6,109 research outputs found
Core hole-electron correlation in coherently coupled molecules
We study the core hole-electron correlation in coherently coupled molecules
by energy dispersive near edge X-ray absorption fine-structure spectroscopy. In
a transient phase, which exists during the transition between two bulk
arrangements, 1,4,5,8-naphthalene-tetracarboxylicacid-dianhydride multilayer
films exhibit peculiar changes of the line shape and energy position of the
X-ray absorption signal at the C K-edge with respect to the bulk and gas phase
spectra. By a comparison to a theoretical model based on a coupling of
transition dipoles, which is established for optical absorption, we demonstrate
that the observed spectroscopic differences can be explained by an
intermolecular delocalized core hole-electron pair. By applying this model we
can furthermore quantify the coherence length of the delocalized core-exciton.Comment: 5 pages, 3 figures, Accepted Version, PRL, minor wording change
Free-carrier screening of polarization fields in wurtzite GaN/InGaN laser structures
The free-carrier screening of macroscopic polarization fields in wurtzite
GaN/InGaN quantum wells lasers is investigated via a self-consistent
tight-binding approach. We show that the high carrier concentrations found
experimentally in nitride laser structures effectively screen the built-in
spontaneous and piezoelectric polarization fields, thus inducing a
``field-free'' band profile. Our results explain some heretofore puzzling
experimental data on nitride lasers, such as the unusually high lasing
excitation thresholds and emission blue-shifts for increasing excitation
levels.Comment: RevTeX 4 pages, 4 figure
Uso de aditivos para obtenção de incrementos na nodulação e no rendimento de grãos de feijoeiros inoculados com rizóbios selecionados.
Spin-down evolution and radio disappearance of the magnetar PSR J16224950
We report on 2.4 yr of radio timing measurements of the magnetar PSR
J16224950 using the Parkes telescope, between 2011 November and 2014 March.
During this period the torque on the neutron star (inferred from the rotational
frequency derivative) varied greatly, though much less erratically than in the
2 yr following its discovery in 2009. During the last year of our measurements
the frequency derivative decreased in magnitude monotonically by 20\%, to a
value of s, a factor of 8 smaller than when
discovered. The flux density continued to vary greatly during our monitoring
through 2014 March, reaching a relatively steady low level after late 2012. The
pulse profile varied secularly on a similar timescale as the flux density and
torque. A relatively rapid transition in all three properties is evident in
early 2013. After PSR J16224950 was detected in all of our 87 observations
up to 2014 March, we did not detect the magnetar in our resumed monitoring
starting in 2015 January and have not detected it in any of the 30 observations
done through 2016 September.Comment: 8 pages, 5 figures, submitted to Ap
Hybrid Quantum Dot-2D Electron Gas Devices for Coherent Optoelectronics
We present an inverted GaAs 2D electron gas with self-assembled InAs quantum
dots in close proximity, with the goal of combining quantum transport with
quantum optics experiments. We have grown and characterized several wafers --
using transport, AFM and optics -- finding narrow-linewidth optical dots and
high-mobility, single subband 2D gases. Despite being buried 500 nm below the
surface, the dots are clearly visible on AFM scans, allowing precise
localization and paving the way towards a hybrid quantum system integrating
optical dots with surface gate-defined nanostructures in the 2D gas.Comment: 4 pages, 5 figures (color
On the X-ray variability of magnetar 1RXS J170849.0-400910
We present a long-term X-ray flux and spectral analysis for 1RXS
J170849.0-400910 using Swift/XRT spanning over 8 years from 2005-2013. We also
analyze two observations from Chandra and XMM in the period from 2003-2004. In
this 10-yr period, 1RXS J170849.0-400910 displayed several rotational glitches.
Previous studies have claimed variations in the X-ray emission associated with
some of the glitches. From our analysis we find no evidence for significant
X-ray flux variations and evidence for only low-level spectral variations. We
also present an updated timing solution for 1RXS J170849.0-400910, from RXTE
and Swift observations, which includes a previously unreported glitch at MJD
56019. We discuss the frequency and implications of radiatively quiet glitches
in magnetars.Comment: 9 pages, 2 figures, accepted for publication in Ap
Synthetic Spectra and Color-Temperature Relations of M Giants
As part of a project to model the integrated spectra and colors of elliptical
galaxies through evolutionary synthesis, we have refined our synthetic spectrum
calculations of M giants. After critically assessing three effective
temperature scales for M giants, we adopted the relation of Dyck et al. (1996)
for our models. Using empirical spectra of field M giants as a guide, we then
calculated MARCS stellar atmosphere models and SSG synthetic spectra of these
cool stars, adjusting the band absorption oscillator strengths of the TiO bands
to better reproduce the observational data. The resulting synthetic spectra are
found to be in very good agreement with the K-band spectra of stars of the
appropriate spectral type taken from Kleinmann & Hall (1986) as well. Spectral
types estimated from the strengths of the TiO bands and the depth of the
bandhead of CO near 2.3 microns quantitatively confirm that the synthetic
spectra are good representations of those of field M giants. The broad-band
colors of the models match the field relations of K and early-M giants very
well; for late-M giants, differences between the field-star and synthetic
colors are probably caused by the omission of spectral lines of VO and water in
the spectrum synthesis calculations. Here, we present four grids of K-band
bolometric corrections and colors -- Johnson U-V and B-V; Cousins V-R and V-I;
Johnson-Glass V-K, J-K and H-K; and CIT/CTIO V-K, J-K, H-K and CO -- for models
having 3000 K < Teff < 4000 K and -0.5 < log g < 1.5. These grids, which have
[Fe/H] = +0.25, 0.0, -0.5 and -1.0, extend and supplement the color-temperature
relations of hotter stars presented in a companion paper (astro-ph/9911367).Comment: To appear in the March 2000 issue of the Astronomical Journal. 60
pages including 15 embedded postscript figures (one page each) and 6 embedded
postscript tables (10 pages total
Magnetic field geometry and chemical abundance distribution of the He-strong star CPD -57 3509
The magnetic field of CPD -57 3509 was recently detected in the framework of
the BOB (B fields in OB stars) collaboration. We acquired low-resolution
spectropolarimetric observations of CPD -57 3509 with FORS2 and high-resolution
UVES observations randomly distributed over a few months to search for
periodicity, to study the magnetic field geometry, and to determine the surface
distribution of silicon and helium. We also obtained supplementary photometric
observations at a timeline similar to the spectroscopic and spectropolarimetric
observations. A period of 6.36d was detected in the measurements of the mean
longitudinal magnetic field. A sinusoidal fit to our measurements allowed us to
constrain the magnetic field geometry and estimate the dipole strength in the
range of 3.9-4.5kG. Our application of the Doppler imaging technique revealed
the presence of He I spots located around the magnetic poles, with a strong
concentration at the positive pole and a weaker one around the negative pole.
In contrast, high concentration Si III spots are located close to the magnetic
equator. Further, our analysis of the spectral variability of CPD -57 3509 on
short time scales indicates distinct changes in shape and position of line
profiles possibly caused by the presence of beta Cep-like pulsations. A small
periodic variability in line with the changes of the magnetic field strength is
clearly seen in the photometric data.Comment: 11 pages, 5 tables, 7 figures, accepted for publication in MNRA
Eficiência simbiótica de bactérias fixadoras de nitrogênio inoculadas em diferentes espécies de adubo verde.
What causes the large extensions of red-supergiant atmospheres? Comparisons of interferometric observations with 1-D hydrostatic, 3-D convection, and 1-D pulsating model atmospheres
We present the atmospheric structure and the fundamental parameters of three
red supergiants, increasing the sample of RSGs observed by near-infrared
spectro-interferometry. Additionally, we test possible mechanisms that may
explain the large observed atmospheric extensions of RSGs.
We carried out spectro-interferometric observations of 3 RSGs in the
near-infrared K-band with the VLTI/AMBER instrument at medium spectral
resolution. To comprehend the extended atmospheres, we compared our
observational results to predictions by available hydrostatic PHOENIX,
available 3-D convection, and new 1-D self-excited pulsation models of RSGs.
Our near-infrared flux spectra are well reproduced by the PHOENIX model
atmospheres. The continuum visibility values are consistent with a
limb-darkened disk as predicted by the PHOENIX models, allowing us to determine
the angular diameter and the fundamental parameters of our sources.
Nonetheless, in the case of V602 Car and HD 95686, the PHOENIX model
visibilities do not predict the large observed extensions of molecular layers,
most remarkably in the CO bands. Likewise, the 3-D convection models and the
1-D pulsation models with typical parameters of RSGs lead to compact
atmospheric structures as well, which are similar to the structure of the
hydrostatic PHOENIX models. They can also not explain the observed decreases in
the visibilities and thus the large atmospheric molecular extensions. The full
sample of our RSGs indicates increasing observed atmospheric extensions with
increasing luminosity and decreasing surface gravity, and no correlation with
effective temperature or variability amplitude, which supports a scenario of
radiative acceleration on Doppler-shifted molecular lines.Comment: Accepted for publication in A&
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