1,178 research outputs found
Noise characterization of an atomic magnetometer at sub-millihertz frequencies
Noise measurements have been carried out in the LISA bandwidth (0.1 mHz to
100 mHz) to characterize an all-optical atomic magnetometer based on nonlinear
magneto-optical rotation. This was done in order to assess if the technology
can be used for space missions with demanding low-frequency requirements like
the LISA concept. Magnetometry for low-frequency applications is usually
limited by noise and thermal drifts, which become the dominant
contributions at sub-millihertz frequencies. Magnetic field measurements with
atomic magnetometers are not immune to low-frequency fluctuations and
significant excess noise may arise due to external elements, such as
temperature fluctuations or intrinsic noise in the electronics. In addition,
low-frequency drifts in the applied magnetic field have been identified in
order to distinguish their noise contribution from that of the sensor. We have
found the technology suitable for LISA in terms of sensitivity, although
further work must be done to characterize the low-frequency noise in a
miniaturized setup suitable for space missions.Comment: 11 pages, 12 figure
Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains
Collisions of mm-size dust aggregates play a crucial role in the early phases
of planet formation. We developed a laboratory setup to observe collisions of
dust aggregates levitating at mbar pressures and elevated temperatures of 800
K. We report on collisions between basalt dust aggregates of from 0.3 to 5 mm
in size at velocities between 0.1 and 15 cm/s. Individual grains are smaller
than 25 \mum in size. We find that for all impact energies in the studied range
sticking occurs at a probability of 32.1 \pm 2.5% on average. In general, the
sticking probability decreases with increasing impact parameter. The sticking
probability increases with energy density (impact energy per contact area). We
also observe collisions of aggregates that were formed by a previous sticking
of two larger aggregates. Partners of these aggregates can be detached by a
second collision with a probability of on average 19.8 \pm 4.0%. The measured
accretion efficiencies are remarkably high compared to other experimental
results. We attribute this to the rel. large dust grains used in our
experiments, which make aggregates more susceptible to restructuring and energy
dissipation. Collisional hardening by compaction might not occur as the
aggregates are already very compact with only 54 \pm 1% porosity. The
disassembly of previously grown aggregates in collisions might stall further
aggregate growth. However, owing to the levitation technique and the limited
data statistics, no conclusive statement about this aspect can yet be given. We
find that the detachment efficiency decreases with increasing velocities and
accretion dominates in the higher velocity range. For high accretion
efficiencies, our experiments suggest that continued growth in the mm-range
with larger constituent grains would be a viable way to produce larger
aggregates, which might in turn form the seeds to proceed to growing
planetesimals.Comment: 9 pages, 20 figure
Defect engineering of polyethylene-like polyphosphoesters: Solid-state NMR characterization and surface chemistry of anisotropic polymer nanoplatelets
Anisotropic materials with very high aspect ratios, such as nanoplatelets, are an exciting class of materials due to their unique properties based on their unilamellar geometry. Controlling their size and surface-functionality is challenging and has only be achieved in some cases for synthetic polymers. We present a general approach to prepare polymer-nanoplatelets with control over the lateral size and basal functionality, by simple polycondensation of precisely spaced and functional phosphate groups in polyethylene-like polymers. Because of the relatively large size of the phosphate groups, they are expelled from the bulk crystal to the basal surface. This allows to control the chain-folding during crystallization, which we analyzed via solid-state NMR and TEM. Furthermore, we present chemistry “on the surface” of the platelets: the pendant ester group at the phosphate offers the possibility to introduce functional groups accessible for further chemical modification on the crystal surface. This is shown by introducing a 2-acetylthioethyl ester group and subsequently cleaving this 2-acetylthioethyl ester group to the free P–OH. Together, these results render polyethylene-like polyphosphoesters a versatile platform for soft-matter nanoplatelets as functional colloids
Optimal Depth, Very Small Size Circuits for Symmetrical Functions in AC0
AbstractIt is well known which symmetric Boolean functions can be computed by constant depth, polynomial size, unbounded fan-in circuits, i.e., which are contained in the complexity class AC0. This result is sharpened. Symmetric Boolean functions in AC0 can be computed by unbounded fan-in circuits with the following properties. If the optimal depth of AC0-circuits is d, the depth is at most d + 2, the number of wires is almost linear, namely n logO(1)n, and the number of gates is subpolynomial (but superpolylogarithmic), namely 2O(logδn) for some δ < 1
Interfaces Within Graphene Nanoribbons
We study the conductance through two types of graphene nanostructures:
nanoribbon junctions in which the width changes from wide to narrow, and curved
nanoribbons. In the wide-narrow structures, substantial reflection occurs from
the wide-narrow interface, in contrast to the behavior of the much studied
electron gas waveguides. In the curved nanoribbons, the conductance is very
sensitive to details such as whether regions of a semiconducting armchair
nanoribbon are included in the curved structure -- such regions strongly
suppress the conductance. Surprisingly, this suppression is not due to the band
gap of the semiconducting nanoribbon, but is linked to the valley degree of
freedom. Though we study these effects in the simplest contexts, they can be
expected to occur for more complicated structures, and we show results for
rings as well. We conclude that experience from electron gas waveguides does
not carry over to graphene nanostructures. The interior interfaces causing
extra scattering result from the extra effective degrees of freedom of the
graphene structure, namely the valley and sublattice pseudospins.Comment: 19 pages, published version, several references added, small changes
to conclusion
Examining the Gay Panic Defense with an Experimental Design
The gay panic defense (GPD) is a type of provocation defense used in criminal trials for the purpose of mitigating a defendant’s culpability to a jury. The current study utilized chi-square tests and logistic regression to investigate the effects of the GPD on jury decision-making and to assess potential associations of personal characteristics of jurors on verdict selection. Contrary to hypotheses, testing resulted in null findings. Limitations, as well as study strengths, are discussed. Findings suggest that methodology and case details may be pertinent in the empirical investigation of the GPD
RNA-inspired intramolecular transesterification accelerates the hydrolysis of polyethylene-like polyphosphoesters
To synthesize new (bio)degradable alternatives to commodity polymers, adapting natural motives can be a promising approach. We present the synthesis and characterization of degradable polyethylene (PE)-like polyphosphoesters, which exhibit increased degradation rates due to an intra-molecular transesterification similar to RNA. An α,ω-diene monomer was synthesized in three steps starting from readily available compounds. By acyclic diene metathesis (ADMET) polymerization, PE-like polymers with molecular weights up to 38 400 g mol(−1) were obtained. Post-polymerization functionalization gave fully saturated and semicrystalline polymers with a precise spacing of 20 CH(2) groups between each phosphate group carrying an ethoxy hydroxyl side chain. This side chain was capable of intramolecular transesterification with the main-chain similar to RNA-hydrolysis, mimicking the 2′-OH group of ribose. Thermal properties were characterized by differential scanning calorimetry (DSC (T(m)ca. 85 °C)) and the crystal structure was investigated by wide-angle X-ray scattering (WAXS). Polymer films immersed in aqueous solutions at different pH values proved an accelerated degradation compared to structurally similar polyphosphoesters without pendant ethoxy hydroxyl groups. Polymer degradation proceeded also in artificial seawater (pH = 8), while the polymer was stable at physiological pH of 7.4. The degradation mechanism followed the intra-molecular “RNA-inspired” transesterification which was detected by NMR spectroscopy as well as by monitoring the hydrolysis of a polymer blend of a polyphosphoester without pendant OH-group and the RNA-inspired polymer, proving selective hydrolysis of the latter. This mechanism has been further supported by the DFT calculations. The “RNA-inspired” degradation of polymers could play an important part in accelerating the hydrolysis of polymers and plastics in natural environments, e.g. seawater
- …