1,599 research outputs found
Doping dependence of spin excitations in the stripe phase of high-Tc superconductors
Based on the time-dependent Gutzwiller approximation for the extended Hubbard
model we calculate the energy and momentum dependence of spin excitations for
striped ground states. Our starting point correctly reproduces the observed
doping dependence of the incommensurability in La-based cuprates and the
dispersion of magnetic modes in the insulating parent compound. This allows us
to make quantitative predictions for the doping evolution of the dispersion of
magnetic modes in the stripe phase including the energy and intensity of the
resonance peak as well as the velocity of the spin-wave like Goldstone mode. In
the underdoped regime we find a weak linear dependence of
on doping whereas the resonance energy significantly shifts to
higher values when the charge concentration in the stripes starts to deviate
from half-filling for . The velocity is non-monotonous with a
minimum at 1/8 in coincidence with a well known anomaly in . Our
calculations are in good agreement with available experimental data. We also
compare our results with analogous computations based on linear spin-wave
theory.Comment: 18 pages, 14 figures, revised and extended versio
ROSAT X-ray Observations of the Cooling Flow Cluster A2597
The cluster A2597 was observed in X-rays with the ROSAT PSPC and HRI
detectors. The X-ray isophotes are oriented similarly to the optical isophotes
of the central cD galaxy and to the isopleths of the galaxy distribution in the
cluster, but are otherwise quite regular, suggesting that this cluster is
reasonably relaxed and in hydrostatic equilibrium. The merged HRI and PSPC
surface brightness profile is not adequately fit by a beta model because of the
central X-ray surface brightness peak, indicating the presence of a cooling
flow. If the central 108 arcsec in radius are excluded, an acceptable fit is
found which gives beta = 0.64 but only an upper limit to the core radius, rcore
< 78 arcsec. Within a radius of 2 Mpc, we found masses of Mgas = 1.2e14 Msun
and Mtot = 6.5e14 Msun, and a gas mass fraction of about 19%. Both the overall
cluster spectrum and the spatially resolved spectra within 300 kpc require the
presence of both hot gas and a cooling flow in the spectrum. The spectrally
determined total cooling rate of Mdot = 344 Msun/yr is in good agreement with
those derived from analyses of the X-ray surface brightness profile from the
Einstein IPC and the ROSAT HRI images. The ROSAT spatially resolved X-ray
spectra indicate that the cooling component is distributed over the inner ~300
kpc in radius of the cluster. We do not detect any significant excess X-ray
absorption toward the center of A2597, and we set a very conservative upper
limit on the excess column in front of the cooling flow region of NH < 1.72e20
cm**-2.Comment: 16 pages, 12 figures, LaTeX, emulateapj style, Astrophysical Journal
in pres
A proposed mechanism for nitrogen acquisition by grass seedlings through oxidation of symbiotic bacteria
In this paper we propose and provide evidence for a mechanism, oxidative nitrogen scavenging (ONS), whereby seedlings of some grass species may extract nitrogen from symbiotic diazotrophic bacteria through oxidation by plant-secreted reactive oxygen species (ROS). Experiments on this proposed mechanism employ tall fescue (Festuca arundinaceae) seedlings to elucidate features of the oxidative mechanism. We employed 15N2 gas assimilation experiments to demonstrate nitrogen fixation, direct microscopic visualization of bacteria on seedling surfaces to visualize the bacterial oxidation process, reactive oxygen probes to test for the presence of H2O2 and cultural experiments to assess conditions under which H2O2 is secreted by seedlings. We also made surveys of the seedlings of several grass species to assess the distribution of the phenomenon of microbial oxidation in the Poaceae. Key elements of the proposed mechanism for nitrogen acquisition in seedlings include: 1) diazotrophic bacteria are vectored on or within seeds; 2) at seed germination bacteria colonize seedling roots and shoots; 3) seedling tissues secrete ROS onto bacteria; 4) bacterial cell walls, membranes, nucleic acids, proteins and other biological molecules are oxidized; 5) nitrates and/or smaller fragments of organic nitrogen-containing molecules resulting from oxidation may be absorbed by seedling tissues and larger peptide fragments may be further processed by secreted or cell wall plant proteases until they are small enough for transport into cells. Hydrogen peroxide secretion from seedling roots and bacterial oxidation was observed in several species in subfamily Pooideae where seeds possessed adherent paleas and lemmas, but was not seen in grasses that lacked this feature or long-cultivated crop species
Tensor Product Approximation (DMRG) and Coupled Cluster method in Quantum Chemistry
We present the Copupled Cluster (CC) method and the Density matrix
Renormalization Grooup (DMRG) method in a unified way, from the perspective of
recent developments in tensor product approximation. We present an introduction
into recently developed hierarchical tensor representations, in particular
tensor trains which are matrix product states in physics language. The discrete
equations of full CI approximation applied to the electronic Schr\"odinger
equation is casted into a tensorial framework in form of the second
quantization. A further approximation is performed afterwards by tensor
approximation within a hierarchical format or equivalently a tree tensor
network. We establish the (differential) geometry of low rank hierarchical
tensors and apply the Driac Frenkel principle to reduce the original
high-dimensional problem to low dimensions. The DMRG algorithm is established
as an optimization method in this format with alternating directional search.
We briefly introduce the CC method and refer to our theoretical results. We
compare this approach in the present discrete formulation with the CC method
and its underlying exponential parametrization.Comment: 15 pages, 3 figure
A Deep Look at the Emission-Line Nebula in Abell 2597
The close correlation between cooling flows and emission-line nebulae in
clusters of galaxies has been recognized for over a decade and a half, but the
physical reason for this connection remains unclear. Here we present deep
optical spectra of the nebula in Abell 2597, one of the nearest strong
cooling-flow clusters. These spectra reveal the density, temperature, and metal
abundances of the line-emitting gas. The abundances are roughly half-solar, and
dust produces an extinction of at least a magnitude in V. The absence of [O
III] 4363 emission rules out shocks as a major ionizing mechanism, and the
weakness of He II 4686 rules out a hard ionizing source, such as an active
galactic nucleus or cooling intracluster gas. Hot stars are therefore the best
candidate for producing the ionization. However, even the hottest O stars
cannot power a nebula as hot as the one we see. Some other nonionizing source
of heat appears to contribute a comparable amount of power. We show that the
energy flux from a confining medium can become important when the ionization
level of a nebula drops to the low levels seen in cooling-flow nebulae. We
suggest that this kind of phenomenon, in which energy fluxes from the
surrounding medium augment photoelectric heating, might be the common feature
underlying the diverse group of objects classified as LINERS.Comment: 33 Latex pages, including 16 Postscript figures, to appear in 1997
September 1 Astrophysical Journa
HST/STIS Spectroscopy of the Lyman-Alpha Emission Line in the Central Dominant Galaxies in A426, A1795, and A2597: Constraints on Clouds in the Intracluster Medium
We report on HST/STIS spectra of the Lyman-alpha emission in the central
dominant galaxies in three rich clusters of galaxies. We find evidence for a
population of clouds in the intracluster medium.We detect 10 Ly-alpha
absorption systems towards the nucleus of NGC1275 with columns of N(HI)
1E12-1E14 cm-2. The detected absorption features are most consistent with
associated nuclear absorption systems. There is very little nuclear absorption
at the systemic velocity in NGC1275. This implies that the large columns
detected in the 21 cm line towards the parsec scale radio source avoid the line
of sight to the nucleus. This gas may be located in a circumnuclear disk or
torus. We detect at least one and possibly two absorption features towards the
extended Ly-alpha in A426. We do not detect absorption towards the extended
Ly-alpha emission in A1795, and A2597 with upper limits N(HI) 1E13 cm-2 for
optically thin absorbers. Our data constrain the covering factor of any high
column density gas in the ICM to be less than 25%. Our results suggest that the
lack of observed intermediate temperature gas is not explained by obscuration.
In addition, the low columns of gas on the 100 kpc scales in the ICM suggests
that (1) the rate at which cold gas accumulates in the ICM on these scales is
very low, and (2) the dense nebulae in the central 10 kpc must have cooled or
been deposited in situ.Comment: 6 figure
Gr1+IL-4-producing innate cells are induced in response to Th2 stimuli and suppress Th1-dependent antibody responses
Alum is used as a vaccine adjuvant and induces T<sub>h</sub>2 responses and T<sub>h</sub>2-driven antibody isotype production against co-injected antigens. Alum also promotes the appearance in the spleen of Gr1+IL-4+ innate cells that, via IL-4 production, induce MHC II-mediated signaling in B cells. To investigate whether these Gr1+ cells accumulate in the spleen in response to other T<sub>h</sub>2-inducing stimuli and to understand some of their functions, the effects of injection of alum and eggs from the helminth, Schistosoma mansoni, were compared. Like alum, schistosome eggs induced the appearance of Gr1+IL-4+ cells in spleen and promoted MHC II-mediated signaling in B cells. Unlike alum, however, schistosome eggs did not promote CD4 T cell responses against co-injected antigens, suggesting that the effects of alum or schistosome eggs on splenic B cells cannot by themselves explain the T cell adjuvant properties of alum. Accordingly, depletion of IL-4 or Gr1+ cells in alum-injected mice had no effect on the ability of alum to improve expansion of primary CD4 T cells. However, Gr1+ cells and IL-4 played some role in the effects of alum, since depletion of either resulted in antibody responses to antigen that included not only the normal T<sub>h</sub>2-driven isotypes, like IgG1, but also a T<sub>h</sub>1-driven isotype, IgG2c. These data suggest that alum affects the immune response in at least two ways: one, independent of Gr1+ cells and IL-4, that promotes CD4 T cell proliferation and another, via Gr1+IL-4+ cells, that participates in the polarization of the response
RXJ1716.6+6708: a young cluster at z=0.81
Clusters of galaxies at redshifts nearing one are of special importance since
they may be caught at the epoch of formation. At these high redshifts there are
very few known clusters. We present follow-up ASCA, ROSAT HRI and Keck LRIS
observations of the cluster RXJ1716.6+6708 which was discovered during the
optical identification of X-ray sources in the North Ecliptic Pole region of
the ROSAT All-Sky Survey. At z=0.809, RXJ1716.6+6708 is the second most distant
X-ray selected cluster so far published and the only one with a large number of
spectroscopically determined cluster member velocities. The optical morphology
of RXJ1716.6+6708 resembles an inverted S-shape filament with the X-rays coming
from the midpoint of the filament. The X-ray contours have an elongated shape
that roughly coincide with the weak lensing contours. The cluster has a low
temperature, kT=5.66{+1.37 -0.58} keV, and a very high velocity dispersion
sigma_{los}=1522{+215 -150} km s^{-1}. While the temperature is commensurate
with its X-ray luminosity of (8.19 +/- 0.43)x10^{44} h_{50}^{-2} erg s^{-1}
(2-10 keV rest frame), its velocity dispersion is much higher than expected
from the sigma-T_X relationship of present-day clusters with comparable X-ray
luminosity. RXJ1716.6+6708 could be an example of a protocluster, where matter
is flowing along filaments and the X-ray flux is maximum at the impact point of
the colliding streams of matter.Comment: Latex file, 18 pages, 4 figures, accepted for publication in the
Astronomical Journa
Lattice anisotropy as microscopic origin of static stripes in cuprates
Structural distortions in cuprate materials offer a microscopic origin for
anisotropies in electron transport in the basal plane. Using a real-space
Hartree-Fock approach, we consider the ground states of the anisotropic Hubbard
(t_x \ne t_y) and t-J (t_x \ne t_y, J_x \ne J_y) models. Symmetrical but
inhomogeneous (``polaronic'') charge structures in the isotropic models are
altered even by rather small anisotropies to one-dimensional, stripe-like
features. We find two distinct types of stripe, namely uniformly filled,
antiphase domain walls and non-uniform, half-filled, in-phase ones. We
characterize their properties, energies and dependence on the model parameters,
including filling and anisotropy in t (and J). We discuss the connections among
these results, other theoretical studies and experimental observation.Comment: 18 pages, 16 figures, 8 table
A Flux-limited Sample of Bright Clusters of Galaxies from the Southern Part of the ROSAT All-Sky Survey: the Catalog and the LogN-LogS
We describe the selection of an X-ray flux-limited sample of bright clusters
of galaxies in the southern hemisphere, based on the first analysis of the
ROSAT All-Sky Survey data (RASS1). The sample is constructed starting from an
identification of candidate clusters in RASS1, and their X-ray fluxes are
remeasured using the Steepness Ratio Technique. This method is better suited
than the RASS1 standard algorithm for measuring flux from extended sources. The
final sample is count-rate-limited in the ROSAT hard band (0.5-2.0 keV), so
that due to the distribution of NH, its effective flux limit varies between
about 3-4 x 10**-12 ergs cm**-2 s**-1 over the selected area. This covers the
Decl<2.5 deg part of the south Galactic cap region (b<-20 deg) - with the
exclusion of patches of low RASS1 exposure time and of the Magellanic Clouds
area - for a total of 8235 deg**2. 130 candidate sources fulfill our selection
criteria for bonafide clusters of galaxies in this area. Of these, 101 are
Abell/ACO clusters, while 29 do not have a counterpart in these catalogs. Of
these clusters, 126 (97%) clusters have a redshift and for these we compute an
X-ray luminosity. 20% of the cluster redshifts come from new observations, as
part of the ESO Key Program REFLEX Cluster Survey that is under completion.
Considering the intrinsic biases and incompletenesses introduced by the flux
selection and source identification processes, we estimate the overall
completeness to be better than 90%. The observed number count distribution,
LogN-LogS, is well fitted by a power law with slope alpha = 1.34 +/- 0.15 and
normalization A = 11.87 +/- 1.04 sr**-1 (10**-11 ergs cm**-2 s**-1)**alpha, in
good agreement with other measurements.Comment: 27 pages, 8 figures and 3 tables included, LaTex, emulateapj.sty and
epsf.sty, accepted for publication in ApJ: scheduled for the March 20, 1999,
Vol.514. The cluster catalog is available at
http://www.merate.mi.astro.it/~degrand
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