910 research outputs found
Inelastic Scattering in Metal-H2-Metal Junctions
We present first-principles calculations of the dI/dV characteristics of an
H2 molecule sandwiched between Au and Pt electrodes in the presence of
electron-phonon interactions. The conductance is found to decrease by a few
percentage at threshold voltages corresponding to the excitation energy of
longitudinal vibrations of the H2 molecule. In the case of Pt electrodes, the
transverse vibrations can mediate transport through otherwise non-transmitting
Pt -channels leading to an increase in the differential conductance even
though the hydrogen junction is characterized predominately by a single almost
fully open transport channel. In the case of Au, the transverse modes do not
affect the dI/dV because the Au d-states are too far below the Fermi level. A
simple explanation of the first-principles results is given using scattering
theory. Finally, we compare and discuss our results in relation to experimental
data.Comment: Accepted in Phys. Rev.
Amine-Gold Linked Single-Molecule Junctions: Experiment and Theory
The measured conductance distribution for single molecule benzenediamine-gold
junctions, based on 59,000 individual conductance traces recorded while
breaking a gold point contact in solution, has a clear peak at 0.0064 G
with a width of 40%. Conductance calculations based on density functional
theory (DFT) for 15 distinct junction geometries show a similar spread.
Differences in local structure have a limited influence on conductance because
the amine-Au bonding motif is well-defined and flexible. The average calculated
conductance (0.046 G) is seven times larger than experiment, suggesting
the importance of many-electron corrections beyond DFT
KIC 8410637: a 408-day period eclipsing binary containing a pulsating red giant
Detached eclipsing binaries (dEBs) are ideal targets for accurate measurement
of masses and radii of ther component stars. If at least one of the stars has
evolved off the main sequence (MS), the masses and radii give a strict
constraint on the age of the stars. Several dEBs containing a bright K giant
and a fainter MS star have been discovered by the Kepler satellite. The mass
and radius of a red giant (RG) star can also be derived from its asteroseismic
signal. The parameters determined in this way depend on stellar models and may
contain systematic errors. It is important to validate the asteroseismically
determined mass and radius with independent methods. This can be done when
stars are members of stellar clusters or members of dEBs. KIC 8410637 consists
of an RG and an MS star. The aim is to derive accurate masses and radii for
both components and provide the foundation for a strong test of the
asteroseismic method and the accuracy of the deduced mass, radius and age. We
analyse high-resolution spectra from three different spectrographs. We also
calculate a fit to the Kepler light curve and use ground-based photometry to
determine the flux ratios between the component stars in the BVRI passbands. We
measured the masses and radii of the stars in the dEB, and the classical
parameters Teff, log g and [Fe/H] from the spectra and ground-based photometry.
The RG component of KIC 8410637 is most likely in the core helium-burning red
clump phase of evolution and has an age and composition very similar to the
stars in the open cluster NGC 6819. The mass of the RG in KIC 8410637 should
therefore be similar to the mass of RGs in NGC 6819, thus lending support to
the most up-to-date version of the asteroseismic scaling relations. This is the
first direct measurement of both mass and radius for an RG to be compared with
values for RGs from asteroseismic scaling relations.Comment: Accepted 20.6.2013 for publication in Astronomy and Astrophysic
Prognostic microRNAs in high-grade glioma reveal a link to oligodendrocyte precursor differentiation.
MicroRNA expression can be exploited to define tumor prognosis and stratification for precision medicine. It remains unclear whether prognostic microRNA signatures are exclusively tumor grade and/or molecular subtype-specific, or whether common signatures of aggressive clinical behavior can be identified. Here, we defined microRNAs that are associated with good and poor prognosis in grade III and IV gliomas using data from The Cancer Genome Atlas. Pathway analysis of microRNA targets that are differentially expressed in good and poor prognosis glioma identified a link to oligodendrocyte development. Notably, a microRNA expression profile that is characteristic of a specific oligodendrocyte precursor cell type (OP1) correlates with microRNA expression from 597 of these tumors and is consistently associated with poor patient outcome in grade III and IV gliomas. Our study reveals grade-independent and subtype-independent prognostic molecular signatures in high-grade glioma and provides a framework for investigating the mechanisms of brain tumor aggressiveness
KIC 4768731: a bright long-period roAp star in theKeplerfield
We report the identification of 61.45 d−1 (711.2 μHz) oscillations, with amplitudes of 62.6 μmag, in KIC 4768731 (HD 225914) using Kepler photometry. This relatively bright (V = 9.17) chemically peculiar star with spectral type A5 Vp SrCr(Eu) has previously been found to exhibit rotational modulation with a period of 5.21 d. Fourier analysis reveals a simple dipole pulsator with an amplitude that has remained stable over a 4-yr time span, but with a frequency that is variable. Analysis of high-resolution spectra yields stellar parameters of Teff = 8100 ± 200 K, log g = 4.0 ± 0.2, [Fe/H] = +0.31 ± 0.24 and v sin i = 14.8 ± 1.6 km s−1.
Line profile variations caused by rotation are also evident. Lines of Sr, Cr, Eu, Mg and Si are strongest when the star is brightest, while Y and Ba vary in antiphase with the other elements. The abundances of rare earth elements are only modestly enhanced compared to other roAp stars of similar Teff and log g. Radial velocities in the literature suggest a significant change over the past 30 yr, but the radial velocities presented here show no significant change over a period of 4 yr
Stretching dependence of the vibration modes of a single-molecule Pt-H2-Pt bridge
A conducting bridge of a single hydrogen molecule between Pt electrodes is
formed in a break junction experiment. It has a conductance near the quantum
unit, G_0 = 2e^2/h, carried by a single channel. Using point contact
spectroscopy three vibration modes are observed and their variation upon
stretching and isotope substitution is obtained. The interpretation of the
experiment in terms of a Pt-H_2-Pt bridge is verified by Density Functional
Theory calculations for the stability, vibrational modes, and conductance of
the structure.Comment: 5 pages, 4 figure
Sounding stellar cycles with Kepler - II. Ground-based observations
We have monitored 20 Sun-like stars in the Kepler field-of-view for excess
flux with the FIES spectrograph on the Nordic Optical Telescope since the
launch of Kepler spacecraft in 2009. These 20 stars were selected based on
their asteroseismic properties to sample the parameter space (effective
temperature, surface gravity, activity level etc.) around the Sun. Though the
ultimate goal is to improve stellar dynamo models, we focus the present paper
on the combination of space-based and ground-based observations can be used to
test the age-rotation-activity relations.
In this paper we describe the considerations behind the selection of these 20
Sun-like stars and present an initial asteroseismic analysis, which includes
stellar age estimates. We also describe the observations from the Nordic
Optical Telescope and present mean values of measured excess fluxes. These
measurements are combined with estimates of the rotation periods obtained from
a simple analysis of the modulation in photometric observations from Kepler
caused by starspots, and asteroseismic determinations of stellar ages, to test
relations between between age, rotation and activity.Comment: Accepted for publication in MNRA
Self-Reported Long COVID in the General Population: Sociodemographic and Health Correlates in a Cross-National Sample.
We aimed to gain knowledge of possible sociodemographic predictors of long COVID and whether long COVID was associated with health outcomes almost two years after the pandemic outbreak. There were 1649 adults who participated in the study by completing a cross-sectional online survey disseminated openly in Norway, the UK, the USA, and Australia between November 2021 and January 2022. Participants were defined as having long COVID based on self-reports that they had been infected by COVID-19 and were experiencing long-lasting COVID symptoms. Logistic regression analyses were used to examine possible sociodemographic predictors, and multivariate analysis of variance was used to examine whether long COVID status was associated with health outcomes. None of the sociodemographic variables was significantly associated with reporting long COVID. Having long COVID was associated with higher levels of psychological distress, fatigue, and perceived stress. The effect of long COVID on health outcomes was greater among men than among women. In conclusion, long COVID appeared across sociodemographic groups. People with long COVID reported worsened health outcomes compared to those who had had COVID-19 but without long-term symptoms. Men experiencing long COVID appear to be particularly vulnerable to experiencing poorer health outcomes; health services may pay extra attention to potentially unnoticed needs for support among men experiencing long COVID
Electronic shell structure and chemisorption on gold nanoparticles
We use density functional theory (DFT) to investigate the electronic
structure and chemical properties of gold nanoparticles. Different structural
families of clusters are compared. For up to 60 atoms we optimize structures
using DFT-based simulated annealing. Cluster geometries are found to distort
considerably, creating large band gaps at the Fermi level. For up to 200 atoms
we consider structures generated with a simple EMT potential and clusters based
on cuboctahedra and icosahedra. All types of cluster geometry exhibit
jellium-like electronic shell structure. We calculate adsorption energies of
several atoms on the cuboctahedral clusters. Adsorption energies are found to
vary abruptly at magic numbers. Using a Newns-Anderson model we find that the
effect of magic numbers on adsorption energy can be understood from the
location of adsorbate-induced states with respect to the cluster Fermi level.Comment: 14 pages, 18 figure
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