1,037 research outputs found
Superconductivity-Induced Transfer of In-Plane Spectral Weight in Bi2Sr2CaCu2O8: Resolving a Controversy
We present a detailed analysis of the superconductivity-induced
redistribution of optical spectral weight in Bi2Sr2CaCu2O8 near optimal doping.
It confirms the previous conclusion by Molegraaf et al. (Science 66, 2239
(2002)), that the integrated low-frequency spectral weight shows an extra
increase below Tc. Since the region, where the change of the integrated
spectral weight is not compensated, extends well above 2.5 eV, this transfer is
caused by the transfer of spectral weight from interband to intraband region
and only partially by the narrowing of the intraband peak. We show that the
opposite assertion by Boris et al. (Science 304, 708 (2004)) regarding this
compound, is unlikely the consequence of any obvious discrepancies between the
actual experimental data.Comment: ReVTeX, 9 pages, 8 encapsulated postscript figures, several typo's
correcte
In-plane optical spectral weight transfer in optimally doped BiSrCaCuO
We examine the redistribution of the in-plane optical spectral weight in the
normal and superconducting state in tri-layer \bbb (Bi2223) near optimal doping
( = 110 K) on a single crystal via infrared reflectivity and spectroscopic
ellipsometry. We report the temperature dependence of the low-frequency
integrated spectral weight for different values of the cutoff
energy . Two different model-independent analyses consistently show
that for = 1 eV, which is below the charge transfer gap,
increases below , implying the lowering of the kinetic
energy of the holes. This is opposite to the BCS scenario, but it follows the
same trend observed in the bi-layer compound \bb (Bi2212). The size of this
effect is larger in Bi2223 than in Bi2212, approximately scaling with the
critical temperature. In the normal state, the temperature dependence of
is close to up to 300 K
Intraband Optical Spectral Weight in the presence of a van Hove singularity: application to BiSrCaCuO
The Kubo single band sum rule is used to determine the optical spectral
weight of a tight binding band with further than nearest neighbour hopping. We
find for a wide range of parameters and doping concentrations that the change
due to superconductivity at low temperature can be either negative or positive.
In contrast, the kinetic energy change is always negative. We use an ARPES
determined tight binding parametrization of BiSrCaCuO
to investigate whether this can account for recent observations of a positive
change in the spectral weight due to the onset of superconductivity. With this
band structure we find that in the relevant doping regime a straightforward BCS
calculation of the optical spectral weight cannot account for the experimental
observations.Comment: 10 page 9 figure
Constraining the Mass Profiles of Stellar Systems: Schwarzschild Modeling of Discrete Velocity Datasets
(ABRIDGED) We present a new Schwarzschild orbit-superposition code designed
to model discrete datasets composed of velocities of individual kinematic
tracers in a dynamical system. This constitutes an extension of previous
implementations that can only address continuous data in the form of (the
moments of) velocity distributions, thus avoiding potentially important losses
of information due to data binning. Furthermore, the code can handle any
combination of available velocity components, i.e., only line-of-sight
velocities, only proper motions, or a combination of both. It can also handle a
combination of discrete and continuous data. The code finds the distribution
function (DF, a function of the three integrals of motion E, Lz, and I3) that
best reproduces the available kinematic and photometric observations in a given
axisymmetric gravitational potential. The fully numerical approach ensures
considerable freedom on the form of the DF f(E,Lz,I3). This allows a very
general modeling of the orbital structure, thus avoiding restrictive
assumptions about the degree of (an)isotropy of the orbits. We describe the
implementation of the discrete code and present a series of tests of its
performance based on the modeling of simulated datasets generated from a known
DF. We find that the discrete Schwarzschild code recovers the original orbital
structure, M/L ratios, and inclination of the input datasets to satisfactory
accuracy, as quantified by various statistics. The code will be valuable, e.g.,
for modeling stellar motions in Galactic globular clusters, and those of
individual stars, planetary nebulae, or globular clusters in nearby galaxies.
This can shed new light on the total mass distributions of these systems, with
central black holes and dark matter halos being of particular interest.Comment: ApJ, in press; 51 pages, 11 figures; manuscript revised following
comments by refere
Doping Dependence of the Redistribution of Optical Spectral Weight in BiSrCaCuO
We present the ab-plane optical conductivity of four single crystals of
BiSrCaCuO (Bi2212) with different carrier doping
levels from the strongly underdoped to the strongly overdoped range with
=66, 88, 77, and 67 K respectively. We focus on the redistribution of the
low frequency optical spectral weight (SW) in the superconducting and normal
states. The temperature dependence of the low-frequency spectral weight in the
normal state is significantly stronger in the overdoped regime. In agreement
with other studies, the superconducting order is marked by an increase of the
low frequency SW for low doping, while the SW decreases for the highly
overdoped sample. The effect crosses through zero at a doping concentration
=0.19 which is slightly to the right of the maximum of the
superconducting dome. This sign change is not reproduced by the BCS model
calculations, assuming the electron-momentum dispersion known from published
ARPES data. Recent Cluster Dynamical Mean Field Theory (CDMFT) calculations
based on the Hubbard and t-J models, agree in several relevant respects with
the experimental data
Outflow forces of low mass embedded objects in Ophiuchus: a quantitative comparison of analysis methods
The outflow force of molecular bipolar outflows is a key parameter in
theories of young stellar feedback on their surroundings. The focus of many
outflow studies is the correlation between the outflow force, bolometric
luminosity and envelope mass. However, it is difficult to combine the results
of different studies in large evolutionary plots over many orders of magnitude
due to the range of data quality, analysis methods and corrections for
observational effects such as opacity and inclination. We aim to determine the
outflow force for a sample of low luminosity embedded sources. We will quantify
the influence of the analysis method and the assumptions entering the
calculation of the outflow force. We use the James Clerk Maxwell Telescope to
map 12CO J=3-2 over 2'x2' regions around 16 Class I sources of a well-defined
sample in Ophiuchus at 15" resolution. The outflow force is then calculated
using seven different methods differing e.g. in the use of intensity-weighted
emission and correction factors for inclination. The results from the analysis
methods differ from each other by up to a factor of 6, whereas observational
properties and choices in the analysis procedure affect the outflow force by up
to a factor of 4. For the sample of Class I objects, bipolar outflows are
detected around 13 sources including 5 new detections, where the three
non-detections are confused by nearby outflows from other sources. When
combining outflow forces from different studies, a scatter by up to a factor of
5 can be expected. Although the true outflow force remains unknown, the
separation method (separate calculation of dynamical time and momentum) is
least affected by the uncertain observational parameters. The correlations
between outflow force, bolometric luminosity and envelope mass are further
confirmed down to low luminosity sources.Comment: 24 pages, 13 figures, Accepted by A&
An ALMA Survey of Protoplanetary Disks in the Orionis Cluster
The Orionis cluster is important for studying protoplanetary disk
evolution, as its intermediate age (3-5 Myr) is comparable to the median
disk lifetime. We use ALMA to conduct a high-sensitivity survey of dust and gas
in 92 protoplanetary disks around Orionis members with
. Our observations cover the 1.33 mm continuum
and several CO lines: out of 92 sources, we detect 37 in the mm
continuum and six in CO, three in CO, and none in CO.
Using the continuum emission to estimate dust mass, we find only 11 disks with
, indicating that after only a few Myr of
evolution most disks lack sufficient dust to form giant planet cores. Stacking
the individually undetected continuum sources limits their average dust mass to
5 lower than that of the faintest detected disk, supporting theoretical
models that indicate rapid dissipation once disk clearing begins. Comparing the
protoplanetary disk population in Orionis to those of other
star-forming regions supports the steady decline in average dust mass and the
steepening of the - relation with age; studying these
evolutionary trends can inform the relative importance of different disk
processes during key eras of planet formation. External photoevaporation from
the central O9 star is influencing disk evolution throughout the region: dust
masses clearly decline with decreasing separation from the photoionizing
source, and the handful of CO detections exist at projected separations
pc. Collectively, our findings indicate that giant planet formation is
inherently rare and/or well underway by a few Myr of age.Comment: 16 pages, 9 figures; published in AJ; The full machine readable
tables can be obtained by downloading and extracting the gzipped tar source
file listed under "Other formats.
Warm formaldehyde in the Oph IRS 48 transitional disk
Simple molecules like H2CO and CH3OH in protoplanetary disks are the starting
point for the production of more complex organic molecules. So far, the
observed chemical complexity in disks has been limited due to freeze out of
molecules onto grains in the bulk of the cold outer disk. Complex molecules can
be studied more directly in transitional disks with large inner holes, as these
have a higher potential of detection, through UV heating of the outer disk and
the directly exposed midplane at the wall. We use Atacama Large
Millimeter/submillimeter Array (ALMA) Band 9 (~680 GHz) line data of the
transitional disk Oph IRS 48, previously shown to have a large dust trap, to
search for complex molecules in regions where planetesimals are forming. We
report the detection of the H2CO 9(1,8)-8(1,7) line at 674 GHz, which is
spatially resolved as a semi-ring at ~60 AU radius centered south from the
star. The inferred H2CO abundance is ~10^{-8} derived by combining a physical
disk model of the source with a non-LTE excitation calculation. Upper limits
for CH3OH lines in the same disk give an abundance ratio H2CO/CH3OH>0.3, which
points to both ice formation and gas-phase routes playing a role in the H2CO
production. Upper limits on the abundances of H13CO+, CN and several other
molecules in the disk are also derived and found to be consistent with full
chemical models. The detection of the H2CO line demonstrates the start of
complex organic molecules in a planet-forming disk. Future ALMA observations
should be able to push down the abundance detection limits of other molecules
by 1-2 orders of magnitude and test chemical models of organic molecules in
(transitional) disks.Comment: Updated references and minor changes to text, approved by language
edito
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