556 research outputs found
Growth of a dynamical correlation length in an aging superspin glass
We report on zero field cooled magnetization relaxation experiments on a
concen- trated frozen ferrofluid exhibiting a low temperature superspin glass
transition. With a method initially developed for spin glasses, we investigate
the field dependence of the relaxations that take place after different aging
times. We extract the typical number of correlated spins involved in the aging
dynamics. This brings important insights into the dynamical correlation length
and its time growth. Our results, consistent with expressions obtained for spin
glasses, extend the generality of these behaviours to the class of superspin
glasses. Since the typical flipping time is much larger for superspins than for
atomic spins, our experiments probe a time regime much closer to that of
numerical simulations
From Frictional to Viscous Behavior: Three Dimensional Imaging and Rheology of Gravitational Suspensions
We probe the three dimensional flow structure and rheology of gravitational
(non-density matched) suspensions for a range of driving rates in a
split-bottom geometry. We establish that for sufficiently slow flows, the
suspension flows as if it were a dry granular medium, and confirm recent
theoretical modeling on the rheology of split-bottom flows. For faster driving,
the flow behavior is shown to be consistent with the rheological behavior
predicted by the recently developed "inertial number approaches for suspension
flows.Comment: 5 pages, 4 figures, accepted for Phys. Rev. E. (R
Zero-temperature spin-glass freezing in self-organized arrays of Co nanoparticles
We study, by means of magnetic susceptibility and magnetic aging experiments,
the nature of the glassy magnetic dynamics in arrays of Co nanoparticles,
self-organized in N layers from N=1 (two-dimensional limit) up to N=20
(three-dimensional limit). We find no qualitative differences between the
magnetic responses measured in these two limits, in spite of the fact that no
spin-glass phase is expected above T=0 in two dimensions. More specifically,
all the phenomena (critical slowing down, flattening of the field-cooled
magnetization below the blocking temperature and the magnetic memory induced by
aging) that are usually associated with this phase look qualitatively the same
for two-dimensional and three-dimensional arrays. The activated scaling law
that is typical of systems undergoing a phase transition at zero temperature
accounts well for the critical slowing down of the dc and ac susceptibilities
of all samples. Our data show also that dynamical magnetic correlations
achieved by aging a nanoparticle array below its superparamagnetic blocking
temperature extend mainly to nearest neighbors. Our experiments suggest that
the glassy magnetic dynamics of these nanoparticle arrays is associated with a
zero-temperature spin-glass transition.Comment: 6 pages 6 figure
Learning From the Opportunities and Challenges of a Philanthropy-Private Sector Partnership
Cross-sector partnerships are essential for addressing such complex social issues as improving population health. Among such partnerships, a philanthropy-private sector partnership is rare in practice; they may seem incompatible due to differences in their missions and cultures. However, these collaborations can yield positive returns for philanthropy organizations and businesses, as well as the broader community.
This article draws upon an evaluation of a partnership between the Robert Wood Johnson Foundation and Humana Inc. to highlight key insights for forming and implementing a formal partnership between a philanthropy organization and an investor-owned business.
Establishing and maintaining a philanthropy-private sector partnership is highly complex and challenging. For philanthropy staff interested in establishing a private-sector partnership, the findings suggest four key considerations: due diligence in exploring partnership fit, active engagement with philanthropy staff and in addressing key partnership issues, a process of co-creation on partnership activities, and continuous monitoring and assessment.
Within these key considerations, this evaluation highlights unique organizational attributes that have important practical considerations for philanthropy-private sector partnerships. However, these considerations also have relevance for other types of cross-sector partnerships
Texture-induced modulations of friction force: the fingerprint effect
Dry solid friction is often accompanied by force modulations originating from
stick-slip instabilities. Here a distinct, quasi-static mechanism is evidenced
leading to quasi-periodic force oscillations during sliding contact between an
elastomer block, whose surface is patterned with parallel grooves, and finely
abraded glass slides. The dominant oscillation frequency is set by the ratio
between the sliding velocity and the period of the grooves. A mechanical model
is proposed that provides a quantitative prediction for the amplitude of the
force modulations as a function of the normal load, the period of the grooves
and the roughness characteristics of the substrate. The model's main ingredient
is the non-linearity of the friction law. Since such non-linearity is
ubiquitous for soft solids, this "fingerprint effect" should be relevant to a
large class of frictional configurations and might in particular have important
consequences in human (or humanoid) active digital touch.Comment: 4 page
Anisotropy-axis orientation effect on the magnetization of {\gamma}-Fe2O3 frozen ferrofluid
The effect of magnetic anisotropy-axis alignment on the superparamagnetic
(SPM) and superspin glass (SSG) states in a frozen ferrofluid has been
investigated. The ferrofluid studied here consists of maghemite nanoparticles
(\gamma-Fe2O3, mean diameter = 8.6 nm) dispersed in glycerine at a volume
fraction of ~15%. In the high temperature SPM state, the magnetization of
aligned ferrofluid increased by a factor varying between 2 and 4 with respect
to that in the randomly oriented state. The negative interaction energy
obtained from the Curie-Weiss fit to the high temperature susceptibility in the
SPM states as well as the SSG phase onset temperature determined from the
linear magnetization curves were found to be rather insensitive to the
anisotropy axis alignment. The low temperature aging behaviour, explored via
"zero-field cooled magnetization" (ZFCM) relaxation measurements, however, show
distinct difference in the aging dynamics in the anisotropy-axis aligned and
randomly oriented SSG states.Comment: to appear in Journal of Physics D: Applied Physic
Global health partnerships: building multi-national collaborations to achieve lasting improvements in maternal and neonatal health
Abstract Background In response to health care challenges worldwide, extensive funding has been channeled to the world’s most vulnerable health systems. Funding alone is not sufficient to address the complex issues and challenges plaguing these health systems. To see lasting improvement in maternal and infant health outcomes in the developing world, a global commitment to the sharing of knowledge and resources through international partnerships is critical. But partnerships that merely introduce western medical techniques and protocols to low resource settings, without heeding the local contexts, are misguided and unsustainable. Forming partnerships with mutual respect, shared vision, and collaborative effort is needed to ensure that all parties, irrespective of whether they belong to resource rich or resource poor settings, learn from each other so that meaningful and sustained system strengthening can take place. Methods In this paper, we describe the partnership building model of an international NGO, Kybele, which is committed to achieving childbirth safety through sustained partnerships in low resource settings. The Kybele model adapts generic stages of successful partnerships documented in the literature to four principles relevant to Kybele’s work. A multiple-case study approach is used to demonstrate how the model is applied in different country settings. Results The four principle of Kybele’s partnership model are robust drivers of successful partnerships in diverse country settings. Conclusions Much has been written about the need for multi-country partnerships to achieve sustainable outcomes in global health, but few papers in the literature describe how this has been achieved in practice. A strong champion, support and engagement of stakeholders, co-creation of solutions with partners, and involvement of partners in the delivery of solutions are all requirements for successful and sustained partnerships
Diffusion of School-Based Prevention Programs in Two Urban Districts: Adaptations, Rationales, and Suggestions for Change
The diffusion of school-based preventive interventions involves the balancing of high-fidelity implementation of empirically-supported programs with flexibility to permit local stakeholders to target the specific needs of their youth. There has been little systematic research that directly seeks to integrate research- and community-driven approaches to diffusion. The present study provides a primarily qualitative investigation of the initial roll-out of two empirically-supported substance and violence prevention programs in two urban school districts that serve a high proportion of low-income, ethnic minority youth. The predominant ethnic group in most of our study schools was Asian American, followed by smaller numbers of Latinos, African Americans, and European Americans. We examined the adaptations made by experienced health teachers as they implemented the programs, the elicitation of suggested adaptations to the curricula from student and teacher stakeholders, and the evaluation of the consistency of these suggested adaptations with the core components of the programs. Data sources include extensive classroom observations of curricula delivery and interviews with students, teachers, and program developers. All health teachers made adaptations, primarily with respect to instructional format, integration of real-life experiences into the curriculum, and supplementation with additional resources; pedagogical and class management issues were cited as the rationale for these changes. Students and teachers were equally likely to propose adaptations that met with the program developers’ approval with respect to program theory and implementation logistics. Tensions between teaching practice and prevention science—as well as implications for future research and practice in school-based prevention—are considered
Real spin glasses relax slowly in the shade of hierarchical trees
The Parisi solution of the mean-field spin glass has been widely accepted and
celebrated. Its marginal stability in 3d and its complexity however raised the
question of its relevance to real spin glasses. This paper gives a short
overview of the important experimental results which could be understood within
the mean-field solution. The existence of a true phase transition and the
particular behaviour of the susceptibility below the freezing temperature,
predicted by the theory, are clearly confirmed by the experimental results. The
behaviour of the complex order parameter and of the Fluctuation Dissipation
ratio are in good agreement with results of spontaneous noise measurements. The
very particular ultrametric symmetry, the key feature of the theory, provided
us with a simple description of the rejuvenation and memory effects observed in
experiment. Finally, going a step beyond mean-field, the paper shortly
discusses new analyses in terms of correlated domains characterized by their
length scales, as well as new experiments on superspin glasses which compare
well with recent theoretical simulations.Comment: To appear in the proceedings of "Wandering with Curiosity in Complex
Landscapes", a scientific conference in honour of Giorgio Parisi for his 60th
birthday, Roma, September 8-10 2008 (submitted for the special issue of the
Journal of Statistical Physics, 2009
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