2,195 research outputs found
Understanding complex magnetic order in disordered cobalt hydroxides through analysis of the local structure
In many ostensibly crystalline materials, unit-cell-based descriptions do not
always capture the complete physics of the system due to disruption in
long-range order. In the series of cobalt hydroxides studied here,
Co(OH)(Cl)(HO), magnetic Bragg diffraction reveals a
fully compensated N\'eel state, yet the materials show significant and open
magnetization loops. A detailed analysis of the local structure defines the
aperiodic arrangement of cobalt coordination polyhedra. Representation of the
structure as a combination of distinct polyhedral motifs explains the existence
of locally uncompensated moments and provides a quantitative agreement with
bulk magnetic measurements and magnetic Bragg diffraction
Effects of density imbalance on the BCS-BEC crossover in semiconductor electron-hole bilayers
We study the occurrence of excitonic superfluidity in electron-hole bilayers
at zero temperature. We not only identify the crossover in the phase diagram
from the BCS limit of overlapping pairs to the BEC limit of non-overlapping
tightly-bound pairs but also, by varying the electron and hole densities
independently, we can analyze a number of phases that occur mainly in the
crossover region. With different electron and hole effective masses, the phase
diagram is asymmetric with respect to excess electron or hole densities. We
propose as the criterion for the onset of superfluidity, the jump of the
electron and hole chemical potentials when their densities cross.Comment: 4 pages, 3 figure
CHARA/MIRC observations of two M supergiants in Perseus OB1: temperature, Bayesian modeling, and compressed sensing imaging
Two red supergiants of the Per OB1 association, RS Per and T Per, have been
observed in H band using the MIRC instrument at the CHARA array. The data show
clear evidence of departure from circular symmetry. We present here new
techniques specially developed to analyze such cases, based on state-of-the-art
statistical frameworks. The stellar surfaces are first modeled as limb-darkened
discs based on SATLAS models that fit both MIRC interferometric data and
publicly available spectrophotometric data. Bayesian model selection is then
used to determine the most probable number of spots. The effective surface
temperatures are also determined and give further support to the recently
derived hotter temperature scales of red su- pergiants. The stellar surfaces
are reconstructed by our model-independent imaging code SQUEEZE, making use of
its novel regularizer based on Compressed Sensing theory. We find excellent
agreement between the model-selection results and the reconstructions. Our
results provide evidence for the presence of near-infrared spots representing
about 3-5% of the stellar flux
The long-period Galactic Cepheid RS Puppis - II. 3D structure and mass of the nebula from VLT/FORS polarimetry
The long-period Cepheid RS Pup is surrounded by a large dusty nebula
reflecting the light from the central star. Due to the changing luminosity of
the central source, light echoes propagate into the nebula. This remarkable
phenomenon was the subject of Paper I.The origin and physical properties of the
nebula are however uncertain: it may have been created through mass loss from
the star itself, or it could be the remnant of a pre-existing interstellar
cloud. Our goal is to determine the 3D structure of the nebula, and estimate
its mass. Knowing the geometrical shape of the nebula will also allow us to
retrieve the distance of RS Pup in an unambiguous manner using a model of its
light echoes (in a forthcoming work). The scattering angle of the Cepheid light
in the circumstellar nebula can be recovered from its degree of linear
polarization. We thus observed the nebula surrounding RS Pup using the
polarimetric imaging mode of the VLT/FORS instrument, and obtained a map of the
degree and position angle of linear polarization. From our FORS observations,
we derive a 3D map of the distribution of the dust, whose overall geometry is
an irregular and thin layer. The nebula does not present a well-defined
symmetry. Using a simple model, we derive a total dust mass of M(dust) = 2.9
+/- 0.9 Msun for the dust within 1.8 arcmin of the Cepheid. This translates
into a total mass of M(gas+dust) = 290 +/- 120 Msun, assuming a dust-to-gas
ratio of 1.0 +/- 0.3 %. The high mass of the dusty nebula excludes that it was
created by mass-loss from the star. However, the thinness nebula is an
indication that the Cepheid participated to its shaping, e.g. through its
radiation pressure or stellar wind. RS Pup therefore appears as a regular
long-period Cepheid located in an exceptionally dense interstellar environment.Comment: 14 pages, 21 figures. Accepted for publication in A&
Energy Content of Seeds of Common Sunflowers (Helianthus annuus) in the Diet of Scaled Quail (Callipepla squamata) in Southeastern New Mexico
We analyzed the energy content of seeds of common sunflowers (Helianthus annuus) obtained from the crops of scaled quail (Callipepla squamata) collected from plains-mesa sand-scrub habitat in Eddy and Lea counties, New Mexico. Seeds were removed from crops and dried for 48 hours at 60°C to remove moisture and to standardize masses. Seeds were then analyzed for gross caloric value (i.e., energy content) in an oxygen bomb calorimeter. Energy content of seeds of common sunflowers from New Mexico was greater than that of many seeds previously reported from the diet of scaled quail and other granivorous birds and comparable to previous measurements of seeds of the same species made in Kansas
A microfabricated sensor for thin dielectric layers
We describe a sensor for the measurement of thin dielectric layers capable of
operation in a variety of environments. The sensor is obtained by
microfabricating a capacitor with interleaved aluminum fingers, exposed to the
dielectric to be measured. In particular, the device can measure thin layers of
solid frozen from a liquid or gaseous medium. Sensitivity to single atomic
layers is achievable in many configurations and, by utilizing fast, high
sensitivity capacitance read out in a feedback system onto environmental
parameters, coatings of few layers can be dynamically maintained. We discuss
the design, read out and calibration of several versions of the device
optimized in different ways. We specifically dwell on the case in which
atomically thin solid xenon layers are grown and stabilized, in cryogenic
conditions, from a liquid xenon bath
Many-body correlations probed by plasmon-enhanced drag measurements in double quantum well structures
Electron drag measurements of electron-electron scattering rates performed
close to the Fermi temperature are reported. While evidence of an enhancement
due to plasmons, as was recently predicted [K. Flensberg and B. Y.-K. Hu, Phys.
Rev. Lett. 73, 3572 (1994)], is found, important differences with the
random-phase approximation based calculations are observed. Although static
correlation effects likely account for part of this difference, it is argued
that correlation-induced multiparticle excitations must be included to account
for the magnitude of the rates and observed density dependences.Comment: 4 pages, 3 figures, revtex Accepted in Phys. Rev.
Energy Content of Seeds of Texas Doveweed (Croton texensis) from the Diet of Mourning Doves (Zenaida macroura) from Southeastern New Mexico
We analyzed the energy content of seeds of Texas doveweed (Croton texensis) obtained from the crops of mourning doves (Zenaida macroura) collected from plains-mesa sand-scrub habitat in Eddy and Lea counties, New Mexico. Seeds were removed from crops and dried for 48 hours at 60°C to remove moisture and to standardize masses. Seeds were then analyzed for gross caloric value (i.e., energy content) in an oxygen bomb calorimeter. Energy content of seeds of Texas doveweed was greater than many seeds previously reported from the diet of mourning doves
Disease Progression-Dependent Effects of TREM2 Deficiency in a Mouse Model of Alzheimer's Disease
Neuroinflammation is an important contributor to Alzheimer's disease (AD) pathogenesis, as underscored by the recent identification of immune-related genetic risk factors for AD, including coding variants in the gene TREM2 (triggering receptor expressed on myeloid cells 2). Understanding TREM2 function promises to provide important insights into how neuroinflammation contributes to AD pathology. However, studies so far have produced seemingly conflicting results, with reports that amyloid pathology can be both decreased and increased in TREM2-deficient AD mouse models. In this study, we unify these previous findings by demonstrating that TREM2 deficiency ameliorates amyloid pathology early, but exacerbates it late in disease progression in the APPPS1–21 mouse model of AD. We also demonstrate that TREM2 deficiency decreases plaque-associated myeloid cell accumulation by reducing cell proliferation, specifically late in pathology. In addition, TREM2 deficiency reduces myeloid cell internalization of amyloid throughout pathology, but decreases inflammation-related gene transcript levels selectively late in disease progression. Together, these results suggest that TREM2 plays distinct functional roles at different stages in AD pathology
Dynamic correlations in symmetric electron-electron and electron-hole bilayers
The ground-state behavior of the symmetric electron-electron and
electron-hole bilayers is studied by including dynamic correlation effects
within the quantum version of Singwi, Tosi, Land, and Sjolander (qSTLS) theory.
The static pair-correlation functions, the local-field correction factors, and
the ground-state energy are calculated over a wide range of carrier density and
layer spacing. The possibility of a phase transition into a density-modulated
ground state is also investigated. Results for both the electron-electron and
electron-hole bilayers are compared with those of recent diffusion Monte Carlo
(DMC) simulation studies. We find that the qSTLS results differ markedly from
those of the conventional STLS approach and compare in the overall more
favorably with the DMC predictions. An important result is that the qSTLS
theory signals a phase transition from the liquid to the coupled Wigner crystal
ground state, in both the electron-electron and electron-hole bilayers, below a
critical density and in the close proximity of layers (d <~ r_sa_0^*), in
qualitative agreement with the findings of the DMC simulations.Comment: 13 pages, 11 figures, 2 table
- …