1,596 research outputs found
Winter Conditions Influence Biological Responses of Migrating Hummingbirds
Conserving biological diversity given ongoing environmental changes requires the knowledge of how organisms respond biologically to these changes; however, we rarely have this information. This data deficiency can be addressed with coordinated monitoring programs that provide field data across temporal and spatial scales and with process-based models, which provide a method for predicting how species, in particular migrating species that face different conditions across their range, will respond to climate change. We evaluate whether environmental conditions in the wintering grounds of broad-tailed hummingbirds influence physiological and behavioral attributes of their migration. To quantify winter ground conditions, we used operative temperature as a proxy for physiological constraint, and precipitation and the normalized difference vegetation index (NDVI) as surrogates of resource availability. We measured four biological response variables: molt stage, timing of arrival at stopover sites, body mass, and fat. Consistent with our predictions, we found that birds migrating north were in earlier stages of molt and arrived at stopover sites later when NDVI was low. These results indicate that wintering conditions impact the timing and condition of birds as they migrate north. In addition, our results suggest that biologically informed environmental surrogates provide a valuable tool for predicting how climate variability across years influences the animal populations
Microflow valve control system design
A design synthesis for a microflow control system is presented based on the interrogation of an analytical model, testing, and observation. The key issues relating to controlling a microflow using a variable geometry flow channel are explored through the implementation and testing of open and closed-loop control systems. The reliance of closed-loop systems on accurate flow measurement and the need for an open-loop strategy are covered. A valve and control system capable of accurately controlling flowrates between 0.09 and 400 ml/h and with a range of 900:1 is demonstrated
A selfconsistent theory of current-induced switching of magnetization
A selfconsistent theory of the current-induced switching of magnetization
using nonequilibrium Keldysh formalism is developed for a junction of two
ferromagnets separated by a nonmagnetic spacer. It is shown that the
spin-transfer torques responsible for current-induced switching of
magnetization can be calculated from first principles in a steady state when
the magnetization of the switching magnet is stationary. The spin-transfer
torque is expressed in terms of one-electron surface Green functions for the
junction cut into two independent parts by a cleavage plane immediately to the
left and right of the switching magnet. The surface Green functions are
calculated using a tight-binding Hamiltonian with parameters determined from a
fit to an {\it ab initio} band structure.This treatment yields the spin
transfer torques taking into account rigorously contributions from all the
parts of the junction. To calculate the hysteresis loops of resistance versus
current, and hence to determine the critical current for switching, the
microscopically calculated spin-transfer torques are used as an input into the
phenomenological Landau-Lifshitz equation with Gilbert damping. The present
calculations for Co/Cu/Co(111) show that the critical current for switching is
, which is in good agreement with experiment.Comment: 23 pages, 16 figure
Permanent current from non-commutative spin algebra
We show that a spontaneous electric current is induced in a nano-scale
conducting ring just by putting three ferromagnets. The current is a direct
consequence of the non-commutativity of the spin algebra, and is proportional
to the non-coplanarity (chirality) of the magnetization vectors. The
spontaneous current gives a natural explanation to the chirality-driven
anomalous Hall effect.Comment: 7 pages, 4 figures on separate pag
Ferromagnetic Domain Distribution in Thin Films During Magnetization Reversal
We have shown that polarized neutron reflectometry can determine in a
model-free way not only the mean magnetization of a ferromagnetic thin film at
any point of a hysteresis cycle, but also the mean square dispersion of the
magnetization vectors of its lateral domains. This technique is applied to
elucidate the mechanism of the magnetization reversal of an exchange-biased
Co/CoO bilayer. The reversal process above the blocking temperature is governed
by uniaxial domain switching, while below the blocking temperature the reversal
of magnetization for the trained sample takes place with substantial domain
rotation
Severe Coronavirus Disease 2019 Infection in an Adolescent Patient After Hematopoietic Stem Cell Transplantation
© 2020 American College of Chest Physicians Infection with the severe acute respiratory syndrome coronavirus 2 causes severe acute lung injury in approximately 5% of infected adults, but few reports have been made of severe pediatric disease. We present an adolescent patient who contracted severe acute respiratory syndrome coronavirus 2 one week after a paternal haplo-identical hematopoietic stem cell transplant, with development of severe hyperferritinemic acute lung injury and macrophage activation-like syndrome. We present her case and a comparison of her laboratory data with those of a cohort of pediatric patients with coronavirus disease 2019 without severe disease
Microwave excitations associated with a wavy angular dependence of the spin transfer torque : model and experiments
The spin transfer torque (STT) can lead to steady precession of magnetization
without any external applied field in magnetic spin valve where the magnetic
layer have very different spin diffusion length. This effect is associated with
an unusual angular dependence of the STT, called "wavy" (WAD-STT), predicted in
the frame of diffusive models of spin transfer. In this article, we present a
complete experimental characterization of the magnetization dynamics in the
presence of a WAD-STT. The results are compared to the prediction of the
magnetization dynamics obtained by single domain magnetic simulations
(macrospin approximation). The macrospin simulations well reproduced the main
static and dynamical experimental features (phase diagram, R(I) curves,
dependence of frequency with current and field) and suggest that the dynamical
excitations observed experimentally are associated with a large angle
out-of-plane precession mode. The present work validates the diffusive models
of the spin transfer and underlines the role of the spin accumulation and the
spin relaxation effects on the STT
Field dependence of magnetization reversal by spin transfer
We analyse the effect of the applied field (Happl) on the current-driven
magnetization reversal in pillar-shaped Co/Cu/Co trilayers, where we observe
two different types of transition between the parallel (P) and antiparallel
(AP) magnetic configurations of the Co layers. If Happl is weaker than a rather
small threshold value, the transitions between P and AP are irreversible and
relatively sharp. For Happl exceding the threshold value, the same transitions
are progressive and reversible. We show that the criteria for the stability of
the P and AP states and the experimentally observed behavior can be precisely
accounted for by introducing the current-induced torque of the spin transfer
models in a Landau-Lifschitz-Gilbert equation. This approach also provides a
good description for the field dependence of the critical currents
Reorientation of Spin Density Waves in Cr(001) Films induced by Fe(001) Cap Layers
Proximity effects of 20 \AA thin Fe layers on the spin density waves (SDWs)
in epitaxial Cr(001) films are revealed by neutron scattering. Unlike in bulk
Cr we observe a SDW with its wave vector Q pointing along only one {100}
direction which depends dramatically on the film thickness t_{Cr}. For t_{Cr} <
250 \AA the SDW propagates out-of-plane with the spins in the film plane. For
t_{Cr} > 1000 \AA the SDW propagates in the film plane with the spins
out-of-plane perpendicular to the in-plane Fe moments. This reorientation
transition is explained by frustration effects in the antiferromagnetic
interaction between Fe and Cr across the Fe/Cr interface due to steps at the
interface.Comment: 4 pages (RevTeX), 3 figures (EPS
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