22,435 research outputs found
Monotonicity results and bounds for the inverse hyperbolic sine
In this note, we present monotonicity results of a function involving to the
inverse hyperbolic sine. From these, we derive some inequalities for bounding
the inverse hyperbolic sine.Comment: 3 page
Epitaxial graphene on SiC(0001): More than just honeycombs
The potential of graphene to impact the development of the next generation of
electronics has renewed interest in its growth and structure. The
graphitization of hexagonal SiC surfaces provides a viable alternative for the
synthesis of graphene, with wafer-size epitaxial graphene on SiC(0001) now
possible. Despite this recent progress, the exact nature of the graphene-SiC
interface and whether the graphene even has a semiconducting gap remain
controversial. Using scanning tunneling microscopy with functionalized tips and
density functional theory calculations, here we show that the interface is a
warped carbon sheet consisting of three-fold hexagon-pentagon-heptagon
complexes periodically inserted into the honeycomb lattice. These defects
relieve the strain between the graphene layer and the SiC substrate, while
still retaining the three-fold coordination for each carbon atom. Moreover,
these defects break the six-fold symmetry of the honeycomb, thereby naturally
inducing a gap: the calculated band structure of the interface is
semiconducting and there are two localized states near K below the Fermi level,
explaining the photoemission and carbon core-level data. Nonlinear dispersion
and a 33 meV gap are found at the Dirac point for the next layer of graphene,
providing insights into the debate over the origin of the gap in epitaxial
graphene on SiC(0001). These results indicate that the interface of the
epitaxial graphene on SiC(0001) is more than a dead buffer layer, but actively
impacts the physical and electronic properties of the subsequent graphene
layers
Gamma-ray burst contributions to constraining the evolution of dark energy
We explore the gamma-ray bursts' (GRBs') contributions in constraining the
dark energy equation of state (EOS) at high () and at middle
redshifts () and estimate how many GRBs are needed to get
substantial constraints at high redshifts. We estimate the constraints with
mock GRBs and mock type Ia supernovae (SNe Ia) for comparisons. When
constraining the dark energy EOS in a certain redshift range, we allow the dark
energy EOS parameter to vary only in that redshift bin and fix EOS parameters
elsewhere to -1. We find that it is difficult to constrain the dark energy EOS
beyond the redshifts of SNe Ia with GRBs unless some new luminosity relations
for GRBs with smaller scatters are discovered. However, at middle redshifts,
GRBs have comparable contributions with SNe Ia in constraining the dark energy
EOS.Comment: 3 pages, 5 figures. Published in Astronomy and Astrophysics.
Corrected referenc
A comparison of robust Mendelian randomization methods using summary data.
The number of Mendelian randomization (MR) analyses including large numbers of genetic variants is rapidly increasing. This is due to the proliferation of genome-wide association studies, and the desire to obtain more precise estimates of causal effects. Since it is unlikely that all genetic variants will be valid instrumental variables, several robust methods have been proposed. We compare nine robust methods for MR based on summary data that can be implemented using standard statistical software. Methods were compared in three ways: by reviewing their theoretical properties, in an extensive simulation study, and in an empirical example. In the simulation study, the best method, judged by mean squared error was the contamination mixture method. This method had well-controlled Type 1 error rates with up to 50% invalid instruments across a range of scenarios. Other methods performed well according to different metrics. Outlier-robust methods had the narrowest confidence intervals in the empirical example. With isolated exceptions, all methods performed badly when over 50% of the variants were invalid instruments. Our recommendation for investigators is to perform a variety of robust methods that operate in different ways and rely on different assumptions for valid inferences to assess the reliability of MR analyses
Parametric Fokker-Planck equation
We derive the Fokker-Planck equation on the parametric space. It is the
Wasserstein gradient flow of relative entropy on the statistical manifold. We
pull back the PDE to a finite dimensional ODE on parameter space. Some
analytical example and numerical examples are presented
Transport properties and anisotropy in rare earth doped CaFe2As2 single crystals with Tc above 40 K
In this paper we report the superconductivity above 40 K in the electron
doping single crystal Ca1-xRexFe2As2 (Re = La, Ce, Pr). The x-ray diffraction
patterns indicate high crystalline quality and c-axis orientation. the
resistivity anomaly in the parent compound CaFe2As2 is completely suppressed by
partial replacement of Ca by rare earth and a superconducting transition
reaches as high as 43 K, which is higher than the value in electron doping
FeAs-122 compounds by substituting Fe ions with transition metal, even
surpasses the highest values observed in hole doping systems with a transition
temperature up to 38 K. The upper critical field has been determined with the
magnetic field along ab-plane and c-axis, yielding the anisotropy of 2~3.
Hall-effect measurements indicate that the conduction in this material is
dominated by electron like charge carriers. Our results explicitly demonstrate
the feasibility of inducing superconductivity in Ca122 compounds via electron
doping using aliovalent rare earth substitution into the alkaline earth site,
which should add more ingredients to the underlying physics of the iron-based
superconductors.Comment: 21 pages, 7 figure
Remark on Charm Quark Fragmentation to Mesons
The observed mesons have flavor quantum numbers and
spin-parity of the light degrees of freedom . In
the limit the spin of the charm quark is conserved and
the fragmentation process is characterized by the
probability for the charm quark to fragment to a meson with a given
helicity for the light degrees of freedom. We consider the calculated fragmentation functions in the limit as a qualitative model for the fragmentation
functions. We find that in this model charm quark fragmentation to
light degrees of freedom with helicities is favored over fragmentation to light
degrees of freedom with helicities .Comment: 6 pages, CALT-68-192
Constraints on Cosmological Models and Reconstructing the Acceleration History of the Universe with Gamma-Ray Burst Distance Indicators
Gamma-ray bursts (GRBs) have been regarded as standard candles at very high
redshift for cosmology research. We have proposed a new method to calibrate GRB
distance indicators with Type Ia supernova (SNe Ia) data in a completely
cosmology-independent way to avoid the circularity problem that had limited the
direct use of GRBs to probe cosmology [N. Liang, W. K. Xiao, Y. Liu, and S. N.
Zhang, Astrophys. J. 685, 354 (2008).]. In this paper, a simple method is
provided to combine GRB data into the joint observational data analysis to
constrain cosmological models; in this method those SNe Ia data points used for
calibrating the GRB data are not used to avoid any correlation between them. We
find that the CDM model is consistent with the joint data in the
1- confidence region, using the GRB data at high redshift calibrated
with the interpolating method, the Constitution set of SNe Ia, the cosmic
microwave background radiation from Wilkinson Microwave Anisotropy Probe five
year observation, the baryonic acoustic oscillation from the spectroscopic
Sloan Digital Sky Survey Data Release 7 galaxy sample, the x-ray baryon mass
fraction in clusters of galaxies, and the observational Hubble parameter versus
redshift data. Comparing to the joint constraints with GRBs and without GRBs,
we find that the contribution of GRBs to the joint cosmological constraints is
a slight shift in the confidence regions of cosmological parameters to better
enclose the CDM model. Finally, we reconstruct the acceleration
history of the Universe up to with the distance moduli of SNe Ia and GRBs
and find some features that deviate from the CDM model and seem to
favor oscillatory cosmology models; however further investigations are needed
to better understand the situation.Comment: 14 pages, 9 figures, 2 tables; v3: the revised version, fig. 6 and
some discussions added, accepted for for publication in Phys. Rev. D; v4: the
published version (Phys. Rev. D 81, 083518, 2010
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