27,662 research outputs found
Weak values and the Leggett-Garg inequality in solid-state qubits
An implementation of weak values is investigated in solid-state qubits. We
demonstrate that a weak value can be non-classical if and only if a
Leggett-Garg inequality can also be violated. Generalized weak values are
described, where post-selection on a range of weak measurement results.
Imposing classical weak values permits the derivation of Leggett-Garg
inequalities for bounded operators. Our analysis is presented in terms of
kicked quantum nondemolition measurements on a quantum double-dot charge qubit.Comment: 4 pages, 2 figure
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
The progenitors of magnetic white dwarfs in open clusters
According to the fossil-field hypothesis magnetic fields are remnants of the
previous stages of evolution. However, population synthesis calculations are
unable to reproduce the magnetic white dwarf (MWD) sample without binary
interaction or inclusion of a population of progenitor with unobservable
small-scale fields. One necessary ingredient in population synthesis is the
initial-to-final-mass relation (IFMR) which describes the mass-loss processes
during the stellar evolution. When white dwarfs are members of open clusters,
their evolutionary histories can be assessed through the use of cluster
properties. In this work, we assess the cluster membership by correlating the
proper-motion of MWDs with the cluster proper-motion and by analyzing the
candidates spectroscopically with our magnetic model spectra in order to
estimate the effective temperature and radii. We identified SDSS
J085523.87+164059.0 to be a proper-motion member of Praesepe. We also included
the data of the formerly identified cluster members NGC 6819-8, WD 0836+201 and
estimated the mass, cooling age and the progenitor masses of the three probable
MWD members of open clusters. According to our analysis, the newly identified
cluster member SDSS J085523.87+164059.0 is an ultra-massive MWD of mass 1.12
0.11 Msolar. We increase the sample of MWDs with known progenitor masses
to ten, with the rest of the data coming from the common proper motion
binaries. Our investigations show that, when effects of the magnetic fields are
included in the diagnostics, the estimated properties of these cluster MWDs do
not show evidence for deviations from the IFMR. Furthermore, we estimate the
precision of the magnetic diagnostics which would be necessary to determine
quantitatively whether magnetism has any effect on the mass-loss.Comment: 8 pages, 4 figures, accepted for publication in A&
The magnetic fields of hot subdwarf stars
Detection of magnetic fields has been reported in several sdO and sdB stars.
Recent literature has cast doubts on the reliability of most of these
detections. We revisit data previously published in the literature, and we
present new observations to clarify the question of how common magnetic fields
are in subdwarf stars. We consider a sample of about 40 hot subdwarf stars.
About 30 of them have been observed with the FORS1 and FORS2 instruments of the
ESO VLT. Here we present new FORS1 field measurements for 17 stars, 14 of which
have never been observed for magnetic fields before. We also critically review
the measurements already published in the literature, and in particular we try
to explain why previous papers based on the same FORS1 data have reported
contradictory results. All new and re-reduced measurements obtained with FORS1
are shown to be consistent with non-detection of magnetic fields. We explain
previous spurious field detections from data obtained with FORS1 as due to a
non-optimal method of wavelength calibration. Field detections in other surveys
are found to be uncertain or doubtful, and certainly in need of confirmation.
There is presently no strong evidence for the occurrence of a magnetic field in
any sdB or sdO star, with typical longitudinal field uncertainties of the order
of 2-400 G. It appears that globally simple fields of more than about 1 or 2 kG
in strength occur in at most a few percent of hot subdwarfs, and may be
completely absent at this strength. Further high-precision surveys, both with
high-resolution spectropolarimeters and with instruments similar to FORS1 on
large telescopes, would be very valuable
Analysis of the Hydrogen-rich Magnetic White Dwarfs in the SDSS
We have calculated optical spectra of hydrogen-rich (DA) white dwarfs with
magnetic field strengths between 1 MG and 1000 MG for temperatures between 7000
K and 50000 K. Through a least-squares minimization scheme with an evolutionary
algorithm, we have analyzed the spectra of 114 magnetic DAs from the SDSS (95
previously published plus 14 newly discovered within SDSS, and five discovered
by SEGUE). Since we were limited to a single spectrum for each object we used
only centered magnetic dipoles or dipoles which were shifted along the magnetic
dipole axis. We also statistically investigated the distribution of
magnetic-field strengths and geometries of our sample.Comment: to appear in the proceedings of the 16th European Workshop on White
Dwarfs, Barcelona, 200
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
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