23,948 research outputs found
Probing Light Atoms at Sub-nanometer Resolution: Realization of Scanning Transmission Electron Microscope Holography
Atomic resolution imaging in transmission electron microscopy (TEM) and
scanning TEM (STEM) of light elements in electron-transparent materials has
long been a challenge. Biomolecular materials, for example, are rapidly altered
when illuminated with electrons. These issues have driven the development of
TEM and STEM techniques that enable the structural analysis of electron
beam-sensitive and weakly scattering nano-materials. Here, we demonstrate such
a technique, STEM holography, capable of absolute phase and amplitude object
wave measurement with respect to a vacuum reference wave. We use an
amplitude-dividing nanofabricated grating to prepare multiple spatially
separated electron diffraction probe beams focused at the sample plane, such
that one beam transmits through the specimen while the others pass through
vacuum. We raster-scan the diffracted probes over the region of interest. We
configure the post specimen imaging system of the microscope to diffraction
mode, overlapping the probes to form an interference pattern at the detector.
Using a fast-readout, direct electron detector, we record and analyze the
interference fringes at each position in a 2D raster scan to reconstruct the
complex transfer function of the specimen, t(x). We apply this technique to
image a standard target specimen consisting of gold nanoparticles on a thin
amorphous carbon substrate, and demonstrate 2.4 angstrom resolution phase
images. We find that STEM holography offers higher phase-contrast of the
amorphous material while maintaining Au atomic lattice resolution when compared
with high angle annular dark field STEM.Comment: 9 pages, 5 figures in main text, 1 supplemental figure in the
appendi
Discovery of kilogauss magnetic fields in three DA white dwarfs
We have detected longitudinal magnetic fields between 2 and 4 kG in three (WD
0446790, WD 1105048, WD 2359434) out of a sample of 12 normal DA white
dwarfs by using optical spectropolarimetry done with the VLT Antu 8 m telescope
equipped with FORS1. With the exception of 40 Eri B (4 kG) these are the first
positive detections of magnetic fields in white dwarfs below 30 kG. Although
suspected, it was not clear whether a significant fraction of white dwarfs
contain magnetic fields at this level. These fields may be explained as fossil
relics from magnetic fields in the main-sequence progenitors considerably
enhanced by magnetic flux conservation during the shrinkage of the core. A
detection rate of 25 % (3/12) may indicate now for the first time that a
substantial fraction of white dwarfs have a weak magnetic field. This result,
if confirmed by future observations, would form a cornerstone for our
understanding on the evolution of stellar magnetic fields.
Keywords: stars: white dwarfs - stars: magnetic fields - stars: individual:
WD0446-790, WD1105-048, WD2359-434Comment: 15 pages, 7 figures, Astronomy and Astrophysics, in pres
Using HINODE/Extreme-Ultraviolet Imaging Spectrometer to confirm a seismologically inferred coronal temperature
The Extreme-Ultraviolet Imaging Spectrometer on board the HINODE satellite is used to examine the loop system described in Marsh et al. (2009) by applying spectroscopic diagnostic methods. A simple isothermal mapping algorithm is applied to determine where the assumption of isothermal plasma may be valid, and the emission measure locii technique is used to determine the temperature profile along the base of the loop system. It is found that, along the base, the loop has a uniform temperature profile with a mean temperature of 0.89 +- 0.09 MK which is in agreement with the temperature determined seismologically in Marsh et al. (2009), using observations interpreted as the slow magnetoacoustic mode. The results further strengthen the slow mode interpretation, propagation at a uniform sound speed, and the analysis method applied in Marsh et al. (2009). It is found that it is not possible to discriminate between the slow mode phase speed and the sound speed within the precision of the present observations
Phase-resolved far-ultraviolet HST spectroscopy of the peculiar magnetic white dwarf RE J0317-853
We present phase resolved FUV HST FOS spectra of the rapidly rotating, highly
magnetic white dwarf RE J0317-853. Using these data, we construct a new model
for the magnetic field morphology across the stellar surface. From an expansion
into spherical harmonics, we find the range of magnetic field strengths present
is 180-800MG. For the first time we could identify an absorption feature
present at certain phases at 1160A as a ``forbidden'' 1s_0 -> 2s_0 component,
due to the combined presence of an electric and magnetic field.Comment: 15 pages including 4 figures. Accepted for publication in ApJ Letter
Quantum Discord and Quantum Computing - An Appraisal
We discuss models of computing that are beyond classical. The primary
motivation is to unearth the cause of nonclassical advantages in computation.
Completeness results from computational complexity theory lead to the
identification of very disparate problems, and offer a kaleidoscopic view into
the realm of quantum enhancements in computation. Emphasis is placed on the
`power of one qubit' model, and the boundary between quantum and classical
correlations as delineated by quantum discord. A recent result by Eastin on the
role of this boundary in the efficient classical simulation of quantum
computation is discussed. Perceived drawbacks in the interpretation of quantum
discord as a relevant certificate of quantum enhancements are addressed.Comment: To be published in the Special Issue of the International Journal of
Quantum Information on "Quantum Correlations: entanglement and beyond." 11
pages, 4 figure
Combinatorial synthesis and high-throughput photopotential and photocurrent screening of mixed-metal oxides for photoelectrochemical water splitting
A high-throughput method has been developed using a commercial piezoelectric inkjet printer for synthesis and characterization of mixed-metal oxide photoelectrode materials for water splitting. The printer was used to deposit metal nitrate solutions onto a conductive glass substrate. The deposited metal nitrate solutions were then pyrolyzed to yield mixed-metal oxides that contained up to eight distinct metals. The stoichiometry of the metal oxides was controlled quantitatively, allowing for the creation of vast libraries of novel materials. Automated methods were developed to measure the open-circuit potentials (Eoc), short-circuit photocurrent densities (Jsc), and current density vs. applied potential (JâE) behavior under visible light irradiation. The high-throughput measurement of Eoc is particularly significant because open-circuit potential measurements allow the interfacial energetics to be probed regardless of whether the band edges of the materials of concern are above, close to, or below the values needed to sustain water electrolysis under standard conditions. The Eoc measurements allow high-throughput compilation of a suite of data that can be associated with the composition of the various materials in the library, to thereby aid in the development of additional screens and to form a basis for development of theoretical guidance in the prediction of additional potentially promising photoelectrode compositions
An astronomical search for evidence of new physics: Limits on gravity-induced birefringence from the magnetic white dwarf RE J0317-853
The coupling of the electromagnetic field directly with gravitational gauge
fields leads to new physical effects that can be tested using astronomical
data. Here we consider a particular case for closer scrutiny, a specific
nonminimal coupling of torsion to electromagnetism, which enters into a
metric-affine geometry of space-time. We show that under the assumption of this
nonminimal coupling, spacetime is birefringent in the presence of such a
gravitational field. This leads to the depolarization of light emitted from
extended astrophysical sources. We use polarimetric data of the magnetic white
dwarf to set strong constraints on the essential coupling
constant for this effect, giving k^2 \lsim (19 {m})^2 .Comment: Statements about Moffat's NGT modified. Accepted for publication in
Phys.Rev.
QUASAT: An orbiting very long baseline interferometer program using large space antenna systems
QUASAT, which stands for QUASAR SATELLITE, is the name given to a new mission being studied by NASA. The QUASAT mission concept involves a free flying Earth orbiting large radio telescope, which will observe astronomical radio sources simultaneously with ground radio telescopes. The primary goal of QUASAT is to provide a system capable of collecting radio frequency data which will lead to a better understanding of extremely high energy events taking place in a variety of celestial objects including quasars, galactic nuclei, interstellar masers, radio stars and pulsars. QUASAT's unique scientific contribution will be the increased resolution in the emission brightness profile maps of the celestial objects
Assessment worlds colliding? Negotiating between discourses of assessment on an online open course
Using the badged open course, Taking your first steps into Higher Education, this case study examines how assessment on online open courses draws on concepts of assessment used within formal and informal learning. Our experience was that assessment used within open courses, such as massive open online courses, is primarily determined by the requirements of quality assurance processes to award a digital badge or statement of participation as well as what is technologically possible. However, this disregards much recent work in universities that use assessment in support of learning. We suggest that designers of online open courses should pay greater attention to the relationship of assessment and learning to improve participant course completion
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