4,152 research outputs found
Cool White Dwarfs Found in the UKIRT Infrared Deep Sky Survey
We present the results of a search for cool white dwarfs in the United
Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area
Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky
Survey (SDSS) to identify cool hydrogen-rich white dwarf candidates by their
neutral optical colors and blue near-infrared colors, as well as faint Reduced
Proper Motion magnitudes. Optical spectroscopy was obtained at Gemini
Observatory, and showed the majority of the candidates to be newly identified
cool degenerates, with a small number of G- to K-type (sub)dwarf contaminants.
Our initial search of 280 deg2 of sky resulted in seven new white dwarfs with
effective temperature T_eff ~ 6000 K. The current followup of 1400 deg2 of sky
has produced thirteen new white dwarfs. Model fits to the photometry show that
seven of the newly identified white dwarfs have 4120 K <= T_eff <= 4480 K, and
cooling ages between 7.3 Gyr and 8.7 Gyr; they have 40 km/s <= v_tan <= 85 km/s
and are likely to be thick disk 10-11 Gyr-old objects. The other half of the
sample has 4610 K <= T_eff <= 5260 K, cooling ages between 4.3 Gyr and 6.9 Gyr,
and 60 km/s <= v_tan <= 100 km/s. These are either thin disk remnants with
unusually high velocities, or lower-mass remnants of thick disk or halo late-F
or G stars.Comment: To appear in ApJ, accepted April 18 2011. 34 pages include 11 Figures
and 5 Table
Élaboration de normes DRIS provisoires pour des transplants de céleri
Nous avons utilisé une banque de données contenant 215 observations pour obtenir des normes DRIS (Diagnosis and Recommandation Integrated System) provisoires pour des transplants de céleri (Apium graveolens var. Dulce). La détermination des normes s'est faite en considérant un groupe de tête au rendement supérieur ou égal à 1600 g/plant (27 % de la population). Sur 45 rapports nutritionnels mesurés dans la partie aérienne des transplants, 26 ont présenté des rapports de variance permettant de distinguer significativement le groupe produisant des rendements supérieurs. Le coefficient de corrélation entre l'indice de déséquilibre nutritionnel (IDN) et les rendements s'est révélé très significatif. Les normes provisoires ont été confrontées à un ensemble indépendant de données obtenues chez des producteurs de la région du sud de Montréal (Québec). Les IDN calculés sur ces plantes échantillonnées au stade implantation (environ 27 jours après la plantation) ont été significativement corrélés aux rendements, malgré le fait que le tissu échantillonné et le stade de croissance aient été relativement éloignés de ceux correspondant aux normes. Les normes ont permis d'identifier la cause probable d'un problème de croissance induit par l'utilisation d'une solution fertilisante ne convenant pas à la production de transplants.A data bank of 215 observations was used to elaborate preliminary DRIS norms for celery (Apium graveolens var.Dulce) transplants. The threshold yield for high yielders was set at 1600 g/plant (27% of the population). Of the 45 nutrient ratios obtained, 26 presented significant variance ratios between low and high yielders. The correlation coefficient between nutrient imbalance index (Nil) and yield was highly significant. The norms were tested on an independent set of data obtained from celery growers located south of Montreal (Quebec). The Nil obtained from plants at the implantation stage (about 27 days after transplanting) was significantly correlated to yield even if the sampled tissue was different from that used for norm calculations. The norms were also used to identify the likely cause of a growth problem induced by a nutrient solution not suited for transplant production
Arc-spot Welds for Multi-overlap Roof Deck Panels
Roof deck construction often incorporates cold-formed steel panels that are connected to the underlying framing with the use of arc-spot welds. The welds are commonly located in areas where multiple layers of roof deck exist, such as at sidelaps or endlaps. CSA S136 restricts the use of multi-layer connections to being less than 2.5 mm thick; as well, the thickness of the supporting steel must exceed 2.5 times the aggregate thickness of the deck. In effect, the standard does not allow for the use of arc-spot welds for 18 ga (1.21 mm) and 16 ga (1.52 mm) roof deck panels. Nonetheless, it is not unusual for these deck panels to be used in construction; a solution to the arc-spot weld restrictions and a new welding protocol was needed. This situation led to the initiation of a research program on the shear resistance and tension resistance of multi-layer arc spot welds. The paper describes the welding protocol that was developed to obtain adequate quality and size arc-spot welds in up to four layers of 16 ga. deck. Weld test specimens were fabricated through one, two or four layers of steel sheets with thicknesses ranging from 22 ga. (0.76 mm) to 16 ga. (1.52 mm). Various sheet steel / weld configurations found in roof deck construction were included. A total of 72 tension tests and 107 shear tests were completed. Adequate weld quality could be achieved in all cases except that welds were undersized when the total sheet thickness becomes twice as large as the thickness of the underlying material. The results were compared with the current provisions of CSA S136 and modifications to the existing design equations are recommended
Magnetic and pair correlations of the Hubbard model with next-nearest-neighbor hopping
A combination of analytical approaches and quantum Monte Carlo simulations is
used to study both magnetic and pairing correlations for a version of the
Hubbard model that includes second-neighbor hopping as a
model for high-temperature superconductors. Magnetic properties are analyzed
using the Two-Particle Self-Consistent approach. The maximum in magnetic
susceptibility as a function of doping appears both at finite
and at but for two totally different physical reasons. When
, it is induced by antiferromagnetic correlations while at
it is a band structure effect amplified by interactions.
Finally, pairing fluctuations are compared with -matrix results to
disentangle the effects of van Hove singularity and of nesting on
superconducting correlations. The addition of antiferromagnetic fluctuations
increases slightly the -wave superconducting correlations despite the
presence of a van Hove singularity which tends to decrease them in the
repulsive model. Some aspects of the phase diagram and some subtleties of
finite-size scaling in Monte Carlo simulations, such as inverted finite-size
dependence, are also discussed.Comment: Revtex, 8 pages + 15 uuencoded postcript figure
Variability of potato petiole nitrogen in response to nitrogen fertilizer, implications for variable management
Non-Peer ReviewedRecent increases in the cost of fertilizer nitrogen have prompted producers to assess the
potential to vary inputs in space and time to produce the highest marketable yield of
potatoes. A study was conducted from 2005 to 2007 near Brandon, Manitoba Canada, to
assess the spatial variability of potato yield in upper, middle and lower landforms on a
sandy loam soil in response to a range of nitrogen fertilizer rates and split application.
Petiole nitrogen, determined late in the growing season, was correlated with potato yield
and was used to assess nitrogen sufficiency through the growing season. Petiole nitrogen
varied with time during the growing season, from uniform levels in June across all
fertilizer treatments, to those which varied with fertilizer treatment in July and August.
Furthermore potato petiole nitrogen was higher in lower landforms during July and
August, where higher total and marketable yields were recorded. The potential for split
application of nitrogen in potatoes based on management zones or sensor readings will
have to be carefully assessed to account for temporal and spatial variability
Mott physics and first-order transition between two metals in the normal state phase diagram of the two-dimensional Hubbard model
For doped two-dimensional Mott insulators in their normal state, the
challenge is to understand the evolution from a conventional metal at high
doping to a strongly correlated metal near the Mott insulator at zero doping.
To this end, we solve the cellular dynamical mean-field equations for the
two-dimensional Hubbard model using a plaquette as the reference quantum
impurity model and continuous-time quantum Monte Carlo method as impurity
solver. The normal-state phase diagram as a function of interaction strength
, temperature , and filling shows that, upon increasing towards
the Mott insulator, there is a surface of first-order transition between two
metals at nonzero doping. That surface ends at a finite temperature critical
line originating at the half-filled Mott critical point. Associated with this
transition, there is a maximum in scattering rate as well as thermodynamic
signatures. These findings suggest a new scenario for the normal-state phase
diagram of the high temperature superconductors. The criticality surmised in
these systems can originate not from a T=0 quantum critical point, nor from the
proximity of a long-range ordered phase, but from a low temperature transition
between two types of metals at finite doping. The influence of Mott physics
therefore extends well beyond half-filling.Comment: 27 pages, 16 figures, LaTeX, published versio
Non-local corrections to dynamical mean-field theory from the two-particle self-consistent method
Theoretical methods that are accurate for both short-distance observables and
long-wavelength collective modes are still being developed for the Hubbard
model. Here, we benchmark against published diagrammatic quantum Monte Carlo
results an approach that combines local observables from dynamical mean-field
theory (DMFT) with the two-particle self-consistent theory (TPSC). This method
(TPSC+DMFT) is relevant for weak to intermediate interaction, satisfies the
local Pauli principle and allows us to compute a spin susceptibility that
satisfies the Mermin-Wagner theorem. The DMFT double occupancy determines the
spin and charge vertices through local spin and charge sum rules. The TPSC
self-energy is also improved by replacing its local part with the local DMFT
self-energy. With this method, we find improvements for both spin and charge
fluctuations and for the self-energy. We also find that the accuracy check
developed for TPSC is a good predictor of deviations from benchmarks. TPSC+DMFT
can be used in regimes where quantum Monte Carlo is inaccessible. In addition,
this method paves the way to multi-band generalizations of TPSC that could be
used in advanced electronic structure codes that include DMFT.Comment: 15 pages, 19 figures. Changes from v1: added reference
First order Mott transition at zero temperature in two dimensions: Variational plaquette study
The nature of the metal-insulator Mott transition at zero temperature has
been discussed for a number of years. Whether it occurs through a quantum
critical point or through a first order transition is expected to profoundly
influence the nature of the finite temperature phase diagram. In this paper, we
study the zero temperature Mott transition in the two-dimensional Hubbard model
on the square lattice with the variational cluster approximation. This takes
into account the influence of antiferromagnetic short-range correlations. By
contrast to single-site dynamical mean-field theory, the transition turns out
to be first order even at zero temperature.Comment: 6 pages, 5 figures, version 2 with additional results for 8 bath
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