5,022 research outputs found
Modeling Magnetic Anisotropy of Single Chain Magnets in Regime
Single molecule magnets (SMMs) with single-ion anisotropies ,
comparable to exchange interactions J, between spins have recently been
synthesized. In this paper, we provide theoretical insights into the magnetism
of such systems. We study spin chains with site spins, s=1, 3/2 and 2 and
on-site anisotropy comparable to the exchange constants between the
spins. We find that large leads to crossing of the states with
different values in the same spin manifold of the limit.
For very large 's we also find that the states of the higher
energy spin states descend below the states of the ground state spin
manifold. Total spin in this limit is no longer conserved and describing the
molecular anisotropy by the constants and is not possible. However,
the total spin of the low-lying large states is very nearly an integer
and using this spin value it is possible to construct an effective spin
Hamiltonian and compute the molecular magnetic anisotropy constants and
. We report effect of finite sizes, rotations of site anisotropies and
chain dimerization on the effective anisotropy of the spin chains
Modeling Molecular Magnets with Large Exchange and On-Site Anisotropies
Spins in molecular magnets can experience both anisotropic exchange
interactions and on-site magnetic anisotropy. In this paper we study the effect
of exchange anisotropy on the molecular magnetic anisotropy both with and
without on-site anisotropy. When both the anisotropies are small, we find that
the axial anisotropy parameter in the effective spin Hamiltonian is the
sum of the individual contributions due to exchange and on-site anisotropies.
We find that even for axial anisotropy of about , the low energy spectrum
does not correspond to a single parent spin manifold but has intruders states
arising from other parent spin. In this case, the low energy spectrum can not
be described by an effective Hamiltonian spanning the parent spin space. We
study the magnetic susceptibility, specific heat as a function of temperature
and magnetization as a function of applied field to characterize the system in
this limit. We find that there is synergy between the two anisotropies,
particularly for large systems with higher site spins.Comment: 30 pages, 11 figures and 3 tables. Supporting information included
after the main articl
The Rockefeller Foundations International Health Board and the Attempt to Eradicate Yellow Fever
Beginning in 1914, the Rockefeller Foundation's International Health Commission (which became the International Health Board in 1916 and the International Health Division in 1927) committed itself to the project of eradicating yellow fever. Its efforts were modeled on the sanitary techniques deployed by US sanitarians in Havana in 1901 and, more importantly, during the construction of the Panama Canal between 1904 and 1914, with mosquito control preeminent among them. William C. Gorgas, who led these campaigns and then came to work for the Rockefeller Foundation, argued for a key center approach to yellow fever eradication that targeted the remaining urban endemic foci of infection, with the assumption that once these seed beds of the disease were eliminated, yellow fever would fade from the planet. But as the IHB conducted campaigns in South America, Central America, and West Africa during the late 1910s and 1920s, they discovered that yellow fever's ecology and epidemiology were more complicated than they had assumed, and that a "key center" approach would not work to eradicate the disease. By the 1930s, and particularly with Fred Soper's discovery of sylvan or jungle yellow fever, the Rockefeller Foundation gave up on their eradicationist dream
Evidence for Adsorption of Chlorine Species on Iron(III) (hydr)oxides in the Sheepbed Mudstone, Gale Crater, Mars
Chlorine is a widespread element on Mars present in dust, soils and rocks, including the Sheepbed mudstone at Yellowknife Bay, Gale crater. Combined elemental and volatile analyses of two drilled samples, Cumberland and John Klein, indicated that chloride (Cl-) and perchlorate (ClO4 -) are likely present in the mudstone. The nature of chlorine species in Sheepbed mudstone is still not well constrained. It has been proposed that both are present as amorphous or crystalline salts physically mixed with mudstone minerals. We alternatively hypothesize that adsorbed perchlorate and chloride exist in the mudstone and adsorption could occur, in particular, on Fe(III) (hydr)oxide phases as supported by laboratory observations on terrestrial materials. Mineralogical and compositional analyses of the drilled Cumberland mudstone sample revealed the presence of ~30 wt% of a Fe-rich X-ray amorphous phase. Ferrihydrite has been proposed as a component of the Fe-rich X-ray amorphous material. The objectives of this work were to determine adsorption of perchlorate and chloride on ferrihydrite and to enable data comparison by characterizing adsorbed chloride and perchlorate with thermal and evolved gas analysis run under operating conditions similar to the SAM instrument onboard the Curiosity rover
Predicting outcome in adults with status epilepticus
Status epilepticus (SE) is a life-threatening state of persisting or repetitive seizure activity with often permanent altered level of consciousness. Despite its high morbidity and mortality there is no consensus about the best strategy to treat specific forms of SE. The compromise between the danger related to untreated and persistent seizure activity and the possible damage induced by unnecessary aggressive treatments is challenging. Knowledge about the determinants and reliable prediction models of outcome early in the course of SE is fundamental for rapid treatment modulation and for planning the level of monitoring. This review compiles the current evidence for outcome prediction based on clinical determinants in adult SE patient
First measurement of gravitational lensing by cosmic voids in SDSS
We report the first measurement of the diminutive lensing signal arising from
matter underdensities associated with cosmic voids. While undetectable
individually, by stacking the weak gravitational shear estimates around 901
voids detected in SDSS DR7 by Sutter et al. (2012a), we find substantial
evidence for a depression of the lensing signal compared to the cosmic mean.
This depression is most pronounced at the void radius, in agreement with
analytical models of void matter profiles. Even with the largest void sample
and imaging survey available today, we cannot put useful constraints on the
radial dark-matter void profile. We invite independent investigations of our
findings by releasing data and analysis code to the public at
https://github.com/pmelchior/void-lensingComment: 6 pages, 5 figures, as accepted by MNRA
Тепломассоперенос при локальном нагреве и зажигании жидкого топлива сфокусированным потоком излучения
Выполнено численное моделирование комплекса взаимосвязанных процессов тепломассопереноса с фазовыми переходами и химическими реакциями при нагреве и последующем зажигании типичного жидкого топлива сфокусированным потоком излучения. Установлены масштабы влияния процесса поглощения энергии парогазовой смесью и жидкостью на характеристики зажигания
- …