. In addition, we observe a dichotomy between the local
electronic and structural transitions. Nanoscale x-ray diffraction reveals
local, non-monotonic switching of the lattice structure, a phenomenon that is
not seen in the electronic insulator-to-metal transition mapped by near-field
infrared microscopy.Comment: 23 pages including 7 figure
Submillimeter Line Emission from LMC 30Dor: The Impact of a Starburst on a Low Metallicity Environment
(Abridged) The 30 Dor region in the Large Magellanic Cloud (LMC) is the most
vigorous star-forming region in the Local Group. Star formation in this region
is taking place in low-metallicity molecular gas that is exposed to an extreme
far--ultraviolet (FUV) radiation field powered by the massive compact star
cluster R136. We used the NANTEN2 telescope to obtain high-angular resolution
observations of the 12CO 4-3, 7-6, and 13CO 4-3 rotational lines and [CI]
3P1-3P0 and 3P2-3P1 fine-structure submillimeter transitions in 30Dor-10, the
brightest CO and FIR-emitting cloud at the center of the 30Dor region. We
derived the properties of the low-metallicity molecular gas using an
excitation/radiative transfer code and found a self-consistent solution of the
chemistry and thermal balance of the gas in the framework of a clumpy cloud PDR
model. We compared the derived properties with those in the N159W region, which
is exposed to a more moderate far-ultraviolet radiation field compared with
30Dor-10, but has similar metallicity. We also combined our CO detections with
previously observed low-J CO transitions to derive the CO spectral-line energy
distribution in 30Dor-10 and N159W. The separate excitation analysis of the
submm CO lines and the neutral carbon fine structure lines shows that the mid-J
CO and [CI]-emitting gas in the 30Dor-10 region has a temperature of about 160
K and a H2 density of about 10^4 cm^-3. We find that the molecular gas in
30Dor-10 is warmer and has a lower beam filling factor compared to that of
N159W, which might be a result of the effect of a strong FUV radiation field
heating and disrupting the low--metallicity molecular gas. We use a clumpy PDR
model (including the [CII] line intensity reported in the literature) to
constrain the FUV intensity to about chi_0 ~ 3100 and an average total H
density of the clump ensemble of about 10^5 cm^-3 in 30Dor-10.Comment: 11 pages, 8 figures. Accepted for publication in A&
The origin of mid-infrared emission in massive young stellar objects: multi-baseline VLTI observations of W33A
The circumstellar structure on 100 AU scales of the massive young stellar
object W33A is probed using the VLTI and the MIDI instrument. N-band
visibilities on 4 baselines are presented which are inconsistent with a
spherically symmetric geometry. The visibility spectra and SED are
simultaneously compared to 2D axi-symmetric dust radiative transfer models with
a geometry including a rotationally flattened envelope and outflow cavities. We
assume an O7.5 ZAMS star as the central source, consistent with the observed
bolometric luminosity. The observations are also compared to models with and
without (dusty and gaseous) accretion disks. A satisfactory model is
constructed which reproduces the visibility spectra for each (u,v) point. It
fits the silicate absorption, the mid-IR slope, the far-infrared peak, and the
(sub)mm of the SED. It produces a 350 micron morphology consistent with
observations. The 10 micron emission on 100 AU scales is dominated by the
irradiated walls of the cavity sculpted by the outflow. The visibilities rule
out the presence of dust disks with total (gas and dust) masses more than 0.01
Msun. However, optically thick accretion disks, interior to the dust
sublimation radius, are allowed to accrete at rates equalling the envelope's
mass infall rate (up to 10^(-3) Msun/yr) without substantially affecting the
visibilities due to the extinction by the extremely massive envelope of W33A.Comment: Accepted for publication in A&
Galaxy Evolution and Star Formation Efficiency at 0.2 < z < 0.6
We present the results of a CO line survey of 30 galaxies at moderate
redshift (z \sim 0.2-0.6), with the IRAM 30m telescope, with the goal to follow
galaxy evolution and in particular the star formation efficiency (SFE) as
defined by the ratio between far-infrared luminosity and molecular gas mass
(LFIR/M(H2)). The sources are selected to be ultra-luminous infrared galaxies
(ULIRGs), with LFIR larger than 2.8 10^{12} Lsol, experiencing starbursts;
adopting a low ULIRG CO-to-H2 conversion factor, their gas consumption
time-scale is lower than 10^8 yr. To date only very few CO observations exist
in this redshift range that spans nearly 25% of the universe's age.
Considerable evolution of the star formation rate is already observed during
this period. 18 galaxies out of our sample of 30 are detected (of which 16 are
new detections), corresponding to a detection rate of 60%. The average CO
luminosity for the 18 galaxies detected is L'CO = 2 10^{10} K km/s pc^2,
corresponding to an average H2 mass of 1.6 10^{10} Msol. The FIR luminosity
correlates well with the CO luminosity, in agreement with the correlation found
for low and high redshift ULIRGs. Although the conversion factor between CO
luminosity and H2 mass is uncertain, we find that the maximum amount of gas
available for a single galaxy is quickly increasing as a function of redshift.
Using the same conversion factor, the SFEs for z\sim 0.2-0.6 ULIRGs are found
to be significantly higher, by a factor 3, than for local ULIRGs, and are
comparable to high redshift ones. We compare this evolution to the expected
cosmic H2 abundance and the cosmic star formation history.Comment: 11 pages, 13 figures, accepted in A&
Sestrin2 Phosphorylation by ULK1 Induces Autophagic Degradation of Mitochondria Damaged by Copper-Induced Oxidative Stress
Selective autolysosomal degradation of damaged mitochondria, also called mitophagy, is an indispensable process for maintaining integrity and homeostasis of mitochondria. One well-established mechanism mediating selective removal of mitochondria under relatively mild mitochondria-depolarizing stress is PINK1-Parkin-mediated or ubiquitin-dependent mitophagy. However, additional mechanisms such as LC3-mediated or ubiquitin-independent mitophagy induction by heavy environmental stress exist and remain poorly understood. The present study unravels a novel role of stress-inducible protein Sestrin2 in degradation of mitochondria damaged by transition metal stress. By utilizing proteomic methods and studies in cell culture and rodent models, we identify autophagy kinase ULK1-mediated phosphorylation sites of Sestrin2 and demonstrate Sestrin2 association with mitochondria adaptor proteins in HEK293 cells. We show that Ser-73 and Ser-254 residues of Sestrin2 are phosphorylated by ULK1, and a pool of Sestrin2 is strongly associated with mitochondrial ATP5A in response to Cu-induced oxidative stress. Subsequently, this interaction promotes association with LC3-coated autolysosomes to induce degradation of mitochondria damaged by Cu-induced ROS. Treatment of cells with antioxidants or a Cu chelator significantly reduces Sestrin2 association with mitochondria. These results highlight the ULK1-Sestrin2 pathway as a novel stress-sensing mechanism that can rapidly induce autophagic degradation of mitochondria under severe heavy metal stress
Spectral Energy Distributions of 6.7 GHz methanol masers
The 6.7 GHz maser transition of methanol has been found exclusively towards
massive star forming regions. A majority of the masers have been found to lack
the presence of any associated radio continuum. This could be due to the maser
emission originating prior to the formation of an HII region around the central
star, or from the central object being too cool to produce a HII region. One
way to distinguish between the two scenarios is to determine and model the
spectral energy distributions (SEDs) of the masers. We observed a sample of 20
6.7 GHz methanol masers selected from the blind Arecibo survey, from centimeter
to submillimeter wavelengths. We combined our observations with existing data
from various Galactic plane surveys to determine SEDs from centimeter to
near-infrared wavelengths. We find that 70% of the masers do not have any
associated radio continuum, with the rest of the sources being associated with
hypercompact and ultracompact HII regions. Modeling the SEDs shows them to be
consistent with rapidly accreting massive stars, with accretion rates well
above 10^{-3} M_sun/yr. The upper limits on the radio continuum are also
consistent with any ionized region being confined close to the stellar surface.
This confirms the paradigm of 6.7 GHz methanol masers being signposts of early
phases of massive star formation, mostly prior to the formation of a
hypercompact HII region.Comment: 15 pages, 4 figures; Accepted by A&
Observation of Mott Transition in VO_2 Based Transistors
An abrupt Mott metal-insulator transition (MIT) rather than the continuous
Hubbard MIT near a critical on-site Coulomb energy U/U_c=1 is observed for the
first time in VO_2, a strongly correlated material, by inducing holes of about
0.018% into the conduction band. As a result, a discontinuous jump of the
density of states on the Fermi surface is observed and inhomogeneity inevitably
occurs. The gate effect in fabricated transistors is clear evidence that the
abrupt MIT is induced by the excitation of holes.Comment: 4 pages, 4 figure
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