793 research outputs found
On the Proof of Dark Matter, the Law of Gravity and the Mass of Neutrinos
We develop a new method to predict the density associated with weak lensing
maps of (un)relaxed clusters in a range of theories interpolating between GR
and MOND (General Relativity and Modified Newtonian Dynamics). We apply it to
fit the lensing map of the bullet merging cluster 1E0657-56, in order to
constrain more robustly the nature and amount of collisionless matter in
clusters {\it beyond} the usual assumption of spherical equilibrium
(Pointecouteau & Silk 2005) and the validity of GR on cluster scales (Clowe et
al. 2006). Strengthening the proposal of previous authors we show that the
bullet cluster is dominated by a collisionless -- most probably non-baryonic --
component in GR as well as in MOND, a result consistent with the dynamics of
many X-ray clusters. Our findings add to the number of known pathologies for a
purely baryonic MOND, including its inability to fit the latest data from the
Wilkinson Microwave Anisotropy Probe. A plausible resolution of all these
issues and standard issues of Cold Dark Matter with galaxy rotation curves is
the "marriage" of MOND with ordinary hot neutrinos of 2eV. This prediction is
just within the GR-independent maximum of neutrino mass from current
-decay experiments, and is falsifiable by the Karlsruhe Tritium Neutrino
(KATRIN) experiment by 2009. Issues of consistency with strong lensing arcs and
the large relative velocity of the two clusters comprising the bullet cluster
are also addressed.Comment: 4 pages, 1 figure, accepted for publication in ApJL. Added a simple
model of the bullet cluster's high velocity in TeVeS, and discussions of
sterile neutrinos and of non-uniqueness of the lensing deprojectio
Spatial-Temporal Imaging of Anisotropic Photocarrier Dynamics in Black Phosphorus
As an emerging single elemental layered material with a low symmetry in-plane
crystal lattice, black phosphorus (BP) has attracted significant research
interest owing to its unique electronic and optoelectronic properties,
including its widely tunable bandgap, polarization dependent photoresponse and
highly anisotropic in-plane charge transport. Despite extensive study of the
steady-state charge transport in BP, there has not been direct characterization
and visualization of the hot carriers dynamics in BP immediately after
photoexcitation, which is crucial to understanding the performance of BP-based
optoelectronic devices. Here we use the newly developed scanning ultrafast
electron microscopy (SUEM) to directly visualize the motion of photo-excited
hot carriers on the surface of BP in both space and time. We observe highly
anisotropic in-plane diffusion of hot holes, with a 15-times higher diffusivity
along the armchair (x-) direction than that along the zigzag (y-) direction.
Our results provide direct evidence of anisotropic hot carrier transport in BP
and demonstrate the capability of SUEM to resolve ultrafast hot carrier
dynamics in layered two-dimensional materials.Comment: 21 pages, 6 figure
Dirac-Electrons-Mediated Magnetic Proximity Effect in Topological Insulator / Magnetic Insulator Heterostructures
The possible realization of dissipationless chiral edge current in a
topological insulator / magnetic insulator heterostructure is based on the
condition that the magnetic proximity exchange coupling at the interface is
dominated by the Dirac surface states of the topological insulator. Here we
report a polarized neutron reflectometry observation of Dirac electrons
mediated magnetic proximity effect in a bulk-insulating topological insulator
(BiSb)Te / magnetic insulator EuS heterostructure.
We are able to maximize the proximity induced magnetism by applying an
electrical back gate to tune the Fermi level of topological insulator to be
close to the charge neutral point. A phenomenological model based on
diamagnetic screening is developed to explain the suppressed proximity induced
magnetism at high carrier density. Our work paves the way to utilize the
magnetic proximity effect at the topological insulator/magnetic insulator
hetero-interface for low-power spintronic applications.Comment: 5 pages main text with 4 figures; 2 pages supplemental materials;
suggestions and discussions are welcome
Highly efficient visible colloidal lead-halide perovskite nanocrystal light-emitting diodes
Lead-halide perovskites have been attracting attention for potential use in
solid-state lighting. Following the footsteps of solar cells, the field of
perovskite light-emitting diodes (PeLEDs) has been growing rapidly. Their
application prospects in lighting, however, remain still uncertain due to a
variety of shortcomings in device performance including their limited levels of
luminous efficiency achievable thus far. Here we show high-efficiency PeLEDs
based on colloidal perovskite nanocrystals (PeNCs) synthesized at room
temperature possessing dominant first-order excitonic radiation (enabling a
photoluminescence quantum yield of 71% in solid film), unlike in the case of
bulk perovskites with slow electron-hole bimolecular radiative recombination (a
second-order process). In these PeLEDs, by reaching charge balance in the
recombination zone, we find that the Auger nonradiative recombination, with its
significant role in emission quenching, is effectively suppressed in low
driving current density range. In consequence, these devices reach a record
high maximum external quantum efficiency of 12.9% reported to date and an
unprecedentedly high power efficiency of 30.3 lm W-1 at luminance levels above
1000 cd m-2 as required for various applications. These findings suggest that,
with feasible levels of device performance, the PeNCs hold great promise for
their use in LED lighting and displays
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Decimetre-scale multicellular eukaryotes from the 1.56-billion-year-old Gaoyuzhuang Formation in North China
Fossils of macroscopic eukaryotes are rarely older than the Ediacaran Period (635–541 million years (Myr)), and their interpretation remains controversial. Here, we report the discovery of macroscopic fossils from the 1,560-Myr-old Gaoyuzhuang Formation, Yanshan area, North China, that exhibit both large size and regular morphology. Preserved as carbonaceous compressions, the Gaoyuzhuang fossils have statistically regular linear to lanceolate shapes up to 30 cm long and nearly 8 cm wide, suggesting that the Gaoyuzhuang fossils record benthic multicellular eukaryotes of unprecedentedly large size. Syngenetic fragments showing closely packed ∼10 μm cells arranged in a thick sheet further reinforce the interpretation. Comparisons with living thalloid organisms suggest that these organisms were photosynthetic, although their phylogenetic placement within the Eukarya remains uncertain. The new fossils provide the strongest evidence yet that multicellular eukaryotes with decimetric dimensions and a regular developmental program populated the marine biosphere at least a billion years before the Cambrian Explosion
High-Efficiency Light-Emitting Diodes of Organometal Halide Perovskite Amorphous Nanoparticles
Organometal halide perovskite has recently emerged as a very promising family of materials with augmented performance in electronic and optoelectronic applications including photovoltaic devices, photodetectors, and light-emitting diodes. Herein, we propose and demonstrate facile solution synthesis of a series of colloidal organometal halide perovskite CH3NH3PbX3 (X = halides) nanoparticles with amorphous structure, which exhibit high quantum yield and tunable emission from ultraviolet to near-infrared. The growth mechanism and photoluminescence properties of the perovskite amorphous nanoparticles were studied in detail. A high-efficiency green-light-emitting diode based on amorphous CH3NH3PbBr3 nanoparticles was demonstrated. The perovskite amorphous nanoparticle-based light-emitting diode shows a maximum luminous efficiency of 11.49 cd/A, a power efficiency of 7.84 lm/W, and an external quantum efficiency of 3.8%, which is 3.5 times higher than that of the best colloidal perovskite quantum-dot-based light-emitting diodes previously reported. Our findings indicate the great potential of colloidal perovskite amorphous nanoparticles in light-emitting devices. © 2016 American Chemical Society
Increased Hepatic Insulin Action in Diet-Induced Obese Mice Following Inhibition of Glucosylceramide Synthase
Obesity is characterized by the accumulation of fat in the liver and other tissues, leading to insulin resistance. We have previously shown that a specific inhibitor of glucosylceramide synthase, which inhibits the initial step in the synthesis of glycosphingolipids (GSLs), improved glucose metabolism and decreased hepatic steatosis in both ob/ob and diet-induced obese (DIO) mice. Here we have determined in the DIO mouse model the efficacy of a related small molecule compound, Genz-112638, which is currently being evaluated clinically for the treatment of Gaucher disease, a lysosomal storage disorder.DIO mice were treated with the Genz-112638 for 12 to 16 weeks by daily oral gavage. Genz-112638 lowered HbA1c levels and increased glucose tolerance. Whole body adiposity was not affected in normal mice, but decreased in drug-treated obese mice. Drug treatment also significantly lowered liver triglyceride levels and reduced the development of hepatic steatosis. We performed hyperinsulinemic-euglycemic clamps on the DIO mice treated with Genz-112638 and showed that insulin-mediated suppression of hepatic glucose production increased significantly compared to the placebo treated mice, indicating a marked improvement in hepatic insulin sensitivity.These results indicate that GSL inhibition in obese mice primarily results in an increase in insulin action in the liver, and suggests that GSLs may have an important role in hepatic insulin resistance in conditions of obesity
-scaling and Information Entropy in Ultra-Relativistic Nucleus-Nucleus Collisions
The -scaling method has been applied to ultra-relativistic p+p, C+C
and Pb+Pb collision data simulated using a high energy Monte Carlo package,
LUCIAE 3.0. The -scaling is found to be valid for some physical
variables, such as charged particle multiplicity, strange particle multiplicity
and number of binary nucleon-nucleon collisions from these simulated
nucleus-nucleus collisions over an extended energy ranging from = 20
to 200 A GeV. In addition we derived information entropy from the multiplicity
distribution as a function of beam energy for these collisions.Comment: 4 pages, 4 figures, 1 table; to appear in the July Issue of Chin.
Phys. Lett.. Web Page: http://www.iop.org/EJ/journal/CP
Correlated Excitonic Signatures in a Nanoscale van der Waals Antiferromagnet
Composite quasi-particles with emergent functionalities in spintronic and
quantum information science can be realized in correlated materials due to
entangled charge, spin, orbital, and lattice degrees of freedom. Here we show
that by reducing the lateral dimension of correlated antiferromagnet NiPS3
flakes to tens of nanometers, we can switch-off the bulk spin-orbit entangled
exciton in the near-infrared (1.47 eV) and activate visible-range (1.8 to 2.2
eV) transitions with charge-transfer character. These ultra-sharp lines (<120
ueV at 4.2 K) share the spin-correlated nature of the bulk exciton by
displaying a Neel temperature dependent linear polarization. Furthermore,
exciton photoluminescence lineshape analysis reveals a polaronic character via
coupling with at-least 3 phonon modes and a comb-like Stark effect through
discretization of charges in each layer. These findings augment the knowledge
on the many-body nature of excitonic quasi-particles in correlated
antiferromagnets and also establish the nanoscale platform as promising for
maturing integrated magneto-optic devices
Constructing non-stationary Dynamic Bayesian Networks with a flexible lag choosing mechanism
<p>Abstract</p> <p>Background</p> <p>Dynamic Bayesian Networks (DBNs) are widely used in regulatory network structure inference with gene expression data. Current methods assumed that the underlying stochastic processes that generate the gene expression data are stationary. The assumption is not realistic in certain applications where the intrinsic regulatory networks are subject to changes for adapting to internal or external stimuli.</p> <p>Results</p> <p>In this paper we investigate a novel non-stationary DBNs method with a potential regulator detection technique and a flexible lag choosing mechanism. We apply the approach for the gene regulatory network inference on three non-stationary time series data. For the Macrophages and Arabidopsis data sets with the reference networks, our method shows better network structure prediction accuracy. For the Drosophila data set, our approach converges faster and shows a better prediction accuracy on transition times. In addition, our reconstructed regulatory networks on the Drosophila data not only share a lot of similarities with the predictions of the work of other researchers but also provide many new structural information for further investigation.</p> <p>Conclusions</p> <p>Compared with recent proposed non-stationary DBNs methods, our approach has better structure prediction accuracy By detecting potential regulators, our method reduces the size of the search space, hence may speed up the convergence of MCMC sampling.</p
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