409 research outputs found
Mechanical Decoupling of Quantum Emitters in Hexagonal Boron Nitride from Low-Energy Phonon Modes
Quantum emitters in hexagonal Boron Nitride (hBN) were recently reported to
hol a homogeneous linewidth according to the Fourier-Transform limit up to room
temperature. This unusual observation was traced back to decoupling from
in-plane phonon modes which can arise if the emitter is located between two
planes of the hBN host material. In this work, we investigate the origins for
the mechanical decoupling. Improved sample preparation enabled a reduced
background and a 70-fold decrease of spectral diffusion which was so far the
major drawback of defect center in hBN and allowed us to reveal a gap in the
electron-phonon spectral density for low phonon frequencies. This decoupling
from phonons persists at room temperature and explains the observed Fourier
Transform limited lines up to 300K. Furthermore, we investigate the dipole
emission directionality and show a preferred photon emission through the side
of the hBN flakes supporting the claim for an out-of-plane distortion of the
defect center. Our work lays the foundation to a deeper understanding of the
underlying physics for the persistence of Fourier-Transform limit lines up to
room temperature. It furthermore provides a description on how to identify the
mechanically isolated emitter within the large number of defect centers in hBN.
Therefore, it paves the way for quantum optics applications with defect centers
in hBN at room temperature.Comment: 9 pages, 5 figure
Evaluation of a temporary vaccination recommendation in response to an outbreak of invasive meningococcal serogroup C disease in men who have sex with men in Berlin, 2013-2014
Meningococcal serogroup C (MenC) vaccination of men who have sex with men
(MSM) was temporarily recommended to control an outbreak of invasive MenC
disease among MSM in Berlin in 2012–2013. Vaccination was offered to HIV-
infected MSM free of charge; others had to request reimbursement or pay out of
pocket. We aimed to assess (i) awareness and acceptance of this recommendation
through an online survey of MSM, (ii) implementation through a survey of
primary care physicians and analysis of vaccine prescriptions, and (iii)
impact through analysis of notified cases. Among online survey respondents,
60% were aware of the recommendation. Of these, 39% had obtained vaccination
(70% of HIV-infected, 13% of HIV-negative/non-tested MSM). Awareness of
recommendation and vaccination were positively associated with HIV infection,
primary care physicians’ awareness of respondents’ sexual orientation, and
exposure to multiple information sources. Most (26/30) physicians informed
clients about the recommendation. Physicians considered concerns regarding
reimbursement, vaccine safety and lack of perceived disease risk as primary
barriers. After the recommendation, no further outbreak-related cases
occurred. To reach and motivate target groups, communication of a new
outbreak-related vaccination recommendation should address potential concerns
through as many information channels as possible and direct reimbursement of
costs should be enabled
Five Kepler target stars that show multiple transiting exoplanet candidates
We present and discuss five candidate exoplanetary systems identified with
the Kepler spacecraft. These five systems show transits from multiple exoplanet
candidates. Should these objects prove to be planetary in nature, then these
five systems open new opportunities for the field of exoplanets and provide new
insights into the formation and dynamical evolution of planetary systems. We
discuss the methods used to identify multiple transiting objects from the
Kepler photometry as well as the false-positive rejection methods that have
been applied to these data. One system shows transits from three distinct
objects while the remaining four systems show transits from two objects. Three
systems have planet candidates that are near mean motion
commensurabilities---two near 2:1 and one just outside 5:2. We discuss the
implications that multitransiting systems have on the distribution of orbital
inclinations in planetary systems, and hence their dynamical histories; as well
as their likely masses and chemical compositions. A Monte Carlo study indicates
that, with additional data, most of these systems should exhibit detectable
transit timing variations (TTV) due to gravitational interactions---though none
are apparent in these data. We also discuss new challenges that arise in TTV
analyses due to the presence of more than two planets in a system.Comment: Accepted to Ap
Winner-take-all selection in a neural system with delayed feedback
We consider the effects of temporal delay in a neural feedback system with
excitation and inhibition. The topology of our model system reflects the
anatomy of the avian isthmic circuitry, a feedback structure found in all
classes of vertebrates. We show that the system is capable of performing a
`winner-take-all' selection rule for certain combinations of excitatory and
inhibitory feedback. In particular, we show that when the time delays are
sufficiently large a system with local inhibition and global excitation can
function as a `winner-take-all' network and exhibit oscillatory dynamics. We
demonstrate how the origin of the oscillations can be attributed to the finite
delays through a linear stability analysis.Comment: 8 pages, 6 figure
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Self-Assembly of Polymer-Modified FePt Magnetic Nanoparticles and Block Copolymers
The fabrication of nanocomposites containing magnetic nanoparticles is gaining interest as a model for application in small electronic devices. The self-assembly of block copolymers (BCPs) makes these materials ideal for use as a soft matrix to support the structural ordering of the nanoparticles. In this work, a high-molecular-weight polystyrene-b-poly(methyl methacrylate) block copolymer (PS-b-PMMA) was synthesized through anionic polymerization. The influence of the addition of different ratios of PMMA-coated FePt nanoparticles (NPs) on the self-assembled morphology was investigated using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). The self-assembly of the NPs inside the PMMA phase at low particle concentrations was analyzed statistically, and the negative effect of higher particle ratios on the lamellar BCP morphology became visible. The placement of the NPs inside the PMMA phase was also compared to theoretical descriptions. The magnetic addressability of the FePt nanoparticles inside the nanocomposite films was finally analyzed using bimodal magnetic force microscopy and proved the magnetic nature of the nanoparticles inside the microphase-separated BCP films
Self-Assembly of Polymer-Modified FePt Magnetic Nanoparticles and Block Copolymers
The fabrication of nanocomposites containing magnetic nanoparticles is gaining interest as a
model for application in small electronic devices. The self-assembly of block copolymers (BCPs) makes
these materials ideal for use as a soft matrix to support the structural ordering of the nanoparticles.
In this work, a high-molecular-weight polystyrene-b-poly(methyl methacrylate) block copolymer (PSb-PMMA) was synthesized through anionic polymerization. The influence of the addition of different
ratios of PMMA-coated FePt nanoparticles (NPs) on the self-assembled morphology was investigated
using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). The selfassembly of the NPs inside the PMMA phase at low particle concentrations was analyzed statistically,
and the negative effect of higher particle ratios on the lamellar BCP morphology became visible. The
placement of the NPs inside the PMMA phase was also compared to theoretical descriptions. The
magnetic addressability of the FePt nanoparticles inside the nanocomposite films was finally analyzed
using bimodal magnetic force microscopy and proved the magnetic nature of the nanoparticles inside
the microphase-separated BCP films
Two Earth-sized planets orbiting Kepler-20
Since the discovery of the first extrasolar giant planets around Sun-like
stars, evolving observational capabilities have brought us closer to the
detection of true Earth analogues. The size of an exoplanet can be determined
when it periodically passes in front of (transits) its parent star, causing a
decrease in starlight proportional to its radius. The smallest exoplanet
hitherto discovered has a radius 1.42 times that of the Earth's radius (R
Earth), and hence has 2.9 times its volume. Here we report the discovery of two
planets, one Earth-sized (1.03R Earth) and the other smaller than the Earth
(0.87R Earth), orbiting the star Kepler-20, which is already known to host
three other, larger, transiting planets. The gravitational pull of the new
planets on the parent star is too small to measure with current
instrumentation. We apply a statistical method to show that the likelihood of
the planetary interpretation of the transit signals is more than three orders
of magnitude larger than that of the alternative hypothesis that the signals
result from an eclipsing binary star. Theoretical considerations imply that
these planets are rocky, with a composition of iron and silicate. The outer
planet could have developed a thick water vapour atmosphere.Comment: Letter to Nature; Received 8 November; accepted 13 December 2011;
Published online 20 December 201
Point-Contact Conductances at the Quantum Hall Transition
On the basis of the Chalker-Coddington network model, a numerical and
analytical study is made of the statistics of point-contact conductances for
systems in the integer quantum Hall regime. In the Hall plateau region the
point-contact conductances reflect strong localization of the electrons, while
near the plateau transition they exhibit strong mesoscopic fluctuations. By
mapping the network model on a supersymmetric vertex model with GL(2|2)
symmetry, and postulating a two-point correlator in keeping with the rules of
conformal field theory, we derive an explicit expression for the distribution
of conductances at criticality. There is only one free parameter, the power law
exponent of the typical conductance. Its value is computed numerically to be
X_t = 0.640 +/- 0.009. The predicted conductance distribution agrees well with
the numerical data. For large distances between the two contacts, the
distribution can be described by a multifractal spectrum solely determined by
X_t. Our results demonstrate that multifractality can show up in appropriate
transport experiments.Comment: 18 pages, 15 figures included, revised versio
Kepler-21b: A 1.6REarth Planet Transiting the Bright Oscillating F Subgiant Star HD 179070
We present Kepler observations of the bright (V=8.3), oscillating star HD
179070. The observations show transit-like events which reveal that the star is
orbited every 2.8 days by a small, 1.6 R_Earth object. Seismic studies of HD
179070 using short cadence Kepler observations show that HD 179070 has a
frequencypower spectrum consistent with solar-like oscillations that are
acoustic p-modes. Asteroseismic analysis provides robust values for the mass
and radius of HD 179070, 1.34{\pm}0.06 M{\circ} and 1.86{\pm}0.04 R{\circ}
respectively, as well as yielding an age of 2.84{\pm}0.34 Gyr for this F5
subgiant. Together with ground-based follow-up observations, analysis of the
Kepler light curves and image data, and blend scenario models, we
conservatively show at the >99.7% confidence level (3{\sigma}) that the transit
event is caused by a 1.64{\pm}0.04 R_Earth exoplanet in a 2.785755{\pm}0.000032
day orbit. The exoplanet is only 0.04 AU away from the star and our
spectroscopic observations provide an upper limit to its mass of ~10 M_Earth
(2-{\sigma}). HD 179070 is the brightest exoplanet host star yet discovered by
Kepler.Comment: Accepted to Ap
Ageing-associated small RNA cargo of extracellular vesicles
Previous work on murine models and humans demonstrated global as well as tissue-specific molecular
ageing trajectories of RNAs. Extracellular vesicles (EVs) are membrane vesicles mediating the horizontal
transfer of genetic information between different tissues. We sequenced small regulatory RNAs
(sncRNAs) in two mouse plasma fractions at five time points across the lifespan from 2–18 months: (1)
sncRNAs that are free-circulating (fc-RNA) and (2) sncRNAs bound outside or inside EVs (EV-RNA).
Different sncRNA classes exhibit unique ageing patterns that vary between the fcRNA and EV-RNA
fractions. While tRNAs showed the highest correlation with ageing in both fractions, rRNAs exhibited
inverse correlation trajectories between the EV- and fc-fractions. For miRNAs, the EV-RNA fraction was
exceptionally strongly associated with ageing, especially the miR-29 family in adipose tissues.
Sequencing of sncRNAs and coding genes in fat tissue of an independent cohort of aged mice up to
27 months highlighted the pivotal role of miR-29a-3p and miR-29b-3p in ageing-related gene regulation
that we validated in a third cohort by RT-qPCR
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